UHD example using USRP1

Hi All,

I’m using 2 USRP1s to send and receive a simple sine wave using UHD. The
TX/RX frequencies are 2.45 GHz as I have RFX 2400 as daughterboards.

I use the UHD example tx_waveforms to transmit a sine wave of 10 kHz,
rate 250ksps. I received the waveform at the other end using the UHD
rx_samples_to_file to dump the output to a binary file. I used a
sample rate of 250ksps. I used this file as a file source in a grc
flowgraph, hooked up its output to the Oscope sink (after converting the
interleaved short I,Q binary file to complex in the chain). I’m seeing a
clear sine wave, but its frequency is 5kHz, rather than 10 kHz.

Any idea what is actually happening here?



On 02/03/2011 09:44 PM, Shiva ? wrote:

sink (after converting the interleaved short I,Q binary file to

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So, let’s keep in mind what’s going on here. The Tx side generates a
10KHz signal sampled at
250Ksps (which is then up-sampled to the rate for the ADC). The
resulting 10KHz signal is
then mixed up to 2.45GHz(+/- inherent frequency error). Which
produces a final signal frequency
of 2.45GHz(+/-inherent frequency error)+10KHz.

On Rx, the incoming signal arrives at 2.45GHZ(+/-inherent frequency
error)+10KHz, which is then
down-sampled with a local oscillator set at 2.45GHZ(+/-inherent
frequency error). Which means
that your signal converts to baseband at 10KHz(+/- inherent frequency
error). The inherent
frequency error can be a few PPM, which at 2.45GHz means a few KHz
frequency error. So your
Rx signal may arrive up to a few KHz “off” from where you expect it.
This is rather normal for
synthesized transmitters/receivers with crystal oscillator references.

The crystal oscillator on the USRP1 is “good”, but it’s not
$100.00/apiece “good”. But this frequency
error is entirely normal in RF communications gear.

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