Figure 1:
The 24 GHz transverter along with the described 10 MHz  frequency reference.
Click on the image for a larger version.

Why a frequency reference?

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, be able to talk when signals are weak!

The use of microwave frequencies and narrowband modes such as SSB or CW means that the one must maintain pretty good frequency stability and accuracy:

• 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 accurate.  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.  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 error was on the order of several kHz, "hunting" would be required to find the signal and if those signals are weak, it might 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 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.

At this point I might mention that Rubidium frequency standards (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.  Why not always use a Rubidium standard instead of a crystal-based one?  The crystal-based unit is cheaper, easier to package and consume significantly less power than a Rubidium standard - and the stability/accuracy of a good-quality crystal-based reference is more than "good enough" at least through 24 GHz!

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 minutes or so to warm up - even 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 Ebay and similar.

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.  In looking around I noticed that most readily-available low-dropout regulators 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.  Why regulate?  It was noted in testing that slight variations of supply voltage (a few hundred millivolts) would cause measureable disturbances in the oscillator frequency due to the changes of the power applied to the heater, taking up to several minutes to again reach equalibrium.  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, provides a stable voltage reference for U103, a 741 op amp, which is used as an error amplifier.  If the output voltage is too low the voltage on pin 3 drops along with the pin 6, the op amp's output.  This turns on Q103, a P-Channel power MOSFET, which increases the voltage.  Once the voltage on the wiper of R119 reaches 5 volts - that of the reference - the circuit comes to equilibrium.  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 testing, once the oven was warm (a condition in which it was drawing approximately 250 mA at normal "room temperature) the dropout was approximately 50 millivolts.

Figure 2:
Top Left:  Inside the 10 MHz OCXO module showing the oven (left) and the power supply and distribution amplifier board (right.)
Top Right:  End-view of the OCXO module showing the output jacks, the "Status" LED and the power connection.
Bottom Left:  A close-up view of the oven and its shock mounts.
Bottom Right:  Schematic of the OCXO module.
Click on an image for a larger version.

The 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 supply 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.

The regulator seems to work quite well, holding the output voltage steady to within a few millivolts over the range of 11.1 to 17 volts with good transient response.

It should be noted that this status line doesn't indicate that the oven has fully warmed up, but that it's still warming:  At "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.

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.

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.  This was done using some rubberized mounting pillars from scrapped satellite equipment and some "blobs" of silicone were placed on the wall of the die-cast enclosure to prevent the OCXO housing itself from directly impacting it.

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!

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.

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This page and its contents copyright 2010 by Clint, KA7OEI.  Last update:  20100222