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[Discuss-gnuradio] Transmit filtering

From: John Ackermann N8UR
Subject: [Discuss-gnuradio] Transmit filtering
Date: Mon, 05 Sep 2011 10:19:54 -0400
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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.



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