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Re: [Discuss-gnuradio] Problems with the USRP/LFRX at lower frequencies
From: |
Johnathan Corgan |
Subject: |
Re: [Discuss-gnuradio] Problems with the USRP/LFRX at lower frequencies |
Date: |
Wed, 12 Nov 2008 20:37:11 -0800 |
On Wed, 2008-11-12 at 20:00 -0800, dan s wrote:
> I'm puzzled why the number of counts I measure is not constant even
> though I have a constant amplitude sinewave signal fed into the LFRX.
>
It appears you are seeing the non-uniform passband of the decimation
filter in the USRP FPGA. At a decimation of 128, your 250 KHz signal is
right at the edge, and is probably about 6dB down from something near
the center (DC or 5 KHz.) This is a combination of a 4-stage CIC
filter/downsampler and half-band FIR filter without further droop
compensation.
Try setting the decimation lower, to say, 16, to give you 4 Msps over
the USB. If the delta between a 5 KHz signal and a 250 KHz signal
becomes smaller, than this is likely the reason.
-Johnathan