Re: [Discuss-gnuradio] BasicRx Daugterboard Frequency Wrapping

[Ed Criscuolo]

isaacgerg wrote:
> Hi,
>   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