HackRF TX with GNURadio: Why does my spec an look like this?

Question

I have been exploring two things: software defined radios and the frequency domain. Most of my professional career I have worked with high bandwidth oscilloscopes in signal integrity and high speed digital designs. So while I have run my fair share of FFTs and looked at jitter spectrums, my time in the frequency domain has been limited.

I'm using a HackRF direct feed into a R&S FPC1500 spectrum analyzer. GNURadio Companion drives the HackRF with the following flowgraph.

With such a setup, I am not certain I understand what effect the signal source has on the sink. Does it modulate it somehow?

I expected a stable 433.2 MHz carrier on my spectrum analyzer. The green trace is a max hold and the yellow trace is averaged (10).

I don't understand why the signal "jumps up" every once in a while. Is it because of the "signal source" block in my flowgraph? Is it because I set something wrong on my spec an? Is it because the HackRF is doing something weird?

Any thoughts?

Gough Lui · Answer

I suspect you have discontinuities in your signal from the hackRF. Check if you have buffer underflows or overflows - some SDRs have LEDs to indicate this, otherwise you might need to look at your signal in the time domain. It's similar to a sound card when the sample rate isn't quite right - you get a crackle now and then (more often if you're underflowing severely - e.g. in the case of more complex GNURadio flowgraphs or if you have mismatched rates somewhere). - Gough EDIT: Why is your source 32k and the sink running at 64k? Make them both equal would be a good start, but perhaps not necessarily enough as there will still be offset from the PC clock to the SDR's clock to worry about.

HackRF TX with GNURadio: Why does my spec an look like this?

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