Mirrored spectrum around tuning freq

GNU Radio
1

Hi all,

why when I use:
uhd::tune_request_t tr(tuningFreq);
USRP_DEVICE->set_rx_freq(tr);

I get a spectrum that is “mirrored” around the tuninFreq ?
See: http://picpaste.com/sp-zJquS0qK.png
with tuningFreq = 149.8 MHz, input signal at F = 150 MHz (from signal
generator)

If I use a LO offset different from zero the mirror effect disappear.

Why ?

Thanks,
Luca

2

My first thought was “IQ imbalance”, but it’s pretty strong. Which USRP
and daughterboard do you have?

M

3

Could be intermodulation due to driving the RX amplifier into
nonlinearity;
Luca, what gain are you using? Can you further reduce it? Can you reduce
the amplitude of your test signal?

Best regards,
Marcus