How to perform downconversion using a DSP I read that to perform downconversion, first, I need to generate a sine wave with the same phase and frequency as the transmitter's and then multiply the signal received by the generated sine wave. I don't know how to do that, I think I have a way to generate the synchronized sine wave, but  I don't know how to perform the multiplication. 

I read that I can use a mixer or a nonlinear circuit to perform the multiplication, but I have a DSP and it doesn't have a frequency mixer available. Can I do it digitally ? The carrier has 125kHz and the modulating signal has 20kHz. I can use a sampling rate up to 1MHz and the DSP can reach up to 70 MIPS.

I'm sorry if my explanation is incomplete, I am really studyng the problem, so if you need more information you can ask and I will try to make the question more complete.

With my best regards,

Daniel.
Yes, you can do the operation digitally; this is a very common operation in modern software-defined radio systems. The implementation is straightforward: you just implement the same steps that you would see in a hardware implementation. First, multiply by a sinusoid at the carrier frequency, then apply a lowpass filter to remove any images.