Re: [Discuss-gnuradio] bug in wfm_rcv_pll and fix, who has stereo FM specs/info and noise and gain issues

[Martin Dvh]

Matt Ettus wrote:
>>>I would try this with the regular (non-PLL) FM demod.  I think the
>>>rolloff of the PLL tracking loop is what is giving you the lower gain.
>>>    
>>
>>Yes, that seems to be the problem.
>>For now I switched to using the non-PLL FM-demod.
>>
>>Do you know if/how I can make a PLL which does not have this high roll-of.
>>I played with the pll-parameters but that doesn't seem to help.
>>It is critically damped, so that is probably the best we can get with this 
>>pll.
>>Would I need another type of pll with a higher order loop-filter or something 
>>like that.
>>  
> 
> 
> 
> I think it's just a question of bandwidth.  Have you confirmed that the
> PLL has at least 60 kHz of bandwidth?
Bandwidth should be 200 kHz.

But when I look at the FFT of the output I see about 10 dB/octave roll-of for 
the pll_demod.
The quadrature_demod has a straight freq response (as expected)

See testcode below:

I wrote a small loopback test fm_mod ->fm-demod
http://www.olifantasia.com/pub/projects/gnuradio/mdvh/testcode/wfm_loopback_test.py
The main code is:
        #mod
        max_dev=75e3
        mod_gain = 2 * math.pi * max_dev / self.sample_rate
        self.fm_mod = gr.frequency_modulator_fc (mod_gain)
        #demod pll
        bandwidth = 200e3
        alpha = 0.25*bandwidth * math.pi / self.sample_rate
        beta = alpha * alpha / 4.0
        max_freq = 2.0*math.pi*100e3/self.sample_rate
        self.fm_demod_pll = gr.pll_freqdet_cf (alpha,beta,max_freq,-max_freq)
        #demod quadrature
        fm_demod_gain = 1.0/mod_gain
        self.fm_demod_quadrature = gr.quadrature_demod_cf (fm_demod_gain)

When I look at the FFT of the output I see about 10 dB/octave roll-of for the 
pll_demod.
http://www.olifantasia.com/pub/projects/gnuradio/mdvh/testcode/wfm_loopback_screenshot.gif
Should this be right?

The quadrature_demod has a straight freq response (as expected)

Martin
> 
> Matt
>