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Original question:

TMUXHS4412: BIAS VOLTAGE IN TWO CONNECTED DEMUX

TMUXHS4412: FPD-Link IV Signal Lock and Video Display Issues with Cascaded TMUXHS4412

Samuele Lorenzoni
Prodigy 40 points
Part Number: TMUXHS4412

Tool/software:

Hello Texas Instruments,

I am writing to you again regarding the use of two TMUXHS4412 multiplexers configured in cascade for FPD-Link IV signal routing.

Previously, you confirmed the feasibility of this configuration. However, I am experiencing operational issues with my PCB implementation. Specifically, when an FPD-Link IV signal is transmitted from a TX device through my board and received by an RX device (also equipped with an FPD-Link IV deserializer), the latter is unable to stably lock the signal, and consequently, there is no video display.

I have also tried with the FPD-Link III standard, but I still see nothing and do not have a stable lock of the video signal. It is important to note that when using Gigabit Ethernet, the multiplexer functions correctly.

It is important to emphasize that a bias tee was planned for bias voltage supply.

I have already attempted to resolve the issue by removing all EMI filters present at both the input and output of the signal path, without success.

In this regard, I would like to ask you some specific questions:

  1. Is it possible that the multiplexer, despite being bidirectional, cannot handle the forward channel and backchannel simultaneously with FPD-Link IV signals?

  2. Are we certain that this device (TMUXHS4412) can be used with the FPD-Link IV standard?

Attached below are the images of the electrical schematic for the multiplexers and the bias tee.

I would be grateful if you could provide support in diagnosing this malfunction.

Thank you in advance for your assistance.

  • 0
    TI__Genius 17665 points

    Hello Samuele,

    Today is a TI U.S holiday.
    Please expect a response next week.

    Thanks,

    Nir 

  • 0 in reply to Nir Gilgur
    TI__Genius 17665 points

    Hello Samuele,

    Apologize for the late response.

    Can you provide the link to the thread you are mentioning?

    What do you mean by "stably lock the signal" is the FPD-Link signal not being passed through the mux? Can you probe the inputs and outputs of the mux?
    I find it odd that it works for one protocol, but not the other. The TMUXHS4412 is a high speed passive mux, so it doesn't really "care" what is being passed through it as long as it doesn't exceed its limits.  

    You mentioned " the multiplexer functions correctly", what exactly does that mean? The signal passes through it and you can see it on the output without much distortion? 

    1. The TMUX4412 is bidirectional, if the forward and backwards signals are being passed on sperate channels of the mux, then there shouldn't be any issues.

    2. As long as the signals you are passing through the mux are within its specifications then it should be fine. In other words as long as the FPD-Link IV signals you are trying to pass don't exceed the bandwidth and voltage levels of the mux, then there will be no issues.

    Note, cascading these multiplexers will degrade their bandwidth. For more information please see app note.   

    Thanks,

    Nir