Re: Demodulating QPSK


"John E. Hadstate" <jh113355@xxxxxxxxxxx> wrote in message news:Y4idneGes7ZgcZTVnZ2dnUVZ_qmlnZ2d@xxxxxxxxxxxxxxxx
I have a general-purpose PLL implemented as a primitive in a language where I can easily run signals through it, plot the outputs, and tweak its parameters: initial frequency, loop gain, loop bandwidth and I/Q filter bandwidth.

I noticed something today. If I feed my PLL an unmodulated sine wave or a frequency modulated sine wave, it will acquire lock and track precisely over a very wide range of initial frequency errors and loop gain/bandwidth settings. However, if I feed in a QPSK-modulated signal [SIN=COS=-SIN=-COS=...] the acquisition and tracking ability falls to pieces. Where I could easily acquire and track an FSK signal at 2000+/-200 Hz. from a starting point of 1500 Hz., with the QPSK-modulated signal, I can only acquire lock within +/- 1 Hz at 2400 Hz. Having obtained lock, I can see the demodulated data on the filtered I and Q arms out of the product mixer.

Does this correspond with anyone else's experience? If so, what were you able to do about expanding the acquisition range?

I should mention that to get this to work at all, I introduced a kludge into the basic quadrature PLL. After getting the I/Q phase angle, I decide which quadrant I'm in and use the center of that quadrant as the "Phase Setpoint". I then use the difference between the measured I/Q phase angle and the phase setpoint as an error signal to adjust the VFO's phase offset (driving the demodulated I/Q phase to the center of the quadrant). I also square this error and combine the result with a fixed gain factor when calculating adjustments to the VFO's frequency. The net effect of this is that when the loop is locked in the center of one quadrant, a sudden jump to the center of another quadrant will not cause much change in the VFO's frequency (because the squared phase error is near zero) and it will stay phase-locked on the new baud.

I am not married to this kludge and would happily try a better solution, especially if it expands the range over which I can acquire lock. Any suggestions?

.

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