Hi ,
I designed the signal processing circuit and started testing. I would like to get some help on this. The input signal is 16mV AC @ 50Hz with lots of noise like 500-700KHz. My circuit contains only one amplifier with an effective gain of 127, a buffer of unity gain and an LPF. The output of the signal is a sine wave at 50Hz with a very very low noise. But the amplitude is changing from 1.6 V o 2 V periodically . It is like the 50Hz sine wave is overlapped with another sine wave according to which the peak to peak of normal 50Hz signal is varying from 1.6V to 2 V. I don't know what is the name of this particular transient response. Is it because of the common board assembly or the circuit itself has a problem? I don't know how to post a pdf file here otherwise I would have attached my circuit diagram here. But this problem was not there when we tested this in our lab set up which has less noise. The test environment was a generator room where two generators of 1240Kwatts are running.

Could any one of you pls help me to find out the root cause of this problem and how can I resolve this

Thanks in advance
Manju