Internal Harmonic?

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Internal Harmonic?

Post by mando238 » Mon Oct 24, 2016 9:55 am

First things first: though I’ve been experimenting with SDR (and lurking on these forums) for a while now but I’m still pretty much a noob – so I apologize if this question has an obvious answer. I tried to search the forum but couldn’t find any thread related to the problem.

I have a Balderf operating in full duplex that transmits a tone at a constant frequency while receiving on a different frequency (separated by at least 800Mhz). The transmitted signal is passed through multiple low pass filters before the antenna to crush any harmonics and the receive end has high pass filters to strongly attenuate the transmitted signal (mainly for device protection/avoiding interference). This is all being controlled by GNU radio (osmocom source/sink).

Initial issue:
Here is where things get interesting: when I tuned the RX end a few Mhz away from 2*Ft a very weak signal appears at 2*Ft, which I was guessing was the 2nd harmonic of the transmitted signal. However running through the expected performance of the filters and TX amplifier, I shouldn’t really have been able to detect any harmonic. Nonetheless I thought perhaps I didn’t do the math correct or the strong transmit signal (though it was severely attenuated) could be causing some sort of harmonic at the RX end.

Troubleshooting (where things get interesting):
So I turned off the TX amplifier – but there was no change in the power of the received harmonic…
Not expecting that, I took the TX end, and rather than routing it to the antenna, terminated it with an approximate 50 Ohm terminator. At this point nothing at all should be being transmitted into the air, yet still the RX end would see a harmonic at 2*Ft (again with the same power). Finally I disabled the transmit (osmocom sink) block, and now the harmonic vanished.

This weak harmonic of the transmitted signal on the RX end doesn’t seem to come from any received signal but seems to be internal within the Bladerf. So given that would anyone be able to explain where it comes from/what is generating it? Is there any way to mitigate it?

If any more details are needed or there’s something else I should try to verify this please let me know.

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