Re: [Discuss-gnuradio] Frequency shift in the received and generated signals of the USRP

[Brian Padalino]

On Thu, Nov 20, 2008 at 5:31 AM, Bruhtesfa Ebrahim <address@hidden> wrote:
> Hey all,
>
> I am using USRP with two XCVR2450 Trancievers(working at 2.4-2.5GHz and
> 4.9-5.0GHz).
> I generated a carrier at 2.45GHz from the USRP using :
>  $ python usrp_siggen.py -TA -f2.45e9
> But when i see the generated signal in a spectrum analyzer, i see that  it
> is shifted by 12.5KHz
> from the center frequency( i see the carrier at 2.45GHz minus 12.5KHz). I
> try for other carrier frequencies but the
> shift remain almost the same.
>
> Also, I generated a carrierl of 2.45GHz from a signal generator , received
> it in the usrp and I observed the spectrum.
> Again I see that the signal that is demodulated by the USRP and received in
> the computer through the USB is not a baseband signal as expected. Rather it
> is a carrier of about 12.5KHz.
>
> This means their is a fequency shift of about 12.5KHz both in transmission
> and in reception.When I use a carrier of frequency 4.9-5.0GHz, the frequency
> shift almost doubles to 25KHz.
> Does anyone experienced such a problem before?
> So, What can i do to correct this frequency shift?   -Thanks

Unfortunately, the problem is inherent in all RF communications
systems due to the fact all radios have an independent time base.

Because each local oscillator is not exactly 64.000000000MHz, when
that signal is multiplied up by the PLL - so does the "offset".  You
can reduce this by synchronizing yourself to a very reliable timebase,
but you still get some residual shift.

The fortunate part for you is that it is very well documented and
written about extensively.  Take a look in your favorite wireless
communications systems book and you will most likely see a good
discussion with regards to time synchronization.

Enjoy your reading, and good luck!

Brian