Hi Guys! Can someone help me with this RF question please:

I am using a redpitaya with two OUT and two IN channels configured with this setup: http://pavel-demin.github.io/red-pitaya-notes/sdr-transceiver/ and I am doing a simple experiment of producing a signal on OUT1 and receiving it in IN1 and IN2 as depicted in the figures below.

OUT1 should produce a modulated signal by using a center frequency of fc=50kHz from a base band complex signal of f=5kHz (single peak on the right side of the spectrum). ‘F’ is depicted as “TX before modulation”.

IN2 is receiving the modulated signal and demodulates it with a null center frequency fc=0Hz. Depicted in “RX modulated signal”

IN1 is receiving the modulated signal and demodulates it with a center frequency of fc=50kHz, which produces the signal “RX after demodulation”.

My question is regarding “RX after demodulation”. I was hoping to see a signal that would look like the transmitted one, with a single peak on the right side of the spectrum at f=5kHz, but instead I get a few more peaks on the negative side of the spectrum that I can’t explain (f1=-55kHz and f2=-105kHz). Seems like f1 is equal to -fc and f2 is equal to -2 x fc + f.

I suspect that the modulation being used here is SSB(?)

Can someone help me understand this result? Is this result correct and my expectation wrong?


Block diagram:
https://pasteboard.co/HqB3IL1.png

Base-band signal (complex signal f=5kHz):
https://pasteboard.co/HqAZ4sL.png

Modulated signal (fc=0kHz):
https://pasteboard.co/HqAYQG1.png

Demodulated signal (fc=50kHz):
https://pasteboard.co/HqAYYD4.png