Hi Marcus and lists,
Thank you for the fast reply, and I appreciate your insight. I agree that the 1kHz offset of the pure TX waveform is within the ppm spec of the board, and the spectrum analyzer is a Keysight N9020B. I was only thinking that offset might have been relevant because it was the same offset as the spectral artifacts seen during the playback. I am feeding into the B200 the 500MHz signal at -30dBm, although I find it unlikely this was leading to non-linearities though because the input into GNU Radio was fairly clean and at the correct frequency when plotted during the input. I did as you suggested and plotted the waveform during output:
Seemingly no spectral artifacts present, and to ensure this wasn’t specific to the spectrum analyzer, I went ahead and plotted the B200 output on a different spectrum analyzer:
This was also recreated at 200MHz, again no noise in GNU Radio Frequency Sink during playback, but plenty more artifacts present on spectrum analyzer:
Am I doing something wrong here? Or is this amount of spectral artifacts expected? I’d love to know if someone could recreate this on another B series, I’d hate to have to purchase another B200 just to replicate this issue.
Thanks again for any more help,
Nick
From: Discuss-gnuradio <discuss-gnuradio-bounces+nickrestivo=cornictech.com@gnu.org> On Behalf Of Marcus D. Leech
Sent: Thursday, January 27, 2022 2:48 PM
To: discuss-gnuradio@gnu.org
Subject: Re: Signal Capture and Playback Noise
On 2022-01-27 17:30, nickrestivo@cornictech.com wrote:
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
Working backwards here, an offset of 1kHz in 500MHz is about 2PPM, which is easily within the bounds of what is to be expected from a device with
a 2.5PPM master oscillator. Don't know what the specs are of your analyser, either.
What amplitude level are you feeding into the B200 from your signal generator? Anything significantly higher than what you'd expect from an
antenna can lead to non-linearities.
If you take your capture file, and "play" it in a Gnu Radio flow-graph, do you see the spectral artifacts or not?
“lists”:
“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. “