Lock-in Amplifier at a distance from the signal source (ref)

A lock-in is precisely equivalent to a very narrowband filter followed by a phase-sensitive detector. Your trouble is that the modulation carrier is getting out of phase with the lock-in reference.

You can do several things. From simplest to hardest:

1. If you have a two-phase lock-in, just switch to "amplitude/phase" mode and use the amplitude output. This is exactly equivalent to a narrowband filter followed by an AM detector, which is probably what you want.

1. Amplify the daylights out of your received signal and put it into both the signal and reference inputs of the lock-in. If your modulation is fast enough, and the lock-in doesn't use a phase-frequency detector, you may get it to lock to the carrier.

2. Build a phase-locked loop with a very narrow loop bandwidth (much slower than the modulation of the carrier), lock it to the received carrier, and use that for the lock-in phase reference.

Cheers,

Phil Hobbs