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Re: [Discuss-gnuradio] Is there a reason why this wouldn't shift the si

From: Jonathan Coveney
Subject: Re: [Discuss-gnuradio] Is there a reason why this wouldn't shift the signal that is demodulated?
Date: Sun, 9 Aug 2009 17:03:26 -0400

I know you say not to use a throttle, but for some reason it absolutely does not work without one. Any idea why that would be? It sounds very distorted and sped up, and then I get a bunch of xO errors. With throttling it works perfectly.

2009/8/9 Eric Blossom <address@hidden>
On Sat, Aug 08, 2009 at 05:02:55PM -0400, Jonathan Coveney wrote:
> My goal is to be able to read a frequency other than the frequency that was
> tuned to draw samples using usrp_rcv_cfile (ie I used 93.3M, and want to be
> able to demodulate 92.5M...I'd try to use lessons learned from wfm_rcv2 but
> that uses set_rx_freq in the pipelines, which obviously I don't have access
> to)
> Taking a cue from this
> http://www.nabble.com/Frequency-Hopping-td20538730.html
> I tried to shift by -.8Mhz
>         mix_freq = -.8e6
>         Cosine = gr.sig_source_f(usrp_rate,gr.GR_COS_WAVE,mix_freq,1,0)
>         Sin    = gr.sig_source_f(usrp_rate,gr.GR_SIN_WAVE,mix_freq,1,0)

Consider using gr.sig_source_c.  It generates a complex sinusoid.

>         self.connect(Cosine,(self.expjw,0))
>         self.connect(Sin,(self.expjw,1))
>         self.mixer = gr.multiply_cc()
>         self.connect (self.u, (self.mixer, 0))
>         self.connect (self.expjw, (self.mixer, 1))
>         self.connect (self.mixer, gr.throttle(gr.sizeof_gr_complex,
> usrp_rate), chan_filt, self.guts, self.volume_control, audio_sink)
> I also tried using freq_xlating_fir_filter_ccf (which googling looked like
> it was overkill) to no effect. In both cases I just hear a noisy version of
> 93.3M, I can't get it to focus on something that wasn't the center of the
> frequency that my samples were taken.

gr.freq_xlating_fir_filter_ccf is the easiest way to extract one or
more frequency bands from a wider band input.  usrp_wfm_rcv_sca.py
uses it to extract the SCA channel.

If all you want to do is shift a frequency use gr.sig_source_c and a gr.multiply_cc

In virtual all cases you do _not_ want to be using throttle...


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