This kit is an Oven Controlled Crystal Oscillator (OCXO) version of the standard Si5351A Synthesiser module kit. The 27MHz OCXO is kit-built, it does not use an expensive commercial OCXO component. Construction of this kit is more difficult than the other kits, however it does not require any special techniques or equipment, as long as the construction steps are followed carefully.

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This OCXO kit has been discontinued. The QCX-series TCXO module is a good alternative and may be fitted to the Si5351A Synthesizer module

The Si5351A is a relatively recent clock-generator chip manufactured by SiLabs. It can produce three simultaneous independent 50-ohm impedance, squarewave outputs between 8kHz and 200MHz. Configuration of the chip is via a microcontroller with I2C (two-wire) interface.

NOTE: QRP Labs tested the actual frequency range of the Si5351A outputs. The minimum frequency configuration with 27MHz crystal produces 3.515kHz output. The maximum frequency appears to be an internal limitation (not limited by the configuration) and was approximately 292MHz. Of course, outside the specified range of the datasheet, there are no guarantees...

The 27MHz OCXO is a kit-built oven made of PCB pieces. The Supplied PCB measuring 10 x 8cm breaks into 16 pieces: one main PCB and 15 small pieces which make up three boxes, that are used for the oven chamber and external box. The Proportional Integral temperature controller is simple but effective.

This OCXO/Si5351A synthesiser module kit requires a 5V supply. The tiny 0.5mm pin-spaced Si5351A chip is pre-soldered to the PCB at the factory. The remaining components are all through-hole components. The component density is quite high and construction requires following the instruction manual steps carefully. The circuit contains two 3.3V regulator and I2C level converters, for use with the I2C bus in 5V microcontroller systems. The kit has a 2 x 10-pin 0.1-inch header for all connections.

The synthesizer may be used in a variety of projects. The pinout has been designed to be somewhat similar to the popular AD9850 DDS module and may be substituted in certain cases (with appropriate software changes in the controller). This Si5351A module is also designed to be compatible with the Ultimate3/Ultimate3S QRSS/WSPR kits. It can be plugged into the U3 kit with certain minimal hardware modifications to the U3. It is directly compatible with the Ultimate 3S kit without modification. It requires Ultimate3 firmware v3.07 or above, to communicate with the Si5351A via I2C. Note that for use in the original U3 kit, not U3S, (PCB revisions 1 to 4), pin 12 of the header needs to be removed before installation.


Assembly manual, which also contains some brief notes on theory of operation, and some typical performance data of the module.
SiLabs Si5351A datasheet
Sample source code for using the Si5351A


This page describes some background on the design of this kit. Read more...


Click the following photographs for larger sized versions. The photos show the kit contents and built kit. One photo also shows the OCXO/Si5351A Synth kit installed in an Ultimate3S QRSS/WSPR kit.

Temperature adjustment procedure

Adjusting the oven temperature to the optimum temperature is critical to the success of the OCXO, but it can also be tricky and depends on what equipment you have available.

Eddie G3ZJO describes his adjustment procedure in this document.

Bri G0MJI does the adjustment using a receiver monitoring the 2m WSPR frequency (or highest band to be used). Initially there will be some drift report on the WSPR reception decodes. Bri says as the temperature adjustment is rotated right, bit by bit, the WSPR drift report in a 2 minute transmission decreases; when it is zero, this is a reasonably optimum adjustment for the OCXO. All bands lower than 2m will also show 0 drift.

Useful tips from Gareth G4XAT

Gareth G4XAT wrote in with some tips, I found it particularly interesting how he bends the BS170 transistor leads. Gareth writes:

Just a quick email to say that my OXCO kit and filter board arrived on Monday. I spent a pleasant afternoon today, about 4 hours I guess, assembling and testing the OXCO which I am pleased to say works fine. I have yet to calibrate it but I found the instructions spot on, no ambiguity or such.

For bending the transistor legs I used a piece of pcb offcut - waste not, want not ;-) First I bent the middle leg down flat, then I bent it around the PCB. It fits a treat. Pictures attached which you are welcome to use any which way without credit.

I put a very slight taper on the inner edges of the main PCB – I found that helps get it ‘started’ into the big box.

Like the Ultimate 3S, this assembly is an absolute work of art, both electronically and mechanically.

Thanks for making stuff like this available.

Temperature control modification, by Stig OZ8PZ

Stig OZ8PZ had some trouble with the temperature control, and wrote with his solution, which basically amounts to reducing the value of R5. Stig writes:

When I tried to adjust the temperature, I ran into a problem. I cut slowly rise the temperatur, but when it became cold, shutdown, and when I put power on Again, the heater drew too much current. More than 0.5A.

I put a 50K potentiometer parallel to the resistor R5 (56K), and was able to adjust this potentiometer to set the maximum current when the OCXO was cold.

I found that for mine, it is suitable with a 18K resistor parallel to the 56K R5. I put it just under the little box; I have already closed the box, but there was plenty room for the resistor.

Now the current when cold is about 200mA, and when warm about 100mA. And now I can adjust the temperature with no problem.

A nice kit, but a bit tricky to assemble. Very good explanation how to do it. 

Thermal insulation, Alan G8LCO

Alan G8LCO pointed out that if you fill the box with some kind of insulation material, the performance of the temperature stabilisation is greatly improved. Alan suggests cotton wool.

OCXO modifications by Jon Ove LA3JJ

Jon Ove found the temperature regulation difficult to adjust. He has made three modifications, as illustrated in the diagram below. The picture of the three OCXOs he built shows the different current consumption of each module after adjustment (all taken at the same room temperature). The three modifications he made are:

1) Replace the temperature adjustment preset potentiometer with a multi-turn type

2) Use an LED or zener diode to prevent excessive current in the heater by limiting the gate current

3) Use an external 15 ohm resistor in series with the heater, during the adjustment - it helps prevent excessive current in the BS170 heater transistor by limiting the current in the transistors.

OCXO modification by Dick W7WKR

This modification by Dick W7WKR changes the OCXO temperature controller from "Proportional-integral" to just "proportional". Theoretically this does introduce an error term - i.e. the oven temperature does now have a dependence on ambient temperature, in theory. Whether in practice it is important or not, who knows! But Dick feels this makes the OCXO temperature adjustment easier. This is Dick's description of his modification.

"Today I 'tamed' my U3S kit with the OCXO synthesizer option. It was a very simple fix derivative of the circuit in a 1998 QEX article describing the construction of a TCXO. I changed Hans' circuit from a proportional-integral to a straightforward linear feedback control loop by shorting-out the integrator capacitor (C1 = 22uFd) in the OP-Amp's feedback loop.

I also did not bother to use the complicated multi-part soldered-together PC-Board boxes to contain the oven components. Instead, I just poked the module's two rows of interconnection 'pins' through a somewhat heavy piece of paper and formed a paper box around the entire synthesizer module (first adjust the oven temperature). 

"Tape the paper box together and inserted the synthesizer module into the connector on the U3S main assembly and apply power. Wow - no drift even in a drafty environment. The paper makes an adequate thermal insulator and blocks circulating room air currents. (Hans - you could cut U3S manufacturing costs significantly and simplify kit-builder assembly issues using this technique. Have a steel-die cutter cut out the base with pin holes, a top with adjustment hole and wrap around side-wall that can be Super-glued together) 

"Here in the YAHOO Groups several months-past some U3S owners report issues with the OCXO Synthesizer Module oven controller settings. You might consider my solution to this problem. I have found the thermal adjustment is now a very smooth and easily set (no over-shoots, etc) operation.

"My U3S assembly is now being used as a replacement local oscillator (~41.284500-MHz) for an Icom IC-551D that had a dead synthesizer. I now have a dedicated stand-alone 50.294500MHz WSPR beacon that appears to be VERY stable - typically zero to one Hz drift during a two minute TX-window and 1Hz (+/-) long term drift. By using a WSPR-based LO signal, setting the IC-551D to "AM" mode and using the "key" line from the U3S to drive a 2N2222 to toggle the TX-switch, the beacon is totally automatic and recovers unattended from a power failure.

"I am investigating generating a "CW" form of the LO for a stand-alone CW beacon at 50.070-MHz to follow the WSPR transmission. Hopefully the keying will be acceptable in the USA beacon-window."

"Hans - Thank you for developing the U3S kit. EVERY ham-shack should have several (with the GPS receiver) available for the many projects we encounter every day ! I will be ordering spare U3S/GPS kits soon . .