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Original question:

DS90UB954-Q1: Communication issue - Margin Analysis show low margin

DS90UB954-Q1: Problems using STP instead of coaxial cable

Alessandro Cominelli
Intellectual 375 points
Part Number: DS90UB954-Q1

Hi,

I am working on a project involving the couple deserialize DS90UB954 and serializer DS90UB953.
We are working on two different strategies at the same time to discover which is the best one for our application: on one hand we developed a board based on coaxial connection and we had some layout problems we are trying to solve thanks to your suggestion (DS90UB954-Q1: Communication issue - Margin Analysis show low margin - Interface forum - Interface - TI E2E support forums); on the other hand, we are trying a solution based on STP connection.

We found few matherial on datasheet about STP, so I have a few questions:

- concerning layout the only difference between stp and coaxial is that there are differential traces with 100 Ohm impedance, instead of a single-ended line with 50Ohm impedance. Is it correct? All the suggestions you gave me here DS90UB954-Q1: Communication issue - Margin Analysis show low margin - Interface forum - Interface - TI E2E support forums are still valid?

- we decided to use a standard Ethernet cable (4 wire pairs) to connect deserializer and serializer. In particular we want to use a differential pair to carry the signal and the remaining 3 pairs to carry power. in this way we can avoid any PoC-like filter. Do you think it is a good practice or do you expect any problem?

- Different categories of Ethernet cables exist with different shielding layout and different bandwidth. We already developed our custom ser and deser board with ethernet connector and we already tested the differences between a category 5 cable and a category 6 wire: the Margina Analysis test clearly shows a big improvement using the category 6 cable, but unfortunately the margin is still low. Do you have exxperience with standard ethernet cables to connect ser and deser? Which is the minimum required bandwidth and shilding? In other words, which cable category do you suggest to guarantee a proper transmission?

Thank you in advance.

Best regards,

Alessandro

  • 0
    TI__Mastermind 24095 points

    Hello Alessandro,

    Is there a reason the PoC filter is avoided? Referring to the schematic review, I see a comment was made that the PoC solution wasn't exactly as shared in the datasheet. The concern with the cable selection is that all Channel Specificiations and PoC Noise requirements are met. 

    Best,

    Zoe

  • 0 in reply to Zoe Bison
    Intellectual 375 points

    Hi Zoe,

    Concerning the schematic review I received in the related question, it is about a board we developed for a specific application where we decided to use a coaxial cable to connect deser and ser. In that application the current required at ser side is quite low (about 100mA), so we designed the PoC filter by following the design rules suggested in the deserialiser datasheet.

    Now, we have another application where we want to use deser/ser to make a remote communication between CMOS camera and processor. In this case the current involved Is quite high (about 0.5A), so we decided to send power through another wire instead of designing a proper PoC filter. In addition, the application is for an industrial camera, so a standard Ethernet cable is preferred to a coaxial cable.

    We see on deser datasheet that a STP cable can be used instead of the coaxial one. Nevertheless, there is not much material about STP implementation if compared with informations about coaxial implementation.

    Do you have any suggestion about the STP implementation? Is it possibile to use a Ethernet cable instead? (each couple of the ethernet cable can be considered a stp?). Which minimum category of ethernet cable is required?

    Do you have any suggestion about PCB layout and connectors to be used for stp implementation?

    I have a further question, this time about coaxial implementation: do you suggest a particolar coaxial cable, or is any 50-Ohm coaxial adequate for ser-deser communication?

    Thank you in advance.

    Best regards,

    Alessandro

  • 0 in reply to Alessandro Cominelli
    TI__Mastermind 24095 points

    Hi Alessandro,

    Thank you for the clarification. 

    Alessandro Cominelli said:
    Do you have any suggestion about the STP implementation? Is it possibile to use a Ethernet cable instead? (each couple of the ethernet cable can be considered a stp?). Which minimum category of ethernet cable is required?

    Regarding cable selection, we typically do not give categories of cables that can be used as our requirements are based off the Channel Specification. Parameters such as insertion loss and return loss will change with the length of the cable and the cable quality. For instance, with a high quality cable you could have a longer transmission distance than if you used a lower quality cable. I will follow up with the team to see if there are any concerns using an Ethernet cable specifically. 

    Alessandro Cominelli said:
    Do you have any suggestion about PCB layout and connectors to be used for stp implementation?

    Typically, for STP configurations in FPD-Link III devices we will recommend Dacar 686-3 and Dacar 636-2 with HMTD connector type. 

    Alessandro Cominelli said:
    I have a further question, this time about coaxial implementation: do you suggest a particolar coaxial cable, or is any 50-Ohm coaxial adequate for ser-deser communication?

    Some cables we recommend are the Dacar-462, Dacar-301, and Dacar-302 using Fakra connectors. Any coaxial cable can be used as long as the Channel Specification is met. 

    Best,

    Zoe 

  • 0 in reply to Zoe Bison
    Intellectual 375 points

    Thank you very much for your suggestions.

    Zoe Bison said:
    I will follow up with the team to see if there are any concerns using an Ethernet cable specifically. 

    I will wait for this information.
    In the meantime I will check Channel Specifications for cables we want to use.


    Best regards,

    Alessandro

  • 0 in reply to Alessandro Cominelli
    TI__Mastermind 24095 points

    Hi Alessandro, 

    I followed up with the team and we aren't familiar with the use of an ethernet cable on our devices. As mentioned previously, the device specifications do not specify cable categories as the requirements are based off of the Channel Specifications. Any cable can be used so long as these requirements are met. 

    S-parameter measurements for the Serializer / Deserializer would need to be done to look at the IL/RL over the operating frequency of these devices. 

    Please let me know if you have any additional questions. 

    Best,

    Zoe