Thanks everyone for the responses.
Rolf, I do apologize for the incorrect technical term. Yes, its 5 cycles. The input signal doesn't contain any DC level. The signal is the output from the collector of a 2N3904 small-signal amplifier circuit - not sure of the impedance.
MRCecil, could you provide links for the circuit? Would I require feedback (hysteresis)? I tried an LM311 (without hysteresis and single 5V supply) but got a lot of noise at the output and no squarewave. I have attached the implemented circuit. Simulated okay in PSpice Orcad and worked well with input from a signal generator.
Appreciate your suggestions. The input signal is a burst of 5 cycles of ~2Vp-p, 200KHz sinewaves repeated every 5msec.
del