EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: ezalys on March 28, 2022, 06:13:17 pm
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The signal recovery 7124 is a lock-in amplifier with two separate boxes: one is an analog frontend and the other does DSP, handles the user interface, all these things. They're coupled with a handful of fiber optic links.
Crucially there's no clocks in the analog frontend box -- they make a pretty big deal of this. There's therefore no modulation or encoding. There's a calibration screen for calibrating the gain of the fiber chain for goo on the fiber ends or attenuation due to bending, so I'm inclined to believe they're just transmitting pure analog signals. This is certainly what the block diagram suggests. I popped one open and saw a few OPF322A and OPF472 transmitters and receivers in the main section -- so nothing terribly special.
If you look at the manual, it quotes a truly incredible -90 dB THD. I'm curious how one goes about linearizing and temperature stabilizing a pair of these. There's an analog subsection box, but this is enclosed and I didn't have the cojones to take apart the rest of the instrument to get at it. Maybe there's a splitter and a receiver in there and the transmitter runs in a feedback loop -- I don't know.
Has anyone tried to transmit analog signals over a fiber link with high linearity and no modulation?
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It sounds a bit off as the OPF322a doesn't even have a power feedback detector to keep the output power constant.
There is a general article: https://www.digikey.com.au/en/articles/linear-optical-isolation-for-safe-sensor-operation (https://www.digikey.com.au/en/articles/linear-optical-isolation-for-safe-sensor-operation)
I might just say that THD is purely a measure of linearity of the transmitter/receiver pair, I would be more interested in gain stability, which is what will vary most with temperature.