If I have a bandwidth of frequencies from 200 to 500 Hz made from cosine waves and I mix this with a oscillator at 400 Hz which is also a cosine wave, the mathematics of conversions show that I will get a signal at 600-900 Hz and -100 to 200 Hz. Does this negative frequency from the conversion math physically exist? I dont think it does - so where has some of the energy from -100 to 0 gone?
I understand the negative frequency fourier transform concept, but the above different and is tricking me.
Thanks for you help in advance.