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Re: [Discuss-gnuradio] Transmit filtering

From: Michael Ossmann
Subject: Re: [Discuss-gnuradio] Transmit filtering
Date: Mon, 5 Sep 2011 08:35:10 -0600
User-agent: Mutt/1.5.21 (2010-09-15)

On Mon, Sep 05, 2011 at 10:19:54AM -0400, John Ackermann N8UR wrote:
> 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.

That would be a low-pass filter with a cutoff at 40e3.  With
complex-valued signals, a low-pass filter is essentially a band-pass
filter centered around 0 Hz.

> 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.

Yes, you could use an external analog band-pass filter if that center
spur is a problem for you.

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