[Discuss-gnuradio] Re: question about RFX2400

[Nick Foster]

On Tue, 2011-01-04 at 19:25 -0700, Malihe Ahmadi wrote:
> Hi,
> 
> I am testing a transceiver link in ISM band using two USRP2+RFX2400, one 
> configured as TX and the other one as RX. I would like to test my own 
> FPGA program, so for now I have your FPGA codes in which I only changed 
> the signal driving the DAC in TX to be an square wave (+8191 or -8192) 
> with frequency about 400Hz. At the RX side, I am looking (using an 
> oscilloscope) at the input to the ADC and what I see is quite like my 
> square wave modulated with a carrier of couple of MHz. I looked at the 
> schematic of RFX2400 and I didn't find any PLL or clock recovery 
> circuitry! so I wonder how the frequency offset at RX could be avoided?
> 
> Regards,
> Malihe

In a software-defined radio, frequency estimation and clock recovery are
typically done in software, to keep the hardware general-purpose enough
for many different uses. You can either synchronize the two USRP2s by
using a common 10MHz reference input to both devices (and enabling the
reference lock), or deal with the inevitable frequency mismatch in
software.

--n