A warning if you plan to use the FE-5680A as a
10 MHz reference for microwave transverters:
As a 10 MHz reference for microwave transverters, I've found
the "barefoot" FE-5680A to be unsuitable
microwave transverters due to low-level audio-frequency phase
modulation on the output.
To use the FE-5680A with a microwave transverter it
is necessary to "clean up" its output by locking another
crystal oscillator to it as
FE-5680A's in seemingly identical packages:
The complete FE-5680A-based 10 MHz reference.
Click on the image for
a larger version.
There are a number of versions
of the FE-5680A on the surplus market - some of which are very
different from each other but unfortunately, the outsides of
these units appear nearly identical with no obvious means of
telling which version is which. At the time of the
original writing of this page (December 2011) there were two
types of FE-5680A available and which version one got depended
partly on luck and also on how informed the seller of the
device is on which type is being offered.
The most common types of
FE-5680A seem to be:
- Frequency Programmable.
This version uses DDS techniques to produce an output
frequency that is programmable in sub-hertz steps from
around 20 MHz down to audio frequencies (after modification)
and these units have been used by hobbyists as signal
generators and as exciters for LF/MF/HF beacon
transmitters. These units are not generally suitable as
references for microwave transverters or as ultra-precise
arbitrary frequency generators due to the fact that the DDS
techniques used tend to generate low-level spurious signals
and that the exact
frequency being synthesized is limited to small - but
discrete - steps dictated by the finite resolution of the
DDS chip itself. These units can be programmed via the
serial interface in a human-readable manner using a "dumb"
terminal program and other programs to set frequencies more
conveniently (e.g. without a calculator!) are readily
available. This "programmable" variety
is the version described on the the ZL1BPU Rubidium
page. As of late 2011/early 2012, this
type is the more expensive of the two and less common, often
selling in the $60-$90 area. These units are definitely not
suitable for use as a frequency reference for a
microwave transverter without a "clean-up" oscillator
such as the one described below and their finite tuning
resolution may produce slightly "off-frequency" results
when multiplied to microwave frequencies.
This version does not have a synthesizer that can
be steered over a wide range, but rather separate 10 MHz and
1 pps (pulse-per-second) outputs found on the 9-pin D
connector. These units typically require an external 5
volt source to operate some of the internal logic and
programming via the serial interface requires that raw bytes
be sent to "tweak" (by only a tiny fraction of a Hertz!) the
10 MHz precisely
to frequency. Internally, all components are installed
on ONE circuit board. This
"Non-Programmable" unit is the version described on the
VK3UM Rubidium page.
As of late 2011/early 2012, this type appears to be
the less expensive ($30-$45 typically) and more common
variety. Even though these units appear to have
"cleaner" RF outputs than the "programmable" versions, they
directly suitable for use as a 10 MHz frequency reference
for microwave transverters and require a "clean-up"
oscillator as noted above.
- The "8.3886 MHz" version. There seems to be a
version that outputs 1PPS and 8.3866something MHz from a
tiny RF connector on an internal board. I don't
have one of these units, nor do I have any information
as to whether or not these can be made into a useful
It is the
2nd of these, the "Non-Programmable"
version, that is described as being used on this page!
Why a frequency reference?
"I can't see the
One comment frequently seen by those who are
evaluating the FE-5680A is that they are unable to see
the 1PPS output.
If you are using an old, analog type scope, it is very
difficult to see this pulse since it is only
a microsecond or so wide. Since it
occurs only once every second - and because it is so
brief - it is slightly tricky to get even a good-quality
'scope to trigger on it.
To do this, one must very carefully
adjust the triggering threshold and configure the 'scope
so that it sweeps only when triggered
rather than repeatedly. Even with a good, bright
tube in the scope, it often requires that the intensity
be turned way up and the room darkened to see the
"occasional", narrow pulse as it gets painted on the
It's often a bit easier to get a DSO (Digital Storage
Oscilloscope) to display this pulse since, unlike an old
analog scope one can simply set the sweep to show about
a microsecond per division and easily see the pulse once
the 'scope is properly triggered.
Perhaps the easiest way to detect the
1PPS pulse is to connect a small audio amplifier to the
output and turn up the volume. For this I used a
cheap, Radio Shack amplified speaker and was readily
able to hear the once-per-second "tick" of the output
once the unit warmed up!
When operating on the microwave amateur radio bands, narrowband
modes (such as SSB or CW) are often used to maximize the link
margin - that is, being able to talk when signals are weak as
the SSB or CW modes can offer 10's of dB improvement over the
wideband FM modes used with Gunn transceivers.
There is a catch, however: The use of microwave
narrowband modes such as SSB or CW means that the one must
maintain pretty good frequency stability and
- Stability is important!
A drift of even a few hundred Hz at the operating frequency
(in the GHz range!) can affect intelligibility of voice -
or, if CW is being used for weak-signal work, such drifting
can move the received signal outside the receiver's passband
filter. Having to "chase" the frequency around is not
only distracting, but it complicates being able to
communicate in the first place.
- Accuracy is critical!
Because it is important that both parties be confident that
their stated operating frequencies are reasonably close to
where they think they are, frequency-wise, it's important
that the frequency be precisely known. If a contact is
arranged beforehand it is vital that both parties be able to
find each other simply by knowing the intended frequency of
communication and as long as the two parties are within
several hundred Hz of each other, it is more likely that
they will be able to complete the contact. If the
error was on the order of several kHz or 10's of kHz,
"hunting" would be required to find the signal and if those
signals are weak, they might be missed entirely -
particularly if, in addition to tuning around, it was
necessary to move the antennas about as well.
Because achieving such stability and accuracy requires some
effort, it is more convenient if microwave gear is constructed
such that it can use a common, external frequency reference and
lock to it, and this is true for several reasons:
- Only one reference is required. It's better
to expend the effort in putting together just one "master",
stable reference rather than each piece of gear having its
own reference. In this way, the extra money, time and
effort saved can be put toward having this one reference be
as good as you can make it.
- Power savings. Having a common reference can
also be convenient if one is operating portable using
battery power. Using an external reference means that
one doesn't need to keep all of those individual pieces of
gear "warmed up" all of the time to maintain stability,
turning it on (and draining battery power) only when it is
needed. For this reason, many amateur radio operators
(and commercial equipment manufacturers, for that matter)
design their gear to accept a 10 MHz input from a
known-accurate and stable source.
In addition to rubidium references I also have a 10 MHz "ovenized" crystal
that I generally use instead of a "ruby."
While not as accurate, the crystal oscillator's stability and
accuracy is more than adequate for operation at least through 24
GHz (it is within a few hundred Hz at that frequency)
and consuming significantly less power to operate than the
Rubidium reference - an important consideration when operating
from battery power. Nevertheless, it's nice to have
something that is portable and "dead on" frequency in less than
5 minutes after cold startup and can also be used as a backup if
About this frequency reference:
The FE-5680A was originally used in mobile/cellular telephone
sites to provide an accurate frequency and/or timing reference
for network synchronization, etc. and as such, it has very good
intrinsic accuracy as compared to conventional quartz
oscillators. After their end-of-life, the equipment was
sent overseas to be "recycled" and in the process, these units
have appeared on EvilBay with many being shipped back, into the
hands of experimenters.
A rubidium reference - unlike a quartz crystal oscillator which
has no clearly-defined "wear out" period and, if well-designed,
can actually improve
as time goes on - has a definite
lifetime associated with its lamp: As the unit operates,
the Rubidium within the lamp is less-available to be vaporized
and eventually, too little is available for the atomic resonance
to be detected and the unit fails to achieve lock and it is for
this reason that many amateurs who have Rubidium references
them on all of the time. For "base station" use a
GPS-based disciplined quartz oscillator is often used as the
"primary" reference against which the Rubidium unit is compared,
but since a GPS-based disciplined reference is, by its nature,
not "portable" - that is, you can't just move it around unless
you stay in one location for many hours, giving it time to
re-lock and the disciplined oscillator to achieve reasonable
accuracy - a Rubidium reference fills the niche, providing very
high accuracy and stability in a portable package..
At "room temperature" (approx. 68F or 20C) the FE-5680A takes
about 3 minutes to warm up and "lock" (much faster than a
crystal-oven reference!) almost immediately providing accuracy
equal to or better than a good-quality "ovenized" quartz
oscillator. The version of the The FE-5680A described here
has available - via its serial port - a means to make fine
adjustments to the output frequency, allowing "tweaking" of the
10 MHz output frequency to within a few parts of 10E11 under
stable "bench-top" conditions. (The actual adjustment steps are much finer than
that, but the degree of stability noted is more realistic.)
The FE-5680A has an output that goes from high (4.5-5 volts) to
low (0-0.5 volts) when the "physics lock" has been
detected. It should be noted that until this indication is
made the 10 MHz output will be sweeping a few 10's or hundreds
of Hz and it should not be trusted
to provide any sort of accurate frequency
reference, but at the instant
it goes low it will likely
be within about 10E-7, gradually achieving something that's
closer to its ultimate accuracy over the next 5-10 minutes.
This unit also has a 1pps output (present only if a "physics lock" has been
that provides a positive-going pulse that is
about 1 microsecond wide: Unless you are pretty good at
tweaking, you may have a bit of trouble seeing it on an analog
oscilloscope other than by observing that the "trigger" light
would flash every second, so the easiest
way to detect it is
by "listening" to it with an audio amplifier.
Pinout of the FE-5680A:
of the FE-5680A interfaces via a DE-9 connector
- its only
external connector - through which power is
applied and signals (including the 10 MHz) emerge. These
pins are used as follows:
- V+ input (15-18 volts, typical.) The current is more
or less constant across this voltage range with 15 volts
producing less heat and consuming less power.
- Ground - the same as pin 5.
- Lock indication: High (>4 volts) = Error and/or
unlocked; Low (<0.5 volts) = Physics lock (e.g.
- +5 Volts. Most units lack an internal 5 volt supply
and require this voltage to operate. The current
consumption is only a few hundred milliamps, easily supplied
by an external 7805 regulator.
- Ground - the same as pin
- 1 PPS (Pulse Per Second) output. A 1
microsecond-wide pulse appears on this pin once the unit has
achieved lock and the lock indication pin goes low.
This pulse can be difficult to see on some oscilloscopes,
but can be easily "heard" with an audio amplifier. There will
be no output until pin 3 goes low, indicating that a lock
- 10 MHz output. The accuracy of this output should not
be trusted unless there is a lock indication (e.g. pin 3 is
- Serial data RX, RS-232 levels. Binary commands are
used to adjust the unit's frequency to precisely net it to
frequency using a program such as the one linked on VK3UM's
page (see above.)
- Serial data TX, RS-232 levels. Used in conjunction
with pin 8.
Again, before applying power you must
verify that this unit is of the proper type!
Remember: Although the different units are labeled as
being an "FE-5680A", there are a number of variants and it may
be possible to cause damage if one applies power to a unit with
a different pinout or input requirements. To determine the
version that you have, refer to the links at the top of the page
as well as the pictures in Figure 3, below.
Most of the units available at the time of the original
writing of this page (early 2012) are of the "non programmable"
variety - that is, they output only
10 MHz - which are
the type described on this page. Over time, it appears
that other, similar-looking units appear that may or may not be
suitable: One must at least partially rely on the
knowledge and integrity of the seller to be sure that you are
getting the proper unit.
Putting it in a box:
I happen to have a large number of "pre-owned" Hammond 1590D
die-cast aluminum boxes kicking around (they had previously housed Glencom VC-510 units)
so it was a "natural" to cram the FE-5680A and its associated
support circuitry into it. Experience with the Efratom
has shown that this box capable of adequately
dissipating the heat generated and since the overall power
requirements of the '5680A are similar, I figured that the box
would do fine for it as well.
Before mounting it on the lid I used a rotary tool to remove a
few protruding mold marks on the inside that would have
prevented the unit from mounting flat against its surface.
Because of the large area, I didn't really bother with cleaning
off much paint - nor did I smear it with heat sink compound and
have noted that the unit's cover temperature and the top
(outside) of the lid are at about the same temperature when
operating. At room temperature I see a 25-30 degree F
(11-16C) rise which indicates a "lid" temperature that is around
105F (around 40C), well with in "safe" operating conditions in
all but high ambient temperatures.
This "used" Hammond box already had several holes in it:
The one on the rear was covered with a piece of tape on the
outside and then filled from the inside with 2-part epoxy (after
removing paint from the inside area around the hole) to seal it
up to keep bugs and dirt out. Other holes on the opposite
end of the box happened to be in convenient locations already
and a few more were drilled to accommodate the three BNC
connectors for the 10 MHz outputs and the status indicator
LED. I cut a small piece of glass-epoxy copper clad board,
secured it with two 4-40 screws and covered a hole on the
opposite end and mounted in it a solder-in feedthrough capacitor
using one of the screws to hold a strain relief clip for the
power connection wire.
Power supply and
- The information in my
write-up about using the Efratom LPRO-101 could also
be used with the FE-5680A and vice-versa, the only important
difference being that the FE-5680A will run happily on 15
volt whereas the LPRO-101 needs a minimum of 19 volts.
It is worth mentioning that the LPRO-101 has an output that
is sufficiently clean as to NOT
require a clean-up oscillator to be used as a reference for
Diagram of the unit integrating the FE-5680A rubidium
frequency reference showing supply protection/filtering,
status indicator, and bandpass filter with distribution
Diagram and layout of the switcher used in the unit.
Note the two paralleled 1 ohm resistors just to the left
of the switch-mode controller chip just below the aluminum
heat sink on the coil.
Inside the 10 MHz rubidium frequency reference using the
FE-5680A. On the left, mounted in the lid, is the
FE-5680A and inside the box is the switching voltage
converter (near the top) and the 10 MHz
filter/distribution amplifier and status indicator near
the bottom. It happens that this particular FE-5680A
has a factory-installed 5 volt switching regulator - see
below for more details. If it had not the internal
regulator, I would have used a 7805 bolted to the box for
the +5 volt supply. The lid of the die-cast aluminum
box provides adequate heat sinking for the FE-5680A.
Close-up view of the switching up converter along with its
added input/output filtering.
Click on an image for a
When I packaged my LPRO-101
I "rolled my own"
switching converter using the LM2577 chip. It was easy
enough to do, but I decided to look around for some pre-built
modules and found on EvilBay some cheap switching up converter
that appeared to have the necessary capabilities to provide an
up-converted voltage for the FE-5680A - all for $2.99 plus
shipping: This was than less
the low quantity cost of just
the LM2577 I'd used with my LPRO-101! If I'd wanted to do
so, I could have simply replicated the same voltage converter
that I'd built for the LPRO-101 and adjusted its output voltage
for 15 volts.
When I received the voltage converter units I noted that the
build quality was "ok" , but not great (what do you expect for $2.99?)
tested to see how well it powered the FE-5680A I found that it
went into current limiting when the '5680A was cold, lengthening
the warm-up time of the unit. The modification to
ameliorate this was simple: Increase the current
capability slightly by soldering a resistor across R1/R2.
In this case I used a pair of paralleled 1 ohm, 1/4 watt units
(because I couldn't find a 0.33 or 0.47 ohm watt resistor in my
resistor collection at the time) to boost the current capability.
Now, the unit still goes into current limiting when cold, but
even at the lower voltage (which is, at worst case, a diode drop
below the supply voltage) the internal heaters quickly warm and
in a few 10's of seconds, current consumption drops and the
voltage rises to the required 15 volts to properly run the
'5680A. As with any "new" switcher it is strongly
recommended that the output voltage be adjusted with at least a
light load (a few hundred ohms will usually do) before
connecting it to what it is to power to verify proper, stable
Even with the period of current-limiting, the '5680A achieves
lock in under 3 minutes at room temperature and is slightly
faster than my LPRO-101.
Because I was stressing the unit even more by increasing its
output current capability and to improve reliability, I soldered
a small piece of copper to the tab of the transistor (the
collector) and epoxied (using "JB Weld") a piece of aluminum to
the top of the inductor as seen in the top-right picture of Figure 2
additional transistor heat sinking lowered its temperature and,
by association, the temperature of the capacitor next to
it. The heat sink on the coil is probably overkill but was
done to help maintain the permeability of its core under all
conditions as well as to keep the capacitor next to it a bit
cooler. Since the circuit board was going to be enclosed
in a box, the extra bit of heat-sinking seemed like a good idea.
Power supply filtering:
Because of the very nature of switching voltage converters and
the fact that it was to be used in an RF-sensitive application,
additional input and output filtering was applied to the
converter to keep switching energy from making its way into the
FR-5680A or back onto the power supply bus. Being that
this switching converter unit was very inexpensive I didn't
trust the quality of the two capacitors on the board in the
presence of high switching currents and elevated operating
temperatures so off-board capacitors are added in parallel with
For additional filtering, 10 uH inductors from a scrapped
computer power supply and more Low-ESR capacitors were used to
complete a low-pass C-L-C "PI" filter network, all being mounted
"dead bug" on a piece of glass-epoxy circuit board. The
switcher itself was solidly mounted to the same piece of
glass-epoxy board using short pieces of #12 copper wire soldered
to the "-IN" and "-OUT" leads and the ground plane itself.
It's worth noting that the additional components may drop the
voltage slightly (perhaps 100 mV) so it is a good idea to make
sure that at least 15.0-15.1 volts is actually reaching the
'5860A once it has warmed up.
For power supply bypassing, "Low-ESR" electrolytic capacitors
were used and these types are absolutely necessary
reasonable filtering and good efficiency! Once
construction was complete, the various components were
mechanically secured in place using silicone (RTV) adhesive
(applied after the picture was taken) to prevent them from
moving around during transport and possibly breaking the leads.
As can be seen from the pictures, the switching converter itself
is located as far away from the distribution amplifier as
possible to minimize possible coupling into the 10 MHz output.
- This voltage converter was adjusted to 15-18 volts for use
with the FE-5680A. The lower limit (close to 15 volts)
is generally preferred as this produces less heat and
consumes less power, but the higher voltage may allow it to
achieve lock slightly faster from a cold start at the
expense of higher overall power consumption.
- The FE-5680A, like may of these Rubidium units, consumes
the most power when first started, cold, as the internal
heater is at full power and once the unit warms up, the
current will drop. As you might expect, the unit will
consume more power in a cold environment than a warm one
since the heater has to consume more power to overcome the
heat loss. It should also be noted that a well
heat-sinked unit will consume more current than one that is
not heat-sinked, given otherwise identical conditions, but a
non-heat sinked unit will likely be thermally stressed as
more of the circuitry will be exposed to higher
temperatures, potentially reducing reliability. If
minimizing power consumption was the goal, a semi-insulated
unit on a good heat sink with a temperature/speed controlled
fan might be appropriate to maintain a unit temperature that
was neither too hot not cool enough to cause more power to
be consumed than necessary.
- Even though the converter depicted in Figure 2 goes into
current limiting for 30-45 seconds when power is applied to
a "cold" unit, it will still achieve lock in under 3 minutes
at room temperature. Note that if the switching
converter is adjusted for a higher voltage, it may take
slightly longer for it to come completely out of current
are other pre-built voltage up converter units that may be
found online - many of them based on the LM2577. It
has been noted that many of these units have far higher
amounts of residual switching energy - either due to
smaller-value filter capacitors on their inputs/outputs,
board layout, or the use of higher-ESR capacitors.
Because of this, it is recommended that an additional
low-ESR capacitor be connected at the inputs and outputs of
these units and a
choke/capacitor be used to provide additional filtering on
any unit that you might use.
Any "grunge" that appears on the output of your frequency
reference - which may come from the switching converter -
may be multiplied many-fold at the local
oscillator frequency, so good filtering and attention to
layout is essential!
- It was noted that if subject to a mechanical shock, the 10
MHz output phase of the FE-5680A will "jump" and not return
to its original position: Keep this in mind if it is
going to be subject to vibration.
As it happened, one of the two FE-5680A units that I got came
with a factory-installed, on-board switching converter to
produce the necessary 5 volt supply (pictured in Figure 3
below) that only had to be connected by installing the
appropriate jumpers. If your unit does not
have an on-board switching converter and you are incorporating
the unit into a larger box, it is recommended that an outboard 5
volt regulator (such as a 7805 bolted to the case in which you
are mounting the '5680A) be used to minimize current consumption
on the 15 volt bus as well as to reduce the amount of heat
generated within the '5680A itself, but if you are adventurous -
and the part of the board with the regulator is blank on your
'5680A - details on populating this portion of the circuit board
are included below.
A "Go, No-Go" status indicator - refer to the diagram in
As noted, one the several signals output by the FE-5680 is the
"Physics Lock" indication - sometimes called the "BITE"
(Built-In Test Equipment) line. This signal, when "high"
indicates that an error condition is being detected by the
unit's internal circuitry, that the unit is still warming up, it
could also indicate that its supply voltage is too low, or that
the unit itself has failed.
Any time this signal is high, one should not
trust frequency output of the unit to be accurate.
the unit is warming up its frequency output should slowly sweep
back-and-forth around 10 MHz as it searches for lock from the
"physics package" - a fancy phrase that refers to the magical
Rubidium lamp and its associated circuitry. Once the lamp
comes to temperature and it can detect an atomic resonance, it
will suddenly "snap" to frequency and it should also be noted
the unit locks, there will NOT
be any output from the 1pps (1 pulse-per-second) output.
If this signal is high, Q105 is turned on which turns Q106 off
allowing current through R114 to illuminate the RED
portion of the dual LED,
D102, indicating an "error" condition. If the "BITE"
status signal goes low, Q105 is turned off, current through R113
to flow into the "green" portion of the dual LED and turn also
on Q106 which, in turn, powers the green LED. This
GREEN indication signifies that the unit is operating
properly and can now be trusted to provide a reasonably
accurate and stable reference.
This diagram shows a 2-lead dual-color LED (red and green) but
one may also use a 3-lead common-cathode dual-color LED as shown
on the LPRO-101 page
: I used a
2-lead LED on this circuit because it was the first red/green
dual-color LED that I happened to find when I opened the LED
At the instant that the LED turns green the 1 PPS output will go
active and the frequency being output by the FE-5680A will be
"only" within a few parts in 10E-8 but will rapidly stabilize,
achieving good accuracy and will be "pretty darn close" in about
a minute (probably better than an already-warm quartz-based
and after 20-30 minutes it should achieve
something close to its ultimate accuracy - assuming that it has
been adjusted properly and that it is being operated under
environmental conditions similar to those under which it was
It's worth mentioning that if one were to remove power from the
FE-5680A module itself, the BITE output would go low giving a
false indication that the unit was working! This condition
is pretty easy to diagnose as one or both of the following would
- If the LED goes green the instant that power is
applied. Even if the power is interrupted briefly, it
may take a minute or so for the '5680A to re-lock, so seeing
it go green immediately on the application of power and
staying that way is a bad sign.
Power supply input filtering/protection:
Finally, the input supply is RF-bypassed using a feedthrough
capacitor and FT101 to prevent the ingress or egress of
extraneous RF along the power lead as well as conduction of
switching supply noise along that same line - this, in addition
to the L/C filtering on the input of the switching
regulator. For power supply short-circuit and
reverse-polarity protection, TH101, a 3 amp, self-resetting PTC
fuse, is used in conjunction with D101 making the unit nearly
fool-proof in the field.
Distribution amplifier and 10 MHz filter for non-microwave applications - refer to figure 2:
Important note if you plan
to use this for a microwave converter frequency reference:
If you want just a plain, simple distribution
amplifier and don't want to mess with this crystal filter
stuff or "clean-up oscillator" stuff, take a look at Figure
2 (above) or the diagram on my LPRO-101 page for
- The crystal filter in the upper-left diagram of figure 2,
above, is not narrow enough to
clean up the output of the FE-5680A for use as a microwave
frequency reference: Read about the regeneration
for a means by which the '5680A may be used as such.
It had been reported that the 10 MHz output of the FE-5680A has
a bit of off-frequency (non-harmonic) "grunge" on it so I
decided to add some narrowband filtering as part of the
multi-output distribution amplifier to clean up the FE-5680A's
output since it was to be used as a frequency reference for
microwave transverters. As it turned out, this filtering
enough to make it usable as a reference for at least 10 GHz
and up and it is probably not even suitable to lock a 23cm
How it works:
The 10 MHz signal from the FE-5680A is first terminated at 50
ohms and attenuated by a 10-ish dB pad consisting of R101-R103
before being applied to T101, a 10.7 MHz IF transformer, the
Mouser 42IF129 with a 100 ohm input and 15k output impedance
with a built-in (fixed) resonating capacitor. It was noted
that this particular unit is no longer stocked by Mouser, but
about any 10.7 MHz IF transformer with a built-in capacitor
should work as most 10.7 MHz IF transformers will tune well
below 10 MHz and above 11.5 MHz and using the resonant circuit,
this simple crystal filter may be tuned to resonance by peaking
the output level with the coil adjustment.
The high impedance side of the transformer is applied to a 10
MHz series-resonant crystal (the case of which is grounded) and
terminated using R104, a 22k resistor which also provides a
ground return for Q101, an MPF102 FET, a source follower.
Because of the voltage gain of the transformer, the signal
emerging from the source of of Q101 will be of higher amplitude
than on the input of T101, effectively amplifying the input
signal. The output from the source follower is then
buffered by independent PNP emitter followers Q102-Q104, each
one being dedicated to a 10 MHz output through a blocking
capacitor. Even though the port-to-port isolation isn't
terribly high with this simple circuit, connecting/disconnecting
loads on one output should have only a very slight effect on the
While the crystal filter removes frequency components not
related to 10 MHz, not
much attention was made to keeping the various follower stages
in the linear range so the 10 MHz output waveforms are somewhat
clipped, introducing harmonics. For most equipment this
harmonic content is not a problem, but the circuit could be
reworked (with a slight increase of complexity) to maintain a
nice-looking sine wave on the outputs were this a priority.
Power supply filtering (R117, C107) assures that residual
components from the 15 volt output of the switching supply will
not find their way into the 10 MHz output. The 15 volt
supply is used here to make the output level from the
distribution amplifier insensitive to changes in the supply
- The "absolute" phase stability of the 10 MHz output was
not considered to be particularly important. If the
phase of the 10 MHz signal is important to your
application it should be noted that the phase of the 10 MHz
bandpass filter will be temperature-sensitive owing to the
very nature of T101 and the crystal. It should also go
without saying that any mechanical adjustment of T101 will
cause a significant phase shift as well.
- As described, the bandwidth of this extremely simple and
cheap 10 MHz crystal bandpass filter is on the order of +/-6
kHz at the -6dB points. If narrower response was
desired for some reason, several crystals could be cascaded
to make a Cohn-type filter, but filter construction is a
topic that is best-covered elsewhere.
- Again, the described filter is not
narrow enough to remove low-level spurs from the FE-5680A
and make it suitable for use as a frequency reference for
microwave transverters - see
below for a method of regenerating the 10 MHz signal.
of the 10 MHz regenerator, distribution amplifier, status
The above circuit as built into the enclosure with
Click on an
image for a larger version.
Regenerating the 10
MHz output of the FE-5680A with a disciplined VCXO to make
it suitable for use as a microwave transverter reference -
see Figure 3:
After constructing the above 10 MHz distribution amplifier and
crystal bandpass filter described above I did some testing and
was disappointed to find that the low-level spurs on the 10 MHz
output of the FE-5680A, when effectively multiplied by 1000 in
frequency-locking the local oscillator of my 10 GHz narrowband
transverter resulted in a signal that was unsuitable
(CW/SSB) use. For the most part, these low-level spurs are
undetectable at 10 MHz or perhaps even 100 MHz, but the
effective 60dB in amplification of these low-level signals made
them both obvious and unacceptable when multiplied to 10 GHz.
As a basis of comparison I also checked it against three other
10 MHz sources: An HP Z3801 GPS reference, a crystal-based reference
using an Isotemp OCXO
, and another rubidium unit using an
. Both crystal references (the Z3801
and Isotemp) were extremely clean and I think that there was
just little bit of "digital sounding" noise on the LPRO-101 that
I'd not really noticed before. It is worth mentioning,
however, that the "long term" stability of the FE-5680A was fine
and that the actual
10 GHz signal (once located amongst
the spectral clutter!)
was both stable and accurate.
At least some of the problem appears to be due to the fact that
low-level noise from the internal synthesizer (perhaps the DDS in the frequency
is making its way onto the output causing
a fairly obnoxious "digital"-sounding buzz consisting of a
number of fundamental frequencies and their harmonics - plus a
fairly strong sub-audible component. The severity of this
modulation when applied to a 10 GHz transverter - where the 10
MHz is multiplied 1000-fold - is enough that, when SSB is used,
its presence actually makes it a bit difficult to properly
zero-beat a carrier. To find out had "bad" it is, read the
article, Performance of
Low-Cost Rubidium Standards
by John Ackerman, N8UR.
Because of the very low frequency nature of some of these
components it was deemed impractical to filter them directly at
10 MHz so the design goal was to "loosely" lock another 10 MHz
oscillator to the output of the '5860A. In this way the
frequency stability of the '5680A could be inherited by the
other oscillator but its undesirable traits would not.
The heart of this is a low-noise oscillator consisting of Q202,
X201 and associated components forming a Butler oscillator and
it is this circuit that ultimately determines the phase noise of
the microwave oscillator once it has been multiplied. As
it turns out, this oscillator is comparatively insensitive to
capacitance when tuning and it was required that D201, the
varactor, be a high-capacitance "Hyperabrupt" type (MVAM109,
NTE618 or equivalent) with about 400 pF at 1 volt and less than
100 pF at 10 volts, providing about 300 Hz of tuning range -
more than enough to compensate for the temperature-related
frequency changes of the oscillator's crystal. It should
be noted that for this type of oscillator, a series-resonant
crystal is required or there will be a significant frequency
It is possible that this oscillator could be reworked to
increase the tuning sensitivity in terms of capacitance and
allow a lower-capacitance varactor to be used but care should be
taken to avoid excess sensitivity to the "capacitive
environment". When initially constructed there was some
concern that the varactor itself could be the cause of
additional phase noise, but a comparison with of the cleanliness
of the CW note (at 10 GHz) between a free-running oscillator without
the varactor and
with the circuit (with varactor) locked to the FE-5680A and also
to one of the other known-good 10 MHz sources mentioned above
didn't reveal any obvious difference.
In the prototype, a cheap (<$1.00) CPU-type 10 MHz
crystal was used and these inexpensive devices tend to have
rather poor frequency accuracy and temperature stability.
If a high grade crystal (typically $10-$20) had been used it is
likely that both temperature stability and frequency accuracy
would have been improved, allowing a somewhat relaxed VCXO
tuning range and, perhaps, somewhat better phase noise
performance with increased crystal "Q" - although all of these
could probably be accomplished with more careful design of the
oscillator itself, even with the "cheap" crystal. Another
benefit of a "better" crystal with a lower temperature drift
would be that D201 could be replaced with a lower capacitance
and/or back-to-back varactors which could further reduce
contribution to phase noise.
If one has a good quality 10 MHz VCXO available it may be used
to replace the oscillator shown, but be aware that many low-cost
VCXO DIP or SMD modules often have rather poor phase noise and
just aren't suitable for narrowband (e.g. SSB/CW operation) when
multiplied 1000 times. If one has the room and doesn't
mind the extra power consumption, an OCXO with external
(electronic) tuning may also be used, but this would probably be
overkill, consume more power, take longer to warm up than the
rubidium and generate even more
and distribution amplifier:
Q203, a source follower, takes a sample of the oscillator's
output to minimize loading effects with Q206 providing some gain
and Q207 and Q208 forming a complimentary driver voltage source
with resistors R223-R225 setting the source impedance and
driving multiple outputs. While only three outputs are
shown, testing showed that it would easily drive half a dozen 50
ohm loads. The isolation between these ports isn't
terribly high (perhaps on the order of 20dB) but it is unlikely
that there will be much interaction between outputs and what
effects there might be would likely occur only at the instant
other devices were connected or disconnected. Since this
was intended to feed the 10 MHz inputs of microwave
transverters, there wasn't the fanatical need for
phase/amplitude stability under all operating conditions or even
very high port-to-port isolation.
With the amplification circuitry shown, the fidelity of the
reasonably "clean" sine wave from the oscillator isn't terribly
well-preserved, but it's much more "sine-like" than
square. As it happens, a waveform with
harmonically-related content isn't likely to cause a problem
with the 10 MHz input on synthesizers used in microwave gear and
test equipment, but what can
be a problem is if the waveform is very "ringy" - something that
can happen with square-ish waves as the device could erroneously
"see" the ringing portion of a waveform, falsely trigger on it,
and causes errors - but this unit's waveform wasn't at all
Q204 and Q205 amplify the VXCO's output to logic levels feeding
it to U203, a 74HC4060 binary ripple counter that reduces
comparison frequency and loop gain and provides the square wave
required for optimal operation of an XOR-type phase detector
while a similar amplifier and divider consisting of Q201 and
U201 form a divider chain with this counter's output - and that
of U203 - being phase compared by U202, a 74HC4046. In the
'4046, only the XOR gate phase comparator is being used and the
comparison frequency output is filtered using R203 and C204 and
then amplified (and filtered) by U202a and C205/R204 to a 0-10
volt level and applied to the VCXO tuning line through R211
being further filtered by C214-C216 to set a fairly low loop
bandwidth. A pair of '4060-type counters were used because
I had plenty of them on hand and also because the multiple
divider taps allowed by to try different comparison frequencies.
A pair 74HC4040 counters could have been used (with appropriate
wiring changes) or even a single 74HC4520 - which I would have
used if I'd had one onhand! If you happen to need 5, 2.5
or even 1.25 MHz outputs for some reason (such an input to test
equipment) then one could take a sample of one of the divider's
outputs (especially in the case of the '4040 or '4520), buffer
it and use it, but make sure you use the counter connected to
the output from the crystal oscillator and not the rubidium.
Because of the nature of the XOR-type phase detector, the
"tuning sense" (e.g. whether increasing tuning voltage causes
the frequency to go up or down) or even which signal input (VCXO
or Rubidium) is connected to the divider inputs inputs at Q201
or Q204 is irrelevant: If the tuning goes the "wrong way"
the waveform on the XOR phase detector will simply "slide" 180
degrees and invert itself. The '4046 also contains other
phase/frequency detectors, but the advantage of the XOR fed with
square waves is that its output can never
contain any spectral
components that are lower than that of the comparison frequency
- and it usually outputs energy at twice that! In
comparison, the flip-flop phase/frequency detector in the '4046
(the so-called "PC 2" detector) has the nasty habit of, when in
lock, bouncing around at a relatively low frequency which may
be easily removed by a loop filter.
I could also have used a 74HC86 quad XOR gate instead of the
74HC4046 as the phase detector. If this had been done, two
of the three "extra" gates might have been usable as "gate
amplifiers" to bring the two 10 MHz inputs up to "logic level"
instead of using Q201/Q204: I've done this in the past
with good results but I didn't do it for this project because I
wanted to play around with the other phase detectors on the
It is worth noting that the simple PLL/loop filter shown (e.g. a non-integrating type)
have a constant phase relationship
between the '5680A and VCXO signal when locked, but that is not
important with a relatively stable oscillator when used only as
a frequency reference for microwave transverters. Finally,
the loop design itself was intended to be fairly "insensitive"
and slow because we wanted to pass on the stability of the
FE-5680A's output to the 10 MHz VCXO, but as little of the
low-level, low-frequency phase noise that afflicts it as
One of the difficulties with using an XOR-type phase detector is
that an "unlocked" condition is more difficult to detect than
with a more complicated phase detector, so this circuit takes
advantage of the fact that if the PLL is not
locked up the XOR's output will "flap" back and forth between
high and low at a rate related to the difference between the
frequencies of the VCXO, the rubidium, and the divisor ratios
used. U202b is a comparator that "looks" at the tuning
voltage output from U202a and as it crosses the threshold
voltage - the "middle" of the tuning voltage range - the
comparator "snaps" back and forth in unison. This output
is capacitively-coupled (via C207) and then via R228 so if the
PLL is unlocked, the voltage changes from 0 to V+ as the phase
comparison inputs to U202 "slide" past each other and cause the
status indicator to switch rapidly back and forth between red
and green, providing a visual indication that the PLL is
unlocked by causing the LED to flash/flicker or - if the
frequency is way
turn orange/yellow when both the red and green appear to the eye
to be illuminated simultaneously.
The only peculiarity with this circuit is that if the PLL tuning
voltage happens to cross U202b's threshold during normal
operation, the status indicator may flash, but this should
happen rarely and it will have no effect at all on the 10 MHz
output. Because the cost of implementing this lock
indicator was very low (requiring only a few inexpensive
components, utilizing the "unused" op amp section and the
existing status indicator) this minor deficiency can be
Note that the PLL may be unlocked while the rubidium unit is
warming up and its output frequency is sweeping back and forth,
possibly out of the lock range of the "clean-up" VCXO so it
wouldn't be unusual to see this LED turn yellow or frequently
No-Go indicator, voltage converter and power supply
Aside from the addition of the PLL unlock indicator to the "go,
no-go" circuit, the operation of the voltage converter and power
supply filtering are identical to that shown in figure 2 - in
fact, the original crystal bandpass/distribution amplifier board
was simply replaced with the new board that regenerates the 10
- If one is planning to use the 10 MHz output of the
FE-5680A for precise timing rather than a frequency
reference it is worth noting that the "absolute" phase of
the regenerated 10 MHz output will be affected by
temperature, but at a slow enough rate that the frequency
shift - even at microwave frequencies - is not likely to be
detectable. If both a "clean" 10 MHz signal suitable
for multiplication and a phase-stable signal were required,
the two could be taken independently.
- As can be seen from the pictures, the oscillator and
amplifier portions were built "dead-bug" on a piece of
glass-epoxy circuit board while the "digital" portions and
the PLL filter were constructed on a piece of perforated
proto board. It is possible that a circuit board could
be designed on which both circuits could reside, but I have
no current plans to do so. If you
want to do it, let me know so that the design can be
shared with others who might be interested!
- There are more comments on using this "cleaned up" signal
on microwave transverters, below - click here
to jump directly to that section.
Using an internal 5
volt supply with the FE-5680A:
The "non-programmable" version of the FE-5680A
typically requires that a source of +5 volts be supplied
externally via pin 4 of the DE-9 connector - this typically
being provided by the user via a 7805 linear regulator - without
which, the unit will not function.
Upon receipt of two "identical" FE-5680A's, I opened them up and
noticed an obvious difference: One had components
installed for an internal 5 volt switching regulator and the
other did not. Interestingly, even though the components
were fitted on one of the units, neither
the input or output
of the +5 volt regulator was connected. A bit of poking
around with an ohmmeter revealed to me that it was necessary to
install two jumpers to enable this supply, making the FE-5680A
an entirely self-contained unit - see Figure 4 for the
location of those jumpers.
On the "other" unit I simply soldered a 7805 to the ground plane
in the area in which the switching supply components were
fitted. I then applied heat-sink grease to the case of the
7805 and also the insulating sheet to transfer at least some
of the heat generated
by it to the bottom plate and wired jumpers to the V+ input of
the regulator from the 15 volt line and the 5 volt output of the
to pin 20 of the 74ACT240 nearby. Testing indicates that
with the bottom plate heat-sinked, the unit does not run
Comment about using an on-board
linear regulator and heat generation:
for the on-board 5 volt switching regulator:
- It should be noted
that if the unit needs to be entirely self-contained, the
addition of an internal linear regulator makes sense.
If, however, you are going to include the '5680A in another
project, an external 5 volt supply would be better as it
wouldn't contribute to internal heat generation and less
power would be wasted as the 7805 could be powered from a
lower-voltage supply. In other words, if you are going to convert from
12 up to 15 volts, it would be better to run the 7805 from
the 12 volt supply instead of the 15 volt supply.
- I have NOT done
the modifications suggested and I strongly suspect that
there are one or two other jumpers that need to be
- This information was obtained
through the use of an ohmmeter, data sheets, some
measurements, and a bit of guessing and is probably mostly correct.
- Use of this information will
be at your own risk! If you are successful in
performing these modifications - or even if unsuccessful
- please let me know!
Poking around the two FE-5680A's that I have, I have divined a
bit of information as to what it might entail to install the
necessary components to allow the use of the internal 5 volt
Figure 4 - middle-left
what the populated 5 volt regulator looks like - all components
being located within the border of the ground trace. Based
on the Linear LT1376 regulator, this efficiently (90% or so)
converts the 15-18 volt supply voltage down to the +5 volts
required for proper operation while producing only a small
amount of extra heat to be dissipated by the unit.
Figure 4 - lower
shows what is believed to be the diagram of
the switching regulator with the picture in Figure 4 - middle right
being an annotated version of the picture showing the locations
of the installed components. While some of the components
in the schematic are noted on the board's silkscreen, some are
not and those are indicated using "a" and "b" designations.
Top left: Close up of the
details in soldering and wiring a 7805 inside the
FE-5680A, the tab being soldered to the ground plane for
heat sinking. Heat sink compound was put on the case
of the 7805 and the insulating sheet to more-effectively
transfer heat to the bottom cover. The input voltage
comes from the jumper on the left and the +5 volt output
connects above the 20 pin chip on the right.
Yes, it's just a 7805 and two capacitors!
Middle left: Close-up of the board showing
the section (within the ground plane) containing the
components of the 5 volt switching regulator.
The same picture, annotated with the parts designations
used in the schematic. Also note the added jumper
indicated - see text and warnings!
Believed to be the schematic of the regulator.
Another jumper that was required to be installed on the
"top" of the board to enable the 5 volt regulator - see
text and warnings!
Click on an image for a
- U504 is an
LT1376HVIS - the "adjustable" version with the output
voltage selected using resistors Ra and Rb. If a "5
volt" version of the LT1376 is used one may omit both Ra and
Rb, jumpering the Ra position with a piece of wire.
Note that this chip is available in several different
packages and it is the 16
pin version that is required for this application.
- L500 is believed
to be a Coilcraft DO33088-153 15uH, 1.9 amp (RMS) SMD
coil. This particular coil has a somewhat larger
footprint than most coils of similar type but a smaller unit
could be used if care were taken to avoid shorting to the
ground plane beneath. The main requirements of this
coil are that it be capable of handling the current and
protrude only about 3-3.5mm above the board in order to
clear the lid. Inductor values of 10-20uH should be
usable in this application.
- CR501 is an
MBR5360T3 60 volt, 3 amp Shottky diode.
- C511, C521:
The most critical components are C511 and C521 which are
both 120 uF, 20 volt solid tantalum capacitors, Vishay P/N
594D127X0020R2T (Digi-Key P/N: 718-1007-2-ND).
Note that because this switcher operates at about 500 kHz
and is in an RF-sensitive environment, one CANNOT
use electrolytic capacitors in this application! It
should be possible to use high-value ceramic chip capacitors
(one or more units to add
up to 100 uF) but one must keep in mind the
component height requirement! Looking closely at the
picture you'll note that these particular capacitors have a
"point" that indicates the positive lead.
- Da is a
surface-mount 1N914 - nothing special: About any
reasonably fast silicon diode with a >40 volt rating at
>50 mA should be suitable.
- Ca and Cb are standard chip
capacitors in 0805 packaging.
- Resistors Ra and
Rb (0805 packaged
devices) are used to set the voltage output. The
values noted in the schematic are those on the board but one
can reference the LT1376 data sheet for other values.
As noted above, if a 5 volt version of the LT1376 is used
one may omit both resistors and simply jumper the Ra
- As noted, there are at least two jumpers (shown in Figure
4) that needed to be installed on the unit that already had
the switcher's components installed in order to power it
up. It is unknown, however, if the "unpopulated" board
will require these same jumpers, require that some existing
jumpers be removed or additional jumpers added. What
would be "safe" to do, however, would be to simply connect a
jumper wire to the positive side of C511 from the V+ input
of the unit (or from the jumper position depicted in the upper-left picture in Figure 4) and connect
pin 20 of the nearby 74ACT240 (at the junction of the two
capacitors as shown in the same picture) to the converter's
V+ output at C521.
As noted, the version of FE-5680A depicted on this page is
the "non-adjustable" type, so-called because unlike some versions, the output
is "fixed" at 10 MHz rather than adjustable from audio
frequencies (if the unit is
appropriately modified) to around 20 MHz in sub-Hertz
steps. The "adjustable" units have a simple user
interface via the serial port that produces human-readable
results, more or less.
The version used above has its output at 10 MHz and it, too,
has the ability to be controlled via the serial port, but the
output frequency (10 MHz) can be tweaked in steps of several
parts in 10E-13 with a total range of well under 1 Hz.
Unlike the command set in the "adjustable" version, the
command bytes cannot easily be entered directly by the user so
a program has been written to do this. The format of the
commands to control this version of the FE-5680A are detailed
in the FE-5680A
technical manual, available from the VK3UM
page with the program itself, written by VK4XV, being linked here.
Having a known-accurate frequency reference (an HP-Z3801)
available, I compared the 10 MHz output of the FE-5680A with
that of the Z3801 and, using a dual-trace oscilloscope, found
that it took about 6 minutes for the relative phases to change
by 360 degrees once the unit had been operating for an hour or
so. Firing up the program, I read the offset that had
been programmed into the FE-5680A (from either the factory or
the previous owner) and found it to be set to zero.
Using the program, I did a "binary search" by first selecting
rather large offset values to determine which polarity of
offset (positive or negative) reversed the direction of the
offset and then started cutting those values in half, always
staying on the side of reduce rate-of-change of the phase
change. Once this value is found, one may write the
value "permanently" into the unit and it is
recommended that his number be written (in indelible ink) on
the unit itself for future reference.
To be sure, at single-digit offsets I used the scope's
maximum magnification and it took several minutes to see the
very slow rate-of-change which was, when I was done, about 80
minutes for 360 degrees ending up with a final offset value of
+190 for this particular unit. I repeated the same
procedure for another FE-5680A and ended up with an offset
value of -384, but this unit's sensitivity seemed to be a bit
higher per-step and resulted in a 360 degree change over about
20 minutes. It was interesting to note that in both
cases, the units arrived with an offset of zero indicating that
whoever used them was satisfied with the "out-of-the-box" accuracy of the units.
One caveat with the current version of the VK4XV program is
that it does not
automatically fill in the "offset" value to be entered in
the unit with that already programmed into it - even
if you read the currently-programmed values from the
'5680A. What this means is that the field you enter will
always be zero and
one must decipher the HEX values displayed and convert them to
decimal to determine what the current offset is. Again,
the easiest way around this is to keep notes and, using an
indelible marker, write the final offset on the unit's label
the phase stability of the FE-5680A:
The observations below were made using an
oscilloscope triggered from an HP Z3801 GPS-locked
While I was at it, I decided to take a look at how changes in
voltage affected the output of the FE-5680A and noticed that
changes in voltage within the allowable range (15-18 volts)
caused a noticeable shift in phase - as did a rapid change in
temperature. Longer term, these changes did not cause a large offset
in the stability of the unit (at least as far as I was
concerned as a microwave band operator) but rather it seemed
as though these were, in fact, just shifts in the phase of the
output. When these changes were "un-done" it appeared
that the phase moved back to where it had been - taking into
account the unit's frequency offset, of course.
Poking around, I noted that the heated crystal within these
units appeared to be the cause main of these phase
offsets: A change in temperature of this crystal seemed
to cause a repeatable phase shift and it is likely that the
power applied to the heater attached to the crystal itself
changes slightly as the input voltage changes. This
observation also seemed to explain, at least in part, why a
change in temperature of the unit also caused a phase shift.
These observations indicate that if phase stability is
important to you, the operating voltage of the FE-5680A should
be well regulated and the temperature remain constant.
In my case - where I will be using the FE-5680A as a precise
10 MHz reference - the slow phase changes (which would
correlate with small frequency offsets at microwave
frequencies) that would accompany temperature changes are less
important, representing only a few Hz of error at GHz-range
I also noted that a gentle (but firm) tapping of the either
of my FE-5680's caused "permanent" jumps in the phase of the
10 MHz output on the order of 10's of degrees - probably from
an oscillator/counter comparison "slipping some cogs"
internally in response to microphonics. While not likely
to be an issue for those units sitting on a shelf somewhere,
those units being exposed to vibration (such as in a car) may
suffer brief "jumps" in frequency. The LPRO-101, by
comparison, did not seem to be fazed by any physical abuse I
dared inflict upon it.
Comparing the FE-5680A to the LPRO-101, I find the former to
be somewhat inferior in its susceptibility to changes in its
physical environment and operating voltage, but this is just
my perception. As far as holding their adjustments under
similar conditions, they seem to be comparable. The
FE-5680A (unmodified) has its fine adjustments made only via
the serial port while the LPRO-101 has an onboard
potentiometer and an external "C-Field" lead to allow
disciplining and/or environmental compensation and when it
comes to making very fine adjustments, it is far easier to
"tweak" the FE-5680A to "dial in" the frequency via computer
than to adjust the potentiometer in the LPRO-101 where one
doesn't have as good a "feel" for the magnitude of the
adjustments that one is making.
To re-reiterate, the output of the FE-5680A is NOT
directly suitable for use as a frequency reference for
microwave transverters due to low-level phase noise -
particularly when frequency multiplication worsens its
effects. As noted above, an outboard crystal oscillator
"disciplined" by the the FE-5680A using a circuit such as that
described by Figure 3.
"cleaned-up" FE-5680A on the microwave bands - some
As noted above, the "barefoot" FE-5680A is not
suitable for use on the higher microwave bands (e.g. above 1
GHz) owing to the multiplication of low-level phase modulation,
hence the building of the "clean-up" oscillator. On the
lower VHF/UHF bands (e.g. 2 meters, 70cm) the "raw" output of
the '5680A should be "OK" - if you have equipment that uses a 10
MHz reference input, that is! Many hams have modified
commercial gear to frequency-lock the radio's reference to an
external 10 MHz source and these circuits may (or can be made
to) be able to adequately filter any low-level "grunge" that
might appear on the reference input, anyway.
In testing the '5680A with the above "clean-up" oscillator on 10
GHz SSB I found it to be quite usable - although there was a
slight amount of "warble" that appears to be due to
low-frequency instability that was proportionally worse on 24
GHz SSB. The cause of this has yet to be traced out just
yet, but a few possibilities come to mind:
- Low "Q" of the cheap 10 MHz CPU-type crystal that was
used. This crystal was less than $1, so I wasn't
expecting it to work perfectly. If I were to replace
it with a higher quality ($20-$30) crystal I would expect
that it might perform better because it would have better
characteristics, including better "Q" and temperature
- Too much VCXO frequency control. As
noted, I wasn't terribly happy with the amount of
capacitance required to achieve the amount of VCXO voltage
tuning required to assure that the crystal oscillator would
be in range over a wide variety of temperatures and this
required the use of a "hyperabrupt" varactor. More
ideally, back-to-back varactors would be used to minimize
the effects of a single varactor's impact on phase noise,
but this was not possible while still keeping the desired
tuning range. It is possible that a re-working of the
capacitor/inductor values in the crystal oscillator will
allow a reduction of the amount of varactor capacitance
required. What would really help,
however, would be the use of a higher quality crystal so
that far less electronic frequency control range would be
needed and the lower voltage-versus-frequency sensitivity
would thereby minimize any contributions of low-level,
low-frequency phase modulation that might be coming from the
phase detector and amplifier.
- 1/f noise from the op amp/phase detector. As
one approaches DC, there is an increase in the random noise
from electronic circuits and it is likely that very near DC,
the phase detector and/or op-amp is contributing to the
low-frequency warble because of this. By re-working
the phase detector and, perhaps, using a "quieter" op amp
the warble may be reduced. The better solution to this
would be to use a better-quality crystal as mentioned above
as this would reduce the required tuning capability which
would therefore make it less sensitive to low-frequency
"noise" on the tuning line.
- 1/f noise from the voltage regulator. I need
to look into this, but in the past I've observed that
3-terminal regulators (such as the 7805) can produce a bit
of low-level noise on its output - which could then find its
way into everything else. Again, using a better
crystal and reducing voltage-frequency tuning sensitivity
would probably be the best solution.
Other links pertaining to Rubidium references:
- A better oscillator! It should be possible to
find a decent, off-the-shelf VCXO or OVCXO that has good,
low phase-noise characteristics. I found an
inexpensive ovenized miniature 10 MHz oscillator (with
electronic tuning) on EvilBay that is known to be good when
multiplied to 10 GHz and above that I will try some
time. The obvious downside is that not only would this
increase the power consumption, but it would likely lengthen
the "lock-up" time of the entire reference from the normal 3
minutes of the '5680A alone to 5-10 minutes - this,
depending on the warm-up time of the chosen oscillator.
FAQ at KO4BB - This
page has a lot of links and useful information about the
various incarnations of the FE-5680A in one place.
If you are having trouble getting your '5680A working, you
take a look! For more about precision timing and
many other things, take a look at KO4BB's Wiki.
page about the LPRO-101 - A suspiciously similar page that describes a box
built around the Efratom LPRO-101. Almost everything
on the LPRO-101 page could be applied to the FE-5680A and
vice-versa, the only difference being that the FE-5680A is
happy with +15 volts while the LPRO-101 needs at least 19
Rubidium Lamps - The rubidium lamp in these
devices has only a finite lifetime, but this page explains
how you may be able to get more life out of
it if it quits working! Note that while this page
doesn't address the FE-5680A specifically, the same general
technique may be applicable.
"Time Nuts" Mailing list and archive - Covering all
sorts of nerdy topics related to frequency and time
measurement, the archives of this list contain a wealth of
information about this and other frequency references.
While anyone may peruse the archives, you must join the list
in order to participate.
of Low-Cost Rubidium Standards by John Ackerman, N8UR
- This article compares a number of low-cost (e.g. surplus)
rubidium units to determine best short and long-term
stability. From this article, one sees why the
FE-5680A by itself does NOT make a good
reference for microwave transverters! The winner among
Rb's? The Efratom LPRO-101.
- Stability and Noise
Performance of Various Rubidium Standards by John
Miles, KE5FX - Another excellent article comparing the
important parameters of various Rubidium devices available
on the surplus market. From this page you can readily
see why the LPRO-101 works "barefoot" as a microwave
reference and an FE-5680 does not!
The usual warnings:
- As with any electronic project, anything described
or referenced above should only be done by those
familiar with the techniques involved.
- Please observe all safety precautions when dealing
with voltage, heat, or potentially dangerous materials
such as Rubidium.
You have been warned!!!
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