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

From: bjoernm
Subject: Re: [Discuss-gnuradio] UCLA ZigBee and the Capture Effect
Date: Fri, 24 Feb 2012 10:28:39 +0100
User-agent: Internet Messaging Program (IMP) H3 (4.2)


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

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.
gr-user . ovillatx.sytes.net

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