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[Discuss-gnuradio] RFI considerations with aliasing

From: Marcus D. Leech
Subject: [Discuss-gnuradio] RFI considerations with aliasing
Date: Mon, 20 Sep 2010 21:29:29 -0400
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I have a narrowband receiving system (250KHz to 750KHz wide, depending
on local requirements) which receives at the low-end of VHF.

I'm using a Basic_RX card, which is fed with:

    o a crossed-dipole antenna
    o a 23.5MHz 3rd order high-pass filter (components I had lying
around--nothing magical about 23.5MHz!)
    o 20dB of gain
    o a 5th order 48MHz low-pass filter
    o 20dB of gain
    o another  5th order 48MHz low-pass filter

The initial high-pass is to attenuate lower-frequency signals (broadcast
band in particular, but also HF broadcasters and amateur radio), and
  keep them out of the gain stages.  The 48MHz filters are designed as
anti-alias filters--since I'm using a USRP2 which samples at 100Msps,
  I should be able to "see" up to 50MHz alias free.  One of my main
frequencies is 38.125MHz, which puts the first alias at 61.875Mhz, which
  is part of on-air TV channel 3, and I do see RFI in the evenings, but
it isn't consistent with a TV carrier, and it comes up and down over a
  several-tens-of-seconds cycle.

I'm wondering if I have "enough" low-pass anti-alias filtering in this

How good is the filtering that happens during digital downconversion in
the FPGA?

Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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