Designing a 20Mbps data link over 100m long micro coax

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Anon9599

Joined Nov 13, 2019
1
Hello,

Apologies if this is in the wrong forum. I'm trying to design a data link for transmitting 1080p compressed video at a bit rate of 18Mbps through a 100m long micro coaxial cable (36 AWG, 50 Ohm, such as the Pico Coax line made by Axon). The attenuation of such a cable at 10MHz is 32dB, and at 20MHz is 40dB. Because of the small cable diameter, frequencies higher than 20MHz are severely attenuated, so I don't think I can use that part of the spectrum without a lot of equalization/de-equalization. Ideally, this would be a half duplex link with 18Mbps coming from the camera and a small amount of control data (less than 2Mbps) going to the camera. I'm thinking that 40dB is a feasible amount of attenuation to work with, and that I could set up a single channel operating at 20MHz to get the 20Mbps data rate - no need for fancy frequency division multiplexing etc.

On the camera side, I envision that the logic level video data needs to feed into an amplifier/line driver which drives the coax, and on the other end of the coax I need a receiver IC/circuit of some kind to allow me to get the data back to logic level so I can feed it into a microcontroller or some other IC. Can anyone offer some advice on what types of amplifiers/line drivers, receivers, and protocols I should be looking into using? I'm aware of VDSL, which seems like it's using the right part of the spectrum for my cable, but I think that only pertains to differential signals, which won't work on my 2-conductor coax. And the FPD-Link SerDes series from TI seem to use parts of the spectrum well beyond 20MHz to achieve their huge Gbps data rates, which I don't need and which probably won't work on my long length micro coax.

Thanks in advance for your replies.
 

Papabravo

Joined Feb 24, 2006
12,562
Running so close to the hard limit tells me you have failed to take into account your need for some margin. As an example, if a wire can safely carry 20 Amperes and you say I'm going to have it carry 18 Amperes you are asking for a reliability problem over time.
 

SteveSh

Joined Nov 5, 2019
104
If it were me, I would try to go with a larger diameter cable which would give you lower loss. As you probably know, with a 38 AWG center conductor you're going to have a significant about of loss just due to the DC resistance of the cable, irrespective of any high frequency attenuation.
 

Marley

Joined Apr 4, 2016
294
If you want to transmit data at 18Mbs the actual frequency content of the signal will have to be considerably higher. At least 2X. this is so that you can also transmit clock synchronization and have a reasonable method of recovering the data.

Still, doable with 50dB of attenuation.

Presumably, you have to use this micro-coaxial cable? Transmitting 1080p with CAT 6 network cable over 100m is possible with off-the shelf parts. See here.
 
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