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Saturday, 19 May 2012

More WSPRing and QRSSing


After my last ramblings here:

I've been doing some travelling with work, so progress is not so good. However, I have entered into a dialogue with a number of folk over my observation that the amplitude of the DSS reduces as the frequency increases. Some suggestions to overcome this mainly include AGC type circuitry or utilising pin 12 of the AD9851 to feedback a voltage to set the output amplitude based on a sample of the output (recursion see recursion). The most interesting feedback that I got, however, was not that the amplitude was dropping with an increase in frequency, but that my scope wasn't reading the signals correctly. The theory being that as the frequency increases my scope sensitivity drops off....

I ended up plotting this graph:

This is implying that my scope starts to take a dive, accuracy wise, after about 1MHz, this is the blue line with the y-axis indicating how many dB my scope is "deaf" by. The yellow line is the same signal but through the on-board low pass filter from the DDS module. This seems to imply that the LPF is rather badly designed also. If this theory is correct I need to subtract my scope deafness from the drop in signal out of the LPF. If at this stage we remember that 6dB is half voltage, this is not insignificant!

All rather confusing? My scope is a 100MHz rated fluke which I thought was supposed to be a good quality instrument. So, is this behaviour typical or is my scope a pile of dingos kidneys? I wonder...

I've made a bit of progress boxing the WSPR and QRSS beacon project:

So far, I have tried a number of output amplifiers, the first based on the circuit in my original QRSS beacon, from back in December 2010:

This gives me a nice clean 4v P-to-P voltage out, but I wanted more than that, so either I add another stage or do something else.

I have also tried an output amp based on the Analogue Devices AD8008, this works OK also. I'm off on my travels again tomorrow, so I'll pick this up again when I get back.

All good fun though, egh?

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