I created a simple sketch in GNU Radio that multiples two signals. One is a sine and the other is cosine. I paste a screenshot of it and its results below.
I have heard that in DSP, we can do almost perfect signal processing. Hence, I assume phenomena such as LO leakage and doubly balanced mixers are not applicable when already digitized signals are multiplied such as in the presented case.
However, after multiplying the following two consine signals in GNU Radio, I expect to see the sum & difference frequencies; nothing else.
To my surprise, I am seeing a big peak at the sum frequency followed by a series of multiple peaks on the FFT. I'm curious to know what I am missing here.