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

Discussion in 'Electronic Design' started by Jeepster, Mar 7, 2007.

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  1. Jeepster

    Jeepster Guest

    I'm trying to use a lock-in amplifier at a big distance from the
    signal source (laser communication between two buildings) and hence
    cannot use my reference input.

    When I punch in the frequency of the transmitter (signal source), the
    lock-in signal output keeps oscillating.

    Is this because:
    The frequency I punched in is not exact? or
    The signal phases are not synced? or
    The lock-in just cannot do this?

    Any help would be much appreciated,
    Jim
     
  2. Phil Hobbs

    Phil Hobbs Guest

    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.

    2. 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.

    3. 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
     
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