[Discuss-gnuradio] PI/4 DQPSK demodulator - Progress update

[Ian Wraith]

Hello All
         Progress on my PI/4 DQPSK demodulator has been stalled recently
due to technical problems and lack of time. A couple of group members had
pointed out my previous I & Q waveforms looked incorrect and that I should
look for problems in my hardware. Well last week I had the opportunity to
briefly borrow a Agilent signal generator which was capable of generating
RF using various modulation schemes (unfortunetly not PI/4 DQPSK). But it
was ideal to check out if my quadrature demodulator was working correctly.
It only took 10 minutes for me to discover the circuit had big problems
and was doing horrible things to the waveforms. It didn't take me long to
find and fix ..

1) My 21.4 MHz crystal oscillator which I calibrated using my ancient
home frequency counter was in fact running a huge 7 KHz high.

2) Thanks to me forgetting to connect the voltage offset from the
RF2711 to my op-amps nasty things were happening to my I & Q waveforms.

 Once these were fixed I connected it all up and took in some data using
my partially written code. While looking at this data I discovered I had
captured what may be syncronisation downlink burst containing a frequency
correction field which a run of 0's. I created a graph showing the I & Q
waveforms plus another graph showing this data as a constellation. These
can be seen here ..

http://borg.shef.ac.uk/~ianw/improv_data.html

I am sure you will agree this data looks much better. Although you will
notice from the constellation that my receiver was slightly off frequency
when the data was gathered. In addition this constellation shows all I & Q
data points rather than just those at the correct symbol timing moments.
At the moment I am working on another list members suggestions for symbol
timing and hope to be able to report good news some time soon.

Regards

Ian