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Re: [Discuss-gnuradio] Re: Residual Frequency?

From: Johnathan Corgan
Subject: Re: [Discuss-gnuradio] Re: Residual Frequency?
Date: Mon, 21 Apr 2008 15:16:38 -0700

On Mon, Apr 21, 2008 at 2:21 PM, Wireless Monster
<address@hidden> wrote:

> After the gmsk modulation (blks2.gmsk_mod) I have to scale the value (for
> example by multiplying it by 8000) before sending it to the USRP to get the
> resulting signal centered at the right frequency. If don't do it the output
> signal is shifted... I assume some minimum signal level is needed in the
> FPGA blocks to work correctly.

The "full scale" output to the RFX boards is +-32767.0.  It is not
recommended that you actually use this, as it will drive the RFX
output stages into a strongly non-linear operating range, increasing
noise and spurious signals significantly.  From empirical
measurements, the RFX boards can only get to about 15% full scale
before they go into gain compression.  That would be about +-5000 into

It's likely that what you were seeing before was a spur from the local
oscillator feedthrough of the mixer, can be up to 4 MHz away from your
requested frequency.  Once you sent a waveform of significant
amplitude out to the card, you then saw it instead of the spur (which
was still there, but much lower in power).  So it looks like one
signal "shifted" somehow, but was really two different signals.

Johnathan Corgan
Corgan Enterprises LLC

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