# Detector question on lock-in circuits

Started by February 2, 2014
```I see demodulator circuits that to me look like a full wave rectifier, they take an AC signal and flip its negative part above the line. This is the last stage of a lock-in before the integration-cleanup phase.

In situations where you control the light source, I have thought of using square wave pulses, with measurements taken during 'on' and 'off' periods.

It makes sense to me to subtract the 'dark' signals from the 'on' signals, and collect the 'on' signals for DSP.

But some lock-in circuits describe a switching network that essentially does a full wave rectification of an AC signal.

Can anyone explain the difference here?

JB

```
```On Sunday, 2 February 2014 21:59:22 UTC+11, haitic...@gmail.com  wrote:
> I see demodulator circuits that to me look like a full wave rectifier, they take an AC signal and flip its negative part above the line. This is the last stage of a lock-in before the integration-cleanup phase.
>
> In situations where you control the light source, I have thought of using square wave pulses, with measurements taken during 'on' and 'off' periods.
>
> It makes sense to me to subtract the 'dark' signals from the 'on' signals, and collect the 'on' signals for DSP.

That's what you have just described as "flipping it's negative part above the line". Multiplying a negative-going signal by -1 (that is, subtracting it rather than adding it) does "flip it above the line".

> But some lock-in circuits describe a switching network that essentially does a full wave rectification of an AC signal.

It's only full-wave rectification if the AC signal is in phase with modulating signal. Think about what it would look like if they were in quadrature.

--
Bill Sloman, Sydney
```