I need to make a synchronous demodulator using basic components (op-amps, etc...) and I was wondering if any one has currently made one. I want to avoid using an custom IC chip unless there is one available. I was looking into the Analog's AD630 and TI's INA143 (which they claim can be used as one) but I'm not exactly sure how to hook them up and I think it might be cheaper to actually just build one. Currently I have the following setup but I'm wondering if anyone has a more efficient design or setup: I have the input AM signal going into two amplifiers, one non-inverting and the other inverting which go into a voltage controlled switch. The switch oscillator between the two inputs (the outputs of the two amplifiers) based on the carrier frequency (20 kHz). One problem is that I have the output of the switch going into 3 low-pass filters. Two are just basic capacitor-resistor networks and the last one is a Butterworth 2nd order filter. The problem is that the Butterworth filter isn't enough for some reason. I need the two passive low-pass filters before it to get a nice smooth signal. Also even after the Butterworth filter the signal still contains a frequency content of 20kHz, which at this point isn't so bad but I can't figure out why this is happening. Any help is greatly appreciated.