hackrf_transfer -c produces possible buffer underruns

[Sasszem]

What type of issue is this?

permanent - occurring repeatedly

What issue are you facing?

While measuring the output of a clone HackRF one with a proper spectrum analyser, I noticed the power output fluctuating, and with slower sweep times, the spectrum looked exactly like an OOK spectrum. I confirmed the result using time-domain measurements with a low-frequency carrier - the output went to 0 for extended periods (milliseconds) somewhat randomly. I suspected software error, and wrote a small piece of C++ code on my own to transmit. My code did NOT produce the same effect, confirming a software bug in or triggered by hackrf_transfer. I am planning to investigate the real cause, but in the meantime, I'm opening this issue in case anyone else encountered the same effect.

What are the steps to reproduce this?

• run hackrf_transfer -f 10000000 -a 0 -c 127 -x 47 (10 MHz output with maximum gain settings without amplifier)
• measure with spectrum analyser, 10Meg center, 2Meg SPAN and <1khz RBW. You should (or rather should not) see 1/x type OOK spectrum. WARNING: set your reference level high enough or use an external attenuator so you will not damage your instrument!
• measure with oscilloscope, time scale SLOWER so you can see amplitude variations in the millisecond scale, like 1ms / div

Can you provide any logs? (output, errors, etc.)

No response