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Re: [Discuss-gnuradio] BasicRx Daugterboard Frequency Wrapping

From: Ed Criscuolo
Subject: Re: [Discuss-gnuradio] BasicRx Daugterboard Frequency Wrapping
Date: Thu, 22 May 2008 09:58:28 -0400
User-agent: Thunderbird (Macintosh/20071031)

isaacgerg wrote:
  I am hooked up to the BasicRx Daughterboard and wrote a simple applicate
using the GRC to survey the spectrum from 1MHz to 250MHz and run the user
tunable output through the WFM Demodulator.  I can get FM radio stations in
the 88-108MHz region just find.  However, I am also getting copies of these
stations at frequencies higher and lower than this range!!!  It's as if the
frequency I request is out of range and the tuner wraps around.  What's
going on here?  When I tune to 50MHz, I expect to be at 50MHz, not some
other frequency!

What you're experiencing is called aliasing.  The BasicRx daughterboard
has no low pass filters on it, and the USRP is sampling at
64 M-samples/sec. This allows it to unambiguously sample frequencies
up to 32 MHz. However, for any given frequency f, multiples of f
will produce either an identical set of samples as f, or an inverted
set.  This produces the "wrap-around" effect you are seeing.

Check out any text on analog to digital conversion for a more detailed
description of aliasing.

@(^.^)@  Ed

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