Re: Signal Capture and Playback Noise

I think you would need your SDR and signal generator phase locked to get a perfect frequency measurement. Windowed measurements aren't perfect. 

Your signal generator probably has a 10MHz reference input and perhaps output on the back. I don't know about the SDR. 

From: nickrestivo@cornictech.com Sent: January 27, 2022 2:34 PM To: discuss-gnuradio@gnu.org Subject: Signal Capture and Playback Noise

Hi GNU Radio users,

 

I am experiencing some interesting noise captured on a signal analyzer when trying to playback a captured signal using an Ettus B200 with both GNU Radio blocks and UHD example code. The UHD example code I am using is:

• "rx_samples_to_file --rate 5000000 --freq 500000000" for capture
• "tx_samples_from_file --rate 5000000 --freq 500000000 --file /home/path/usrp_samples.dat --gain 30" for playback

The GNU Radio blocks I am using are simply a USRP Source into File Sink for capture and a File Source into USRP Sink for playback (both programs operating at a 10M sample rate).

 

The signal I am capturing is a 500MHz pure sine wave fed directly out of a signal generator into the TX/RX port of the B200. The playback with noise I am receiving is directly out of the B200 TX/RX port into a signal analyzer and is pictured here:

 

 

Is this peak 1kHz from the carrier expected? Could this be the LO or some other noise on the board leaking through due to a lack of filtering? Is this re-creatable on other hardware? I can send the captured data if there isn't an obvious answer to why there is a signal at 499.999MHz.

 

Another piece to the puzzle I feel like I should mention is that when running "tx_waveforms –rate 10000000 –freq 500000000" and feed it directly into signal analyzer the output is only a peak at 499.999MHz…

 

Is this offset within tolerance of B200 hardware, or is there a bigger issue happening here?

 

Thanks in advance for any help,

Nick Restivo