I am basically making a conductometric sensor. The output is a differential voltage whose frequency is the same as that of my input (say 1KHz). Now this signal is in a lot of noise. I need to find the average value of the signal. I read in a journal publication that lock-in amplifiers can help in such situations but I could not design one. Wherever I look on the internet, I get the names of instruments like SR 830 etc. which I cannot buy. Please help if you can, I think I have been thorough with the problem statement this time.