Sunday 15 August 2010

Back on track with WSPR

A few weeks back I reported problems with my attempts to transfer the PIC-based WSPR signal generator to PCB - I was getting very poor frequency stability. More recently, even the breadboarded version started to display exactly the same symptoms (after having been working perfectly). I decided to take another look at the PCB-based WSPR generator.

I discovered that my "keying" arrangements were not just muting the audio signal, as intended. Rather, they were also inadvertently shutting down the oscillator. Thus, each time the system went into "transmit" mode, the oscillator started from cold - little wonder I wasn't getting the best stability.

I disabled the faulty "keying" stage and observed the frequency change after a cold start, seen in this trace generated by the WSPR PC application...

You can see how long the thermal time constant is and appreciate why I was having such trouble with frequency stability when switching on the Tx every ten minutes!

I modified my version of Gene Marcus' circuit again, this time using a quarter of the analog switch to explicitly gate the audio signal, rather than switch my faulty "keying stage". This left the oscillator running all the time - et voila - the desired stability. I quickly finished off populating the PCB and connected it up to the rest of the system...

I hooked up to the rejuvenated g5rv and - although it wasn't the best time for 30m spots - started making my presence felt in a quiet, WSPR-ed way...

It has been very satisfying to get these totally homebrewed, PC-free, double sideband WSPR signals spotted. Perhaps its time to start thinking about putting all the beacon boards into a permanent enclosure - certainly it is a nuisance having them clutter up the bench!

...-.- de m0xpd

1 comment:

  1. congrats! it seems your signal made it into my subharmonic receiver ;-)
    73, Joachim pa1gsj