Hi, Can someone recommend a switch for the attached circuit. I want it to be switched on and off by a square wave reference signal so at the output I get a full-wave rectified sine wave.

 

Have a look here:

http://search.digikey.com/us/en/cat...itches-multiplexers-decoders/2556595?k=switch

A CD4066 would probably do it.

Is there a reason why you could not use a precision full wave rectifier with 2 Opamps?

 

Thanks for the suggestion mate, I just took that out of a book to be honest. I can't figure out the concept of a lock-in amplifier because apparently the AC source for the bridge must also be the synchronised drive signal for the switch.