I'm generating two NBFM signals, offsetting them plus and minus 25 kHz
with FreqXlatingFilters, and combining the output in an adder which then
drives a USRP sink. It seems sensible to put a bandpass filter on the
result.
I have a basic conceptual confusion. I would think that you'd specify a
bandpass filter around zero with a low cutoff of, say, -40e3 and a high
cutoff of +40e3. However, this doesn't work -- the bandpass filter
seems to require a number >0 for the lowpass frequency.
So, in this situation, where I want to filter a spectrum that is
centered around zero, how do I specify the filter lowpass and highpass
frequencies?
A second question -- there is a signal component on the output directly
at the USRP center frequency. It's at most about 30dB below the desired
signals. I assume this is the equivalent of LO leakage. Is there any
way to filter this out or at least reduce its amplitude?
As I'm writing this, I wonder if the solution to both problems is to put
both my signals on one side of the USRP center frequency, say +25 and
+75 kHz. That would allow a sensible bandpass filter that would remove
the zero-frequency component. But the downside of that seems to be
throwing away half the available bandwidth.
Thanks,
John
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