[Discuss-gnuradio] Transmit filtering

[John Ackermann N8UR]

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