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Re: [Discuss-gnuradio] wfm_rcv_pll.py
From: |
Robert McGwier |
Subject: |
Re: [Discuss-gnuradio] wfm_rcv_pll.py |
Date: |
Fri, 24 Nov 2006 16:09:25 -0500 |
User-agent: |
Thunderbird 1.5.0.8 (Windows/20061025) |
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
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