Re: [Discuss-gnuradio] UCLA ZigBee and the Capture Effect

[bjoernm]

Hi LRK,

Thanks for the reply! I thought so too, that this capture effect or  
phase locking or limiting should come from the demodulator or clock  
recovery block, but looking at the c++ files, I couldn't find anything  
related to this. Did I miss something?

The receiver is connected as follows:
        self.fmdemod = gr.quadrature_demod_cf(1)
        self.freq_offset = gr.single_pole_iir_filter_ff(alpha)
        self.sub = gr.sub_ff()
        self.clock_recovery = gr.clock_recovery_mm_ff(omega,  
gain_omega, mu, gain_mu,omega_relative_limit)

        self.connect(self, self.fmdemod)
        self.connect(self.fmdemod, (self.sub, 0))
        self.connect(self.fmdemod, self.freq_offset, (self.sub, 1))
        self.connect(self.sub, self.clock_recovery, self)

But looking at eigher
gr_clock_recovery_mm_ff.cc
http://gnuradio.sourcearchive.com/documentation/3.2.2.dfsg/gr__clock__recovery__mm__ff_8cc-source.html
or
gr.quadrature_demod_cf.cc
http://gnuradio.sourcearchive.com/documentation/3.2.2.dfsg/gr__quadrature__demod__cf_8cc-source.html

didn't help me... any suggestions?
In front of the demodulator is also the Low pass filter:
self.channel_filter = gr.fft_filter_ccc(sw_decim, chan_coeffs)

best regards, B




Quoting LRK <address@hidden>:

> On Thu, Feb 23, 2012 at 03:31:03PM +0100, address@hidden wrote:
> > >  This sounds more like an AGC effect. If the gain gets reduced a bit,
> > >the decoder may not see sync from the weaker jammer. FM or PSK receivers
> > >usually have very fast limiting rather than AGC. Fast limiting is what
> > >you want so you can receive all packets possible.
> >
> > This sounds promissing...! But how would I access the settings from
> > the limiting mechanism or the AGC?
> > I'm using the XCVR2450 Daughterboard on the USRP1. I guess the AGC
> > should be somewhere on there,
>
>   The gain is set in the hardware but the 'automatic' should be in the
> software. I think you said you combined the zigbee and tunnel.py.
>
>   I use the usrp_fft.py to look at the spectrum and adjust the gain
> slider to be just above the point where the noise baseline starts going
> up. Mine shows a setting about 34. Then I set my software receive gain
> there. Obviously the code needs to be similar and I think the UHD code
> may be a lot different.
>
>   Your receiver should not need automatic gain control since it should
> be either fsk or psk. The demodulator should do the limiter part.
>
>
> --
>     The computer is supposed to work for YOU, not the other way around.
> LRK
> gr-user . ovillatx.sytes.net