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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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Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1.12) Gecko/20100907 Fedora/3.0.7-1.fc12 Thunderbird/3.0.7 |
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
setup?
How good is the filtering that happens during digital downconversion in
the FPGA?
--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org
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