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I have an application which requires sending PAL video signal & UART infomation over a 50R coaxial cable without disrupting the PAL protocol while doing it, the total length of the cable is around 200 meters or 656 ft.

I have already decided how to multiplex this signal over the PAL, my issue is that I will need to send signal from both ends of the cable without dirupting the controlled impendance and typical methods for cable termination are not suitable for me.

What I come around, is a design involving drivers with a shutdown pin to let me "disable" the 50R. Since the receivers have "always-on" termination I wanted to control them too so I added some BJTs (still don't know if it's a good design or MOS needed) controlled by some GPIO from a microprocessor which can "translate" the line number, if it's HSYNC or VSYNC and so on.

In case somebody wants to take a deeper look:

  • Driver -> LMH6722(w/SH) - Datasheet
  • Receiver -> same just configured as a Schmitt Trigger for level translation

Can somebody recommend me a better termination technique which I could use ? Also I cannot use two coaxial cables for each line, only half-duplex.

This is just a design, not tested yet, waiting for a second opinion. :)

Final Design

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    \$\begingroup\$ Ah, would've been easier if PAL was at a couple hundred MHz, because a simple duplexer would have done the job then (i.e. a high- and a low-pass filter on each end). \$\endgroup\$ – Marcus Müller Jun 14 at 11:41
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    \$\begingroup\$ What about using your STM32's clock output pin to generate a, let's say, 48 MHz carrier wave (pin->C-R-C bandpass filter->buffer) to multiply that with your UART signal, high-pass to keep the repetitions out of your PAL's bandwidth, add to PAL signal; on the UART-receiving end, add a high-pass filter, do envelope detection (as in good old AM radio receivers)? \$\endgroup\$ – Marcus Müller Jun 14 at 11:46
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    \$\begingroup\$ So, all you need to do now is, on the UART side of things, set the TX multiplier's output to high impedance. \$\endgroup\$ – Marcus Müller Jun 14 at 11:54
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    \$\begingroup\$ That IC is designed for impedances higher than 50 ohms. When I once multiplexed baseband 2-way audio , video and 2 way telemetry, I parsed the BW and used FDM with FM for data and RF for video. \$\endgroup\$ – Tony Stewart EE75 Jun 14 at 12:45
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    \$\begingroup\$ You want 75 ohms for video and notice the BW precompensation needed for cables >50 m . if you can extend that design for 200m maybe ok. \$\endgroup\$ – Tony Stewart EE75 Jun 14 at 13:17
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Seems to me that you are going to a lot of bother when this is likely to work for you: -

enter image description here

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  • \$\begingroup\$ Those drivers are having a shutdown pin, so instead of turning off the driver I should driven with 0V to keep my output at 0V to create a "ground" for the termination resistor , that's what you mean ? \$\endgroup\$ – Stefan Merfu Jun 14 at 12:38
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    \$\begingroup\$ Yes, you don't bother turning the drivers off - just leave one of them idling but active @StefanMerfu \$\endgroup\$ – Andy aka Jun 14 at 12:44
  • \$\begingroup\$ Clever idea I would say. Thank you for your suggestion ! \$\endgroup\$ – Stefan Merfu Jun 14 at 12:48
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    \$\begingroup\$ Please feel free to upvote or accept as appropriate. Simplicity is always a good policy. \$\endgroup\$ – Andy aka Jun 14 at 12:48
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    \$\begingroup\$ No, even with coax, you should use a differential receiver to avoid noise problems. A single ended amplifier is a fallacy over a few metres especially in a noisy EMI environment. Coax necessitates a differential amplifier to get best performance. \$\endgroup\$ – Andy aka Jun 14 at 13:12

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