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FPD-Link-II/III Cable & Connectors for commercial application
I am currently using a FPD-Link-II (DS90UR905Q/906Q) chip set, and I am now looking to use a newer FPD-Link-III chip set in next iteration for supporting a WVGA (PCLK = 35 MHz; therefore 1.225Gbps) LCD as well as making use of the I2C and GPIO for a HMI application.
The application is for a commercial project and is cost sensitive; therefore I do not need automotive grade cabling / connectors or H/W. I did consider the commercial Channel-Link-III devices however there functionality did not satify my application requirements; therefore the FPD-Link-III Ser/Des (DS90UB925Q/926Q) was chosen as the most suitable device(s) despite the higher cost.
I'm currently using Belden EIA RS-422 cable 9729 (Which satisfies the 100 ohm differential requirements) but terminating this using standard Phoenix Contact 3.81mm pitch connectors for practical ease of use (I know 100 ohm connectors are recommended). I do experience intermittent loss of the LOCK signal particulally at higher PCLK frequencies when driving higher resolution displays, which I suspect is probably due to the incorrect connector being used. I have accessed the FPD-Link-II (DS90UR905Q/906Q) chip set with the specific Rosenberger HSD connector and cable assembly using Rosenberger mating connectors and Dascar_535 cable for which I did not see any loss of the LOCK signal.
The question is can anybody advise on the connectors / cabling that are best for use in a commercial application ? I have looked around and this seems to be a niche area for which automotive and military dominate.
Thanks in advance
Do you have S-parameter measurements for the cable you want to use (Belden EIA RS-422 cable 9729)? The most important factor with these devices is going to be related to insertion/return loss in the cable at the line rate of interest. My general recommendation would be to minimize insertion/return loss, and one of the first places you can do that is with a balanced differential connector. The LOCK issues you mentioned above could be due to this fact alone.
HSD connectors are very good for this kind of application. We have also seen mini-B style USB connectors. Even though USB is not 100ohm, the connectors are designed with a differential signal in mind (sort of), so this could potentially get the job done in a consumer application where distance and EMI concerns might not be as sensitive as automotive.
To select a cable based on insertion loss and return loss I have on one side the ploted characteristic of the cable (insertion loss vs frequency) and on the other side the bandwith of the LVDS (let's assume 550Mbps).
We have also as input :
- the minimum differential output of the tranmitter (let say 500mV for a pre emphasized FPD Link II)
- the threshold of the LVDS receiver (FPD Link II or III) for the differential voltage 100mV
How to I convert the bandwitdh 550Mbps to a max frequency that I will use after to find the insertion loss that shall be under the ratio in dB between the differential output voltage and differiential input voltage ?
The band occupied by the forward channel on FPD-III will be from ~1/20 the bit rate to ~1/2 of the bit rate - so for a 550Mbps signal, you would be concerned with ~22MHz to 230MHz. With FPD-III there is also the backchannel which occupies a lower frequency band - usually ~1-5MHz, although that varies from part to part.
In addition to just the return loss, you need to be careful of the impedance match since reflections at connectors or cable boundries will both attenuate the signal and can lead to errors, even if the signal amplitude exceeds the input sensitivity.
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