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LMH0394: LMH0394/LMH0302 return loss after adding switch

Luke Sun
Intellectual 1830 points
Part Number: LMH0394
Other Parts Discussed in Thread: TS3USB221, LMH0302, LMH0387, LMH0397, DS25CP102

Hi team,

My customer is using LMH0394 in RX and LMH0302 in TX for SDI application. But now they want to put TS3USB221 before LMH0394/LMH0302 as a TX/RX switch.

The quesiton will be how to evaluate the input/output return loss after adding this switch into the chain. Do you have any idea?

Thank you!

  • 0
    TI__Mastermind 29655 points

    Hi Luke,

    The TS3USB221 is a USB 2.0 switch that operates up to 480 Mbps. This is not enough to be used with our 3G-SDI devices that operate up to 2.97 Gbps. Are they intending to use the device like so?

    I have some thoughts that we should clarify with your customer:

    1. If a switch must be used, we would recommend selecting the DS25CP102, which is a 2x2 crosspoint switch rated up to 3.125 Gbps operation. Alternatively, we would also suggest considering a bidirectional I/O device such as the LMH0397 or LMH0387 that would essentially serve the purpose of the LMH0394, LMH0302, and switch in the above situation.

    2. Measuring return loss according to SMPTE specifications is a requirement for the 75-ohm input or output. Adding a switch on the 100-ohm side will not affect the measurement.

    Regards,

    Michael

  • 0 in reply to Michael Lu (Santa Clara)
    TI__Intellectual 1830 points

    Michael,

    Really thanks for your reply.

    They use TS3USB221 because it has a 1.1GHz Bandwidth. And it should support their HD-SDI application(1.4Gbps)

    And their block diagram will be like this. So TS3USB221 should have some impact on the return loss spec.

    Do we have any way to evaluate such impact?

  • 0 in reply to Luke Sun
    TI__Mastermind 29655 points
    Hi Luke,

    This seems like a strange implementation, since SDI applications often require 75-ohm BNCs and 75-ohm cables. Return loss specifications are also created with the assumption that the signal path to the BNC has 75-ohm impedance. I would like to question this application further, since this implementation is not typical.

    Nevertheless, if you want to measure the impact of return loss, I would simply just enable only one channel of the TS3USB221 at a time, depending on whether the cable driver or the cable EQ is enabled. Therefore, there would be two return loss measurements:
    1. RL when signal path goes to the LMH0394 from the BNC.
    2. RL when signal path goes from LMH0302 to the BNC.

    Again, if the customer has NOT yet begun component selection, I highly recommend they take a look at the LMH0397, which would allow you to accomplish the same concept as above, but with only a single chip instead of 3 chips (TS3USB221, LMH0302, LMH0394) and two impedance matching networks. Moreover, the RL network in the LMH0397 is already integrated.

    Thanks,

    Michael