From: [email protected] <[email protected]> on behalf of Bob Sutton via Groups.Io <zl1rs@...>
Sent: 05 December 2017 16:52
To: [email protected]
Subject: Re: [QRPLabs] U3S drift/chirp - again!

Hi Ian,

I built a 10m LPF kit for my U3S yesterday and ran it up on 28.000.089 FSKCW QRSS.??? With long calibration parameters, a similar 'power-on tail' and almost the same -20 Hz frequency error occurred, along with slow drift during the whole transmission just as the grabber screen shot shows on your U3S.??? It seems that a long calibration time (like 20 240 as in your first posting) causes the standard synth module to cool down too much during the calibration period resulting in the calibration inaccuracy and then frequency drift when it starts up/warms up again.??? Far better results were obtained with short calibration parameters like "Step Time" = 20 020 (along with Park Mode parameters = 2 150.000.000 to keep the Si5351a 'busy and warm' until the start of the next transmission) ... so, despite what the U3S operating instructions say regarding calibration time, at 28 MHz it seems that "less is more"!??? After a couple of transmissions with those settings my U3S had settled to almost drift free FSKCW at .089 (see below).?????????

Perhaps try a "Step Time" = 10 020 which should provide a calibration accuracy of 1 Hz with up to 20Hz of frequency correction at 28 MHz which should be enough to keep a U3S on the display frequency in a stable temperature environment.

FWIW, one of my standard Si5351a synth boards had a bad reference xtal which caused 'jittery/wobbly' FSK tones and a calibration error no matter what the cal settings.????????? I also have a QRP-Labs OCXO/Si5351a module which performed extremely well keeping the U3S right on frequency with no drift regards of the calibration time length.?????? While it can be done manually, the QLG-1 GPS makes initial U3S calibration easier with the OCXO synth, but after the Ref Frq parameters in the U3S are set to match the particular OCXO, it should stay on frequency.??? In the end, the GPS is useful to keep the U3S clock accurate, but perhaps that is not so important for QRSS?

73, Bob??? ZL1RS

Right, had my U3S have 6 minutes calibration between transmissions over night. Still the frequency is 20Hz low when frequency display is reading 28.000.890 and still has the 20Hz power-on 'tail'. Chris's grabber in not 20 Hz out.

I'm running out of options here. Do I assume there is something really wrong and chuck in the towel for 10m QRSS? I don't think there's anything left to adjust.

Is it worth getting the???OCXO/Si5351A synthesizer module kit? Is that still frequency stabilised by the QLG1 GPS receiver module kit or is that not necessary?

Was having a lot of fun with the U3S, but maybe mine is starting to fail...

73,

Ian, G4JQT