Re: [Discuss-gnuradio] Sample Rate

[Brian Padalino]

On Jan 28, 2008 10:43 AM, Richard Jaeger <address@hidden> wrote:
> I have been playing with a number of the examples - FM NB and WB
> receivers, AM receiver, etc.
> The sample rate appears to default to 64 MSPS, but I have been unable
> to find where that default rate is set.
> Where is it defined?  Can it be changed from the examples themselves?

The 64MHz comes from the crystal that runs both the FPGA and other
digital electronics on the USRP.  You cannot change the sample rate
unless you change the crystal (which I suppose isn't recommended, but
hack away if you're willing to!).

There is a programmable decimating CIC filter (up to 128x decimation I
believe) followed by a decimating halfband FIR filter (fixed 2x
decimation) implemented within the FPGA before you get to the host
processing.

> For example, 64MPS is not a proper subsample rate for the FM broadcast
> band.
> It results in aliasing and should be reduced.

You could always put a bandpass filter inline to suppress anything
outside of the band(s) you may be interested in.

Other than aliasing, what other types of issues are you worried about
with regards to the high speed 64MHz sample rate?

Brian