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Re: [Discuss-gnuradio] Homing in on an RA detector chain

From: Marcus Leech
Subject: Re: [Discuss-gnuradio] Homing in on an RA detector chain
Date: Thu, 05 Jan 2006 17:59:39 -0500
User-agent: Mozilla Thunderbird 1.0 (X11/20041206)

Matt Ettus wrote:

Your code looks reasonable, but before you go writing C++ blocks, I
would try to do what you want with what is already there.  The reality
is that the gr_probe_avg_mag_sqrd is rather specialized, and not really
necessary.  Here's what I would do.

1) usrp source complex to gr_complex_to_float to split the I and Q
2) i channel to both inputs of gr_mult_ff to square them
3) do the same for q
4) outputs from the two gr_mult_ff blocks to a gr_add_ff block

You now have instantaneous power.  I would filter how you like, probably
starting with a gr_fir_filter_f.  Pick a relatively high decimation
ratio, M, like 256.  There should be M taps in the filter, each of value
1/M.  This will do an integrate and dump to get you to a lower sample rate.

Given the rather-long integration times characteristic of RA, would you suggest that the integration be "split" between the decimating FIR filter, and a single-pole
 IIR filter?

If you have any interfering signals, you have to remove them by
filtering before step 1 above.  That is also where you'd tap off if you
wanted to do an FFT or waterfall display.

The interferers was my next little thought project. Considered it on the way home. It seems to me that the RA "receiver" can start up analysing a FFT for narrowband interferers (anything bins that's so-many sigma away from the standard deviation), and make those interferers the "notch" in a (multi?) notch FIR filter. Is this feasible with a single FIR filter, or would multiple filters be required? What I'm seeing right now is that with the feedhorn covered in metal, at 1420.405Mhz+/- 2Mhz, I'm seeing a strong (15dB out of the noise) narrowband spur at -500Khz, and a weaker
 one at about +1.8Mhz.

It's possibly being picked up by the coax cable between my LNA and the DBS_RX.
 But my theory is that it's internal to the DBS_RX/USRP "stack".

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