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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 (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".


Erlend Barstad Strand wrote:

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

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