analog_pll_carriertracking_cc bug

[David Hagood]

I was trying to use the PLL carrier tracking block, and found that it 
doesn't seem to work at all. I have a simple flow graph demonstrating 
this - what would be the best way to share it?


It's pretty easy to demonstrate: take a signal generator generating a 
complex sinusoid at some frequency "f". Connect that to the PLL, and set 
the PLL to track that frequency (min freq = .99*f/samp_rate, max freq = 
1.01*f/samp_rate). Feed the input through a complex conjugate and then 
multiply that by the PLL output, and look at the result with a complex 
constellation and/or complex spectrum.


If the PLL is actually frequency locked, you would expect the output to 
be a DC signal at some phase offset, which would show up as a 
constellation cloud at a fixed angle. If the PLL is truely phase locked 
the result should be 1+i0.


It isn't. For my example, I was doing 19kHz tone at 200kHz sample rate, 
and I had a frequency error of 3 kHz - almost 20% of the desired frequency!


I haven't dug into the implementation of the PLL - if it's analogous to 
a hardware PLL, then I would say it's missing an integrator on the 
feedback path.