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LMH1219: Cable Length Problem

Patrick Anderson
Prodigy 40 points

Part Number: LMH1219
Other Parts Discussed in Thread: LMH1228, , USB2ANY

Hello

I designed a 4k-SDI dual 1 to 4 distributor using LMH1219 as equalizer and LMH1228 as driver as you see in the block diagram.

This is my EQ and DR schematic  

In my design I have to use back panel and so I don't have on board BNC connectors.

This is my back panel's picture

You can see my EQ circuit on PCB

And DR circuit:

I tried to have space between EQs and DRs and power section

Also on the back panel I draw a good GND plane between equalizers and drivers

With all this considerations When I enabled all drivers to have output, I can not get more than 25m with Belden-1694 on equalizers while datasheet says it can work with 75m. when I turn off drivers, equalizers will lock more than 60m(I can see on Lock LED). I don't know why drivers effect on equalizer signal and what should I do to prevent this problem. Do you have any advice?

If there is a good simulation tools that I can use to understand where is the problem, please introduce me.

Thanks.

  • 0
    TI__Genius 9209 points

    Hi Patrick,

    Thank you for sending this detailed information. I want to confirm a few things

    • Is OUT_CTRL float for all devices?

    • Instead of simulation for now, try turning off OUT0+ OUT0- on the LMH1228 using register or pin control. Once the 100 ohm SMA outputs are disabled, please check if you get the expected cable reach. We suggest this because the decrease in cable reach may be from crosstalk. Please turn off the other outputs individually and check if you see improvement as well. Let us know which output is causing the problem.

    Let me know if you have any more questions or would like me to elaborate on anything.

    Best Regards,

    Nick

  • 0 in reply to Nick Peabody
    Prodigy 40 points

    Hi Nick

    Thanks for your consideration.

    • OUT_CTRL is float for all drivers and equalizers.
    • How SigCon can help? As I know the best answer occurs with the default registers and with default registers I have problem too.
    • With default registers, OUT0+/- and SDI_OUT2+/- are disabled. In this situation my cable length is better and it is about 25m. When I enabled all outputs of drivers my cable length decreased to 15m.

    Also I simulated my PCB with Ansys SIwave for SYZ and you can see the result in the following image. 

    This is SYZ parameters between 4 output SDI from 2x LMH1228 and 2 input SDI from 2x LMH1219

    With my test results I know that I have problem because of crosstalk and if I could increase space between equalizers and drivers it will be better and I can get about 45m but in this project I can not increase the space. So I want to know is there any solution for me?

    I need to mention that when I turn off driver's power and read lock of equalizer, It can lock more than 70m but I don't know how drivers effect on equalizers.

  • 0 in reply to Patrick Anderson
    TI__Genius 9209 points

    Hi Patrick,

    Thanks again for the information.

    Patrick Anderson said:
    How SigCon can help? As I know the best answer occurs with the default registers and with default registers I have problem too.
    • SigCon is going to be your best bet for visualizing the impact of the crosstalk on each device. SigCon is able to interact with the device's registers to display the signal HEO (UI) and Veo (mV).
    Patrick Anderson said:
    I can not increase the space. So I want to know is there any solution for me?
    • If the board has too much crosstalk there is little to be done from a device perspective to improve the situation. 

    Set the settings with 0 EQ and 0 deemphasis to see if that helps. Otherwise, the changes that you will make will have to be with the layout.

    Best Regards,

    Nick