Re: #wspr U3S for 2m WSPR: How to remove the frequency-swing at start of transmission #wspr

Alan de G1FXB

On 20/11/2018 21:05, mrdynalink wrote:
So ... back from experimenting. What I did:

1.) I connected a very strongl regulated powersupply with thick wires direct to the U3S mainboard. Hmm ... there was no visible change at all. I played with the supply-voltage, reducing the voltage to 4.8V changed the frequency only for 10Hz. But at start of transmission the "chirp" seems to come from "infinity"...

2.) When I put my finger on the crystal I see the frequency jumping for 100Hz or more. It seems to be very temperature-critical. Somewhere I read that it is a good idea to put a heatsink to the crystal to make the temperature-changing lazy. So I soldered a 2 Cent coin direct on the crystal. But again, no influence to the chirp at the beginning of transmission.

3.) Now I tried to measure the 27 MHz Carrier from the crystal. I connected my Perseus antenna-input direct with the 2 Cent coin and I found the crystal oscillating at 27.005 MHz. Then I tuned the Perseus into the wspr window and made a recording with Argo. This was very interesting:

During the pause between the transmissions the crystal frequency makes a big jump. But the crystal should oscillate accurate on one fixed frequency! What is the reason for this behaviour ? Currently I have no idea and have to think about it. Possibly somebody else has an idea for the reason ?

However ... I think it`s a good idea to order the oven-controlled version ... not primary for the oven but for the decoupled 27 MHz oscilator.


Perhaps if you take the OXCO route as you have documented characteristics for the Si5351 kit at VHF, it would be a useful half way step to repeat the testing with the OCXO
but with the heater unpowered to purely to observe the improvement brought about by alternative Osc circuit.Before employing the heater option.
Alternatively as others have stated the cheap TCXO's generally have a robust output not to be affected by loading changes.
The FOX has being used in the past, if you search the complete forum thread, another device characterised is :-

The page title is for the ProgRock, but in actual fact the improvements are made solely to the Si5351 module and is applicable to all QRP-Labs kits using the Si5351 kit.
It's smaller than the Fox alternative, but may be worth consideration if you can work with the device?


73, Henning

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