On 05/09/11 10:19 AM, John Ackermann N8UR wrote: > 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. Make sure that you specify complex-taps. I assume you're using GRC. By default, it uses the version of the bandpass filter with real taps. But if you specify complex taps, you can use negative frequencies.
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