Why a frequency reference?
The 24 GHz
transverter along with the described 10 MHz
Click on the image for a
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, to be able to talk when signals are weak - and
when we use microwave frequencies and
narrowband modes such as SSB or CW
one must maintain pretty good frequency stability and
- Stability is important as 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 also important because it is important that
both parties be confident that their operating frequencies
are reasonably close. 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 likely that they will
be able to find each other if the path "works" in the first
place. If the error was on the order of several kHz,
"hunting" would be required to find the signal and if those
signals are weak, they may be missed entirely.
Because achieving such stability and accuracy requires some
effort, it is more convenient if our gear is constructed such
that it can use a common, external frequency reference and lock
to it: In that way, we need only have one "master"
reference rather than several individual references.
Having one common
frequency reference can also be
convenient if one is operating portable using battery power
since it can mean that one doesn't need to keep all of those
individual pieces of gear "warmed up" all of the time to
maintain stability. If a particular piece of gear can
accept an external 10 MHz input, this would allow one to it on (and
drain battery power)
only when it is needed.
At this point I might mention that Rubidium frequency references
(such as one described here)
are also readily available in the surplus market as well that
provide at least an order or magnitude greater accuracy and
stability and warm up in less time than the crystal reference,
so why not always
use a Rubidium reference
instead of a crystal-based one? The crystal-based unit is
cheaper, easier to package and consume significantly less power
than a Rubidium reference, and the stability/accuracy of a good-quality
crystal-based reference is more than "good enough" at least
through 24 GHz! When I go out in the field to do portable
microwave work, I'll often power up the OCXO after putting it in
the car knowing that by the time that I get to my destination
and set up, it will be warm and on-frequency. (To be sure, I bring the Rubidium
anyway as a "backup" reference!)
About this frequency reference:
The goal for this project was to have a "reasonably stable and
accurate" reference: Based on an Isotemp OCXO 134-10, this
unit seems to be able to hold the 24 GHz local oscillator to
within 500 Hz or better once it has had 15-20 minutes or so to
warm up - and it seems to be fairly stable across a range
ambient temperatures from "hot" to "below freezing." The
Isotemp unit - and others like it - are readily available on
both the new and surplus markets, available via EvilBay and
The oven module itself is rated to operate from 13 volts, +/- 2
volts, implying a minimum of 11.0 volts. Even though
testing indicated that it seemed to be "happy" with a supply
voltage as low as 9.8 volts or so, it was decided to adhere to
the published specifications and in looking around I noticed
that most readily-available low-dropout regulators (and those
that I had onhand) were not specified to handle the maximum
"cold" current of this oven - about 800 mA or so - so I had to
"roll my own" 11 volt "zero-dropout" regulator. More on alternative regulators, below.
A "zero-dropout" regulator:
Why regulate? I noted in testing that slight variations of
supply voltage (a few hundred millivolts) would cause measurable
disturbances in the oscillator frequency due to the changes of
the power applied to the heater, taking several minutes to again
reach equilibrium. Since battery operation was anticipated
it is expected that the supply voltage would change frequently -
between periods of transmit and receive - as well as due to
normal battery discharge.
Referring to the schematic, U101 - a standard 5 volt regulator
(the lower-power 78L05 is a good choice) provides a stable
voltage reference for U103, a 741 op amp, which is used as an
error amplifier. A 7805 was chosen as it is
readily-available but a Zener diode and resistor could have been
chosen: If a Zener is used, a 5.6-6.2 volt unit is
recommended with 2-5 milliamps of bias as this voltage range
offers good temperature stability.
If the output voltage is too low, the voltage on pin 3 drops as
well - along with the pin 6, the op amp's output which turns on
Q103, a P-Channel power MOSFET, which increases the voltage and
once the voltage on the wiper of R119 reaches 5 volts - that of
the reference - the circuit comes to equilibrium. A
P-Channel FET (a less-common device) was used because it takes
3-5 volts of drain-gate voltage to turn on a FET and it would
have been necessary to have at least 16 volt supply to bias the
gate if an N-Channel FET were used. Furthermore, with the
use of a P-Channel power MOSFET the dropout voltage of the
regulator is essentially limited to the channel resistance of
the that FET. In theory a PNP (possibly a Darlington-type
arrangement) could be used instead if one can tolerate closer to
a volt of dropout, but the FET was chosen to minimize the
In testing, once the oven was warm (a condition in which the
OCXO was drawing approximately 250 mA at normal "room
temperature") the dropout of the regulator was approximately 50
millivolts - a voltage drop that is likely to be comparable that
of the resistance of the wires used to power the unit.
This rather simple
regulator seems to work quite well, holding the output voltage
steady to within a few millivolts over the input voltage range
of 11.1 to 17 volts with good transient response.
"Faster warmup" feature:
This OCXO has a "status" output that, when "cold", outputs about
0 volts and in this state, Q101 is turned off, allowing R112 and
R113/D102 to pull its collector high - turning on Q102 - which
pulls the gate of Q103 low through R118, turning it fully
"on." In this state the voltage applied to the oven is
nearly that of the battery supply and this higher voltage
increases the power applied to the oven, allowing it to heat
more quickly. Once the oven's "status" line goes high,
Q101 is turned on, illuminating the LED and turning off Q102,
allowing the regulator to operate normally.
Note: When the unit is warming up, the
OCXO's voltage is unregulated which means that the supply
should be kept below 15.0 volts to stay within the "safe zone"
of the ratings of the oscillator itself.
Does the "boosted" voltage actually help the oven warm up
faster? It's hard to say, but it took only 4 additional
components to add this feature!
Inside the 10 MHz OCXO module showing the oven
(left) and the power supply and distribution amplifier
End-view of the OCXO module showing the output jacks, the
"Status" LED and the power connection.
A close-up view of the oven and its shock mounts.
Schematic of the OCXO module.
Click on an image for a
that this status line doesn't indicate that
the oven has fully
warmed up, but that it's still
"room temperature" it takes at least another 5 minutes before
the frequency will be stable enough for use and another 5
minutes or so after that until it's "pretty close" to the
intended frequency and it can be used at microwave frequencies
without having to chase people around. Why have the
indicator light if it doesn't indicate that the unit is
"ready"? If the light isn't
on, you can be sure that the
frequency output won't
be valid for one reason or
Because the OCXO itself is somewhat load-sensitive, U102 - an
LM7171 - is used as a distribution amplifier to both isolate the
oven from its loads and to provide fan-out to allow multiple
outputs to be driven simultaneously. The LM7171, a
high-output, high-speed op amp, is configured for a gain of 2,
providing about 2 volts peak-to-peak output with the drive
provided by the OCXO.
Mounting the oven:
Because this unit is intended to be used "in the field" it was
decided to mount the OCXO module itself to prevent mechanical
shock from affecting the reliability, frequency stability and
accuracy and this was done using some rubberized mounting
pillars from scrapped satellite equipment while some "blobs" of
silicone were placed on the wall of the die-cast enclosure to
prevent the OCXO housing itself from directly impacting it
should the unit be accidentally dropped.
A few bits of stiff foam could also be used to provide some
shock mounting in the corners of the OCXO but be aware that some
oven-based oscillators have been known to become less
accurate and stable if they are over-insulated, so don't go
Like any crystal oscillator, it is somewhat "position sensitive"
in that a frequency shift of a hundred Hz or so (at 24 GHz) can
be observed if the unit is placed on its side, upside-down,
etc. While this effect is very minor, it's worth noting
when it's being set to frequency and in operation.
DC input protection and
Finally, the input supply is RF-bypassed using a feedthrough
capacitor to prevent the ingress or egress of extraneous RF
along the power lead. For power-supply short-circuit
and reverse-polarity protection, R101, a 1.1 amp, self-resetting
PTC fuse is used in conjunction with D101, a 3-amp diode.
Comment about alternative schemes for low-dropout regulation for the OCXO:
this web page was originally put together a number of "low-dropout"
adjustable regulator ICs have appeared on the market that may
be suitable for your this project - but there are a few caveats.
For example, there is the Linear Technologies LT1086-Adj which is rated
for up to 1.5 amps of current. While much lower dropout than a
conventional adjustable regulator such as an LM317, it does have
approximately 1 volt of dropout which means that if you set the OCXO's
supply voltage to 11.0 volts - the minimum recommended in the
specification - your battery voltage must be at least 12.0 volts:
While this represents a lead-acid battery that mostly depleted
it is likely that a small, but healthy, lead acid could drop to such
a voltage under transmit load - particularly if the resistance of power
leads is taken into account. This 3-terminal regulator is used in
a manner very similar to the LM317 - except that you really must
have some good quality, low-ESR capacitors (probaby tantalum)
very close to the regulator itself - see the data sheet.
Also made by Linear Technologies is the LT1528 that is rated for up to
3 amps that has a (nominal) 0.6 volts of dropout - more typically in
the 0.3 to 0.5 volt area for the amount of current consumed by the
OCXO, particularly once it has warmed up: This extra margin would
keep one in the "safe" region of the OCXO's operating voltage range
down to around 11.5 volts allowing both "deeper" battery discharge and
more voltage drop on connecting wires. This part is somewhat
more complicated to use than the LT1086, above, but it is, overall,
simpler than the op-amp based regulator described earlier in this page.
If the "fast warmup" were to be implemented on either of the above
regulators it would take a different form than the above - likely using
several resistors and a transistor or two to "switch" the
resistor-programmed voltage setting to something higher than the normal
There are a number of other, similar, low-dropout regulators that are
made by different manufacturers, but very few have as low dropout as
the FET/Op-amp circuit described on this page.
It is recommended that one not use a switching regulator to power the OCXO unless
it has been extremely well filtered and bypassed. Refer to the
sorts of techniques used with the Rubidium references found elsewhere
on this web page for suggestion.
If you are interested in an
example of this project being built with an etched PC board
with surface-mount parts, visit VK4ABC's 10 MHz OCXO Web Page.
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This page and its contents copyright 2010-20156by Clint, KA7OEI.. Last update: 20160720