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I have a project on the drawing board that requires information from some sensors to be passed from a rotating platform via IR link.

This is basically going to be inside the generator next to the windings and I was worried about RF interference and the changing magnetic fields so I thought that going with IR would be better than RF.

The distance between emitter and receiver should be no more than 20 cm (it could be low as 10cm, depending on the mechanics of the generator, but I'm being conservative).

The bandwidth requirements (0.8 Mbps average rate per data channel, with possibly 3-4 data channels) necessitate some method of modulation that allow more bits per symbol.

I was considering PAM but it seems that one issue is how to calibrate for the change in brightness as the transmitter moves toward and away from the receiver. It doesn't seem I can use a reference calibration pulse at the beginning of the transmission since the distance between transmitter and receiver distance vary the entire transmission.

How is this normally handled? Is it just not possible with this PAM?

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    \$\begingroup\$ What is your distance BW product? MHz-m? or Mbps-m? Then worst case path loss with motion, angle error. One needs to know SNR, BER and error detection/correction method for proper design \$\endgroup\$ Nov 11 '19 at 21:42
  • \$\begingroup\$ If you're doing 4-PAM (for example) you just need to use some code that (more or less) guarantees 0 and 3 occur with some minimum frequency, and AGC in your receiver. Probably some basic scrambling to generate a (nearly) uniform distribution of codes would work as well. This assuming the rotation rate is much slower than the data baud rate. \$\endgroup\$
    – The Photon
    Nov 11 '19 at 22:01
  • \$\begingroup\$ Some limits on the required bit rate, the rotation rate and the relative received power between the near and far positions would probably be helpful. \$\endgroup\$
    – The Photon
    Nov 11 '19 at 22:03
  • \$\begingroup\$ They have 16Mbps-m chips for line of sight but if you need an omni-directional emitter, then you better go with RF unless only only need 100kbps-m product \$\endgroup\$ Nov 11 '19 at 22:12
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    \$\begingroup\$ FSK, (basically FM binary), would be much more immune to noise and changes in signal amplitude. \$\endgroup\$
    – Nedd
    Nov 11 '19 at 23:08
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Maybe use a discrete ring of IR emitters and possibly this IR Rx, but you don't have enough system specs to choose anything yet.

https://www.digikey.ca/product-detail/en/vishay-semiconductor-opto-division/TFDU8108-TR3/751-1087-2-ND/1681221

But these are rapidly going or are obsolete.

A bigger picture of the actual measurement problem might be helpful.

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  • \$\begingroup\$ Yes, I've run into that component too. And yes, I was going to use a ring because I don't want to have to try and get a 7.2MHz baud rate system working. A ring would let me get away with 800kbps. The research lead basically wants to measure the current on the "DC winding" of a generator. \$\endgroup\$
    – DKNguyen
    Nov 11 '19 at 22:33
  • \$\begingroup\$ If Sharp doesn't make it , no one else does. Vishay bought the license and even they are giving up. Insufficient demand. AM on optical is bad news as the AGC is too slow. \$\endgroup\$ Nov 11 '19 at 22:44
  • \$\begingroup\$ Yeah, that's why I was going to use discrete LEDs and photodiodes The communication link only has to be one way. This application does not have to be eye-safe. So you're basically saying I should go with something else like PPM? or PWM? \$\endgroup\$
    – DKNguyen
    Nov 11 '19 at 22:45
  • \$\begingroup\$ Why can't you use PM-VHF with a longitudinal rotating wire antenna? \$\endgroup\$ Nov 11 '19 at 22:47
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    \$\begingroup\$ Optical is great if the AM Noise is lower f than your frame rate. Which is not your situation. But RF -PM is great when there is significant AM within the burst. \$\endgroup\$ Nov 11 '19 at 22:50
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Forget about PAM. 800 kbps is easily handled with simple binary modulation, using multiple emitters and/or multiple detectors.

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