0
\$\begingroup\$

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

\$\endgroup\$
19
  • 1
    \$\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
    Commented Jun 14, 2021 at 11:41
  • 1
    \$\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
    Commented Jun 14, 2021 at 11:46
  • 1
    \$\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
    Commented Jun 14, 2021 at 11:54
  • 1
    \$\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\$
    – D.A.S.
    Commented Jun 14, 2021 at 12:45
  • 1
    \$\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\$
    – D.A.S.
    Commented Jun 14, 2021 at 13:17

1 Answer 1

Reset to default
1
\$\begingroup\$

Seems to me that you are going to a lot of bother when this is likely to work for you: -

enter image description here

\$\endgroup\$
12
  • \$\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
    Commented Jun 14, 2021 at 12:38
  • 1
    \$\begingroup\$ Yes, you don't bother turning the drivers off - just leave one of them idling but active @StefanMerfu \$\endgroup\$
    – Andy aka
    Commented Jun 14, 2021 at 12:44
  • \$\begingroup\$ Clever idea I would say. Thank you for your suggestion ! \$\endgroup\$
    – Stefan Merfu
    Commented Jun 14, 2021 at 12:48
  • 1
    \$\begingroup\$ Please feel free to upvote or accept as appropriate. Simplicity is always a good policy. \$\endgroup\$
    – Andy aka
    Commented Jun 14, 2021 at 12:48
  • 1
    \$\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
    Commented Jun 14, 2021 at 13:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.