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[Discuss-gnuradio] USRP2 & Basic receiver. Unexpected behavior

From: Jorge Miguel
Subject: [Discuss-gnuradio] USRP2 & Basic receiver. Unexpected behavior
Date: Wed, 15 Sep 2010 17:16:43 +0200


After running my USRP2 with the GRC software I got some results that I didn't expect. My configuration was is simple as an USRP2 source followed by a FFT sink to see the spectrum (around MHz with a 75cm antenna), but:

1) I can only see the sprectrum up to 100MHz. I thought that the LTC2284 ADC had a dynamic range of 575MHz....Why can I only see the spectrum lower than 100 MHz?
(When I say that I can not see the spectrum I mean that at 100MHz I get a mirror of the spectrum between 0-100MHz).

2)The spectrum I can see is much more clear around 98MHz, where I can see a lot of carriers corresponding to FM radio stations that I cannot see for instance around 90Mhz.
3) What is the minimum decimation I can set in the USRP2 block? Experimentally I saw it is 5 but I don't understand why.

My specific configuration of GRC is:
USRP Source block: Decimation=5, Frequency=(tunable to cover FM band)
FFT sink: sample rate=21MHz (105M/5), fft size: 512

In fact when I run my FM receiver (also made in GRC), I can only listen to 89MHz and 108MHz radio stations. I expected to receive the whole FM spectrum.

If some of you have a answers to my questions I will be very glad because I want to understand USRP2 and GNU Radio as soon as possible to became an active member of this community!


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