By modifying the real sensor signal with a quite low switching frequency (which is actually like square signal) and using two lock-in amplifier we can resolve quite low signals shadowed (and /or hidden inside) with the very high level of electrical noise. Unfortunately, I couldnt find the circuit diagram showing the connections of lock-in amplifiers. I remember that we can do this without need of a mixer.
I'm not sure you can do it without mixer. Attached you can find the block diagram illustrating a multi-reference lock-in amplifier using external references.
Dear Salvatore
Could you please give me information for the original article of this circuit...
take a look to the patent entitled:
Double reference lock-in detector
US 20050250465 A1
Best!
There is an old article, on this topic, where they implemented this double lockin technique for weak signals detection from plasma experiment.
http://dusty.physics.uiowa.edu/~goree/papers/goree_RSI_85_double_lockin.pdf