Re: [Discuss-gnuradio] Problems with the USRP/LFRX at lower frequencies

[Johnathan Corgan]

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