TX Modulation Spurs

[kk6rum]

I'm working on a weak signal station based on the BladeRF and noticed broadband modulation spurs with any type of modulation I've applied. In the simplest case, I'm using osmocom_siggen to modulate a sine wave at 3kHz offset and noticed spurs every 160kHz above and below the desired signal on a spectrum analyzer. The first spur is 35 dB below the signal level. I've corrected for DC and IQ via the spectrum analyzer and changed the TX gains with no relative effect. Thanks in advance!

[jynik]

Could you post screenshots of:

• osmocom_siggen in the configuration you're using
• The spur observations

I am interested in seeing the samplerate you're operating the device at. The fact that you noted 160k is interesting to me because that's the lowest samplerate the bladeRF can run at.

However, the lowest LPF setting on the LMS6002D is 1.5 MHz, which implies that if you're not using an external LPF, the lowest samplerate you can use without seeing aliases is 1.5 MHz (I'd recommend using 2-3 MHz as a minimum to account for filter roll-off).

Thus, I am somewhat suspecting that you're seeing aliases. Your screenshots will confirm/deny this hypothesis.

[kk6rum]

The sample rate was set at 160k as a default and I did not set the command line argument to change it for my observation. Changing the sample rate to 2M removed the aliases...thanks!

[jynik]

Excellent - glad it was an easy fix!