Re: [Discuss-gnuradio] wfm_rcv_pll.py

[Robert McGwier]

I looked over my analysis again.  The Z transform version of the usual 
PLL equations leads directly to the alpha, beta calculation contained in 
the code which yields a critically damped phase locked loop for the 
required bandwidth.   A PLL based demodulator was chosen to aid in the 
mitigation of "threshold effect" and the fact that stereo reception is 
not a weak signal proposition.

More importantly,  when I used your 0.8 instead of the 0.25 the math 
yields,  I got horrible high frequency artifacts on strong stations that 
give good stereo demultiplexing.  I enabled the FM post demod graph and 
the noise floor was elevated many dB above the baseline.   When I 
changed it back to 0.25 on local radio stations,   the noise floor 
outside of the stereo encoding and/or the RDS dropped below the baseline 
on the post FM demod system.  This shows a demodulator correctly adapted 
to the signal of interest.

As a final test I put my 8640B in FM modulation mode and observed the 
behavior on 100 MHz.   At the dynamic range limits I considered usable 
for stereo demultiplexing and RDS decoding, the PLL values appear to be 
correct from the power spectrum and from my auditory evaluation (listening).

May I suggest that you attempt to use the gain slider to increase the 
USRP/ADC gain.   If this does not help then, in your case,  0.8 works 
better and you should use it but I consider this suggested change to be 
wrong for the "official code".

Bob



Erlend Barstad Strand wrote:
> Hi
>
> I have experimented with the wfm_rcv_pll.py for broadcast FM, and have 
> som questions regarding it:
>
> - what is the purpose of using the gr_pll_carriertracking_cc on the 
> pilot tone? The way I understand the funtion of 
> gr_pll_carriertreacking_cc is to baseband a signal which lies on a 
> carrier and therefore it should be used on the stereo channel instead.
>
> - is there any particular reason why the alpha i set to 
> 0.25*bandwith*pi/demodrate? I found out that setting this parameter to 
> 0.8*bandwith*pi/demodrate the noise floor drops considerably, which 
> makes the stereo channel appaear more clearly.
>
> best regards
> erlend
> ------------------------------------------------------------------------
>
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>   


--
AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL,
TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair
"If you board the wrong train, it is no use running along the
corridor in the other direction. " - Dietrich Bonhoffer