I recently bought a bladeRF board to demodulate an OFDM signal.
What i found was that I could not recive the OFDM constellation so I decided to investigate on the spurious tones to see if that brakes the orthogonality of the subcarriers.
I injected a single tone (a cw tone generated with a yaesuft-857 ham radio) and I found out that a huge peak appears beside the tone.
The frequency of this peak seems to be at three time the distance from the tone and my bladerf center frequency but in the opposite direction, i mean:
if i'm receiving at 868Mhz and my main tone is at 868.5Mhz (so, center frequency + 500kHz) the spurious tone is at 866.5Mhz (so, center frequency -1500kHz).
As far as i know, his cannot be due to iq imbalance because the tone should be at 867.5Mhz (also, I compensate for this in DSP), and neither to DC tones because this only affects the center frequency.
here is the screenshot from sdr-console https://imagebin.ca/v/3VZzR9PuCNbi
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