Hi, I am having some slight problems with signal mixing and I thought you guys might be able to help. My aim is to mix two frequencies together and then filter out the difference, (f1-f2) using a low pass RC filter. The circuit is comprised of two Colpitts oscillators, one of which oscillates at about 400khz and the other at 405khz, so I am expecting an output of about 5khz. Both these signals are fed into a mixer (the circuit of which can be found here http://electronicdesign.com/site-files/electronicdesign.com/files/archive/electronicdesign.com/files/29/11935/figure_01.gif) The problem I'm having is that the output is a slightly distorted sine wave (I have attached an image of the waveform). I have played around with the RC component values and the attached waveform shows one of the least pronounced 'dips' in the sine wave I could get. I have tested the circuit with waves from a function generator and it worked nicely - I used different test frequencies for that though and should have obviously tried the actual ones. I have a few theories as to why: I thought it could be a result of the since waves from the oscillators not being 'perfect' sine waves and this has resulted in some distortion? I am not sure how large an affect this would have because while the waves are not perfect they are pretty good. I also thought maybe the highish frequencies compared to the gap I am trying to filter out may have some influence? I hope some of you can shine a light on this for me.