03 March, 2021

QRPLabs 25 MHz TCXO performing well in the U3S

I have tried several oscillators in the U3S QRSS/WSPR transmitter from QRPLabs. First an analog devices DDS, then a Silicon Labs oscillator chip running from an ordinary 27 MHz crystal, and then QRPLabs own oven controlled oscillator. I thought I had managed to control the drift when I got the oven controlled oscillator. But then after a while it started drifting again. I didn't want to go through its rather cumbersome calibration procedure once more so I gave it up recently when the TCXO module became available. This tiny module is shown in the first image.

It turned out that it resolved all drift problems immediately and so I recommend it to all perfectionists like me who cannot stand to see a drift of ±2, 3, or 4 Hz/min in the WSPR report. 

Now I have 0 drift both in the 40, 30, 20 and 17 m bands and presumably everywhere else also.

Mounting it was not a big deal. I wanted it to be on top of the oscillator module with the Silicon Labs Si5351A chip and how it was mounted is shown in the next images. The three wires are all stiff and essentially support the module in air.


  1. Hello Sverre,

    I did not know that the TCXO was compatible with the Ultimate 3S - I thought it was only for the QCX?! I can't find anything in the Ultimate 3S instructions mentioning it. Can you provide more detail on the interconnections and any configuration changes necessary to the U3S menu?

    Tnx and 73,
    John AE5X

  2. Hi John,
    This is something I've picked up from the QRPLabs discussion forum at groups.io. My oscillator board is ver 4 and starting with that version there are pads for a TCXO. You'll barely see the letters near the red wire in the second last picture. The pads are for +3.3, ground, and input and I've essentially connected to the same points except that the input pad is the right-most hole for the crystal and ground is on all of the upper row of pins. Setup-wise, the 27 000 kHz setting has to be changed to 25 000 kHz. This change implies a small inaccuracy in the WSPR frequency steps, which, if I remember right, only affects 2m WSPR.

  3. Thanks for the info and confirmation of compatibility with the U3s - that's great to know! I'll open up my u#s tomorrow and see what Rev the board is.

    John AE5X

    1. This is not dependent on rev 4, but it hjelps in identifing where to connect. Ground is easy to find no matter what and +3.3 Volts can be identified as the output of the voltage regulator. Finally, the tcxo output should go to the outermost crystal pad, pin 2 of the SL chip.