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Re: [Discuss-gnuradio] Problem with sinc pulse transmission using RFX240

From: Johnathan Corgan
Subject: Re: [Discuss-gnuradio] Problem with sinc pulse transmission using RFX2400
Date: Wed, 29 Aug 2007 16:33:46 -0700
User-agent: Thunderbird (X11/20070824)

S. Faisal A. Shah wrote:

> ...In fact, the received pulse train seems to be amplitude modulated
> by a low frequency signal (in KHz).

It sounds like you are encountering frequency offset between your
transmitter and receiver.  This is caused by differences in the crystals
between the two USRPs--up to 20 ppm each in new hardware.

At 2.4 GHz, that could be up to 96 KHz in difference between what one is
transmitting and the other is tuned to.

Frequency offset will cause the received signal to "rotate in phase", so
what started out purely in the I channel will be seen in both I and Q,
with the magnitude in either one varying as a sine wave at the
difference in frequency.

Normally, this effect is dealt with using a PLL on the receive side, to
adjust the frequency and phase offset to compensate.  What type of PLL
is dependent on your modulation.

Johnathan Corgan
Corgan Enterprises LLC

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