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DS90UB949A-Q1EVM: How to connect SMA connectors

Prodigy 175 points

Replies: 24

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Part Number: DS90UB949A-Q1EVM

Hi, I have found instructions on how to connect the SMA connectors in this forum for the DS90UB949-Q1EVM, but not for the "949A" variant. It appears on the back-side of the board, next to the P1 FAKRA connector, that there are test-points which could be soldered to with tiny jumper wires. The test-points appear connected to the SMA connectors, so I could use jumper wires to connect them to the corresponding pins on the P1 connector. Is this the intended way of connecting the SMAs? It seems a bit messy and would definitely cause an impedance discontinuity.

On a related note, are there instructions somewhere how to set-up the SerDes connection using unbalanced (rather than differential) coax connections?

  • Hello Alex,

    Yes, your explanation is correct. To connect the SMAs please solder 0 ohm jumper resistors, size 0402 or 0603 on the 4 signals coming from the HSD connector to the SMAs. There will be a small stub going to the HSD connector, but we have tested this connection up to the max supported speed of 949A between EVMs successfully. 

    For setting up coax for 949A, please see datasheet section 8.2.1

    Best Regards,

    Casey 

  • In reply to Casey McCrea:

    Hi Casey,

    Thanks for your reply. I don't believe that will work for the 949A as it does for the 949, as there is no place to put the resistors, rather just tiny test points next to the HSD connector. In my post I was suggesting using very thin wire, maybe 30 gauge or thinner, to jump from the test points to the HSD connector.

    Please see below for an image of the bottom copper layer artwork, showing the four transmission lines going from the side of the board/SMA connectors to test points next to the HSD connector.

    Best,

    Alex

  • In reply to Alex Hegyi:

    Hello Alex,

    The picture didn't come through but I know what you are talking about. It is best to avoid using wires. What we have done in the past is either to bridge solder directly from the pin to the test pad (since they are quite close), or populate a 0 ohm 0402 resistor between the test pad and the connector pin.

    Best Regards,

    Casey

  • In reply to Casey McCrea:

    Hi Casey, I ended up using wires about the size of 0402 resistors to form the bridge, so I would assume that's okay. And, I am able to form a reliable connection to the 948 deserializer, up to a point. I am using a custom EDID to set the timing of the HDMI signal that I plug in, and it works up to a pixel clock of 110 MHz. However, somewhere between 110 MHz and 120 MHz, the link to the deserializer becomes inactive-- the "FPD3 Status" section of the HDMI tab in ALP goes from "Link Ready" and "Link Active" both being true to both being false.

    Any idea what's going on here? I need an overall pixel clock of 148.5 MHz, well within the range on the datasheet. I am using Dual FPD3 link mode as well.

    Also, toggling the "BISTEN" switch on the deserializer never gets the PASS LED to light, which I thought it was supposed to. So maybe I don't have a reliable connection, even though ALP says I do? After running for several minutes I only have a couple dozen CRC Errors.

    Best,

    Alex

  • In reply to Alex Hegyi:

    Hello Alex,

    It doesn't really sound like the connection is reliable if you are getting CRC errors and if the link only works up to a certain PCLK point. I am not so sure about the small wires. I highly recommend just bridging the solder directly over or using proper 0 ohm shunts. Are you using single link or dual link between 949A and 940? 

    Best Regards,

    Casey 

  • In reply to Casey McCrea:

    I'm using dual link. The wires themselves are smaller than the solder bridges--I can try to remove them but the bridges will be about the same size. Here is what it looks like now:

  • In reply to Alex Hegyi:

    Hello Alex,

    I still believe it can be done a little bit more cleanly with 0 ohm jumpers if possible. Like I said, we have done this before successfully up to > 170MHz. 

    Is the same thing being done on the 948 EVM?

    What type of cable and length are you using? Can you share a picture of the full test setup?

    Best Regards,

    Casey 

  • In reply to Casey McCrea:

    Hi Casey,

    Indeed, I was able to clean it up by using direct solder bridges, and now I can get to 148.5 MHz. I also cleaned up as much residual flux as I could between the solder joints. I've attached a pic below of the updated setup for reference. I am using 3" semi-rigid coax connections. 

    However, I'm still not sure what the correct operating mode should be. Referencing section 8.2.1 of the 949A datasheet it appears that the COAX setting is indeed for single-ended connections. Does that mean with my current set-up, I should have the EVMs operating in STP mode, because I am using the same differential connections that would be used for STP?

    Thanks,

    Alex

  • In reply to Alex Hegyi:

    Hello Alex,

    Good news! For this configuration you are using differential (STP). 

    Best Regards,

    Casey 

  • In reply to Casey McCrea:

    Hi Casey,

    I am now switching the link over to single-ended and having some issues. I am doing this by setting Switch 3 of MODE_SEL1 to HI on the serializer, and Switch 4 of MODE_SEL1 to HI on the deserializer. I have not yet changed the physical/wiring configuration from what you see in the post above, and I am able to get the LOCK LED to stay solid green at my pixel clock of 148.5 MHz.

    However, if I remove the coax connections for the negative half of each differential pair of the link, and place 50 ohm SMA terminations on the newly disconnected SMA connectors, I cannot get a stable link at 148.5 MHz (or even at lower frequencies). Is the single-ended mode designed to operate at such high frequencies? Or do you have any other ideas about what might be happening? In this configuration, the LOCK LED looks like it is pulsing very fast.

    Best,

    Alex