The current tether arrangement uses Belden 1353A cable-- a single twisted-pair of 24-ga stranded wire-- to send 10Base-T ethernet bidirectionally. An ETS EBS-10BaseT-2C-ST "Monoline Balun" (ETS calls it a balun but I would call it a hybrid transformer) is used on each end to multiplex the two ethernet directions onto a single twisted pair. ETS claims a range of 100m for this setup using a single Cat-3 twisted-pair. The Belden cable is rated as Cat-5E, so it should be fine.
Previous tether testing has apparently found that tether lengths beyond 50-70m are not reliable. This may be due to changes in the electrical properties of the cable when it is immersed in seawater. Nonetheless, I'm interested in running the ROV out to the full 100m tether length, so I decided to start performing some quantitative tests on the tether system, to try to glean clues as to why it is not working over the full 100m, and how it might be improved. In an ideal world, it might also suggest how to run 100Base-T over the tether as well.
I put together a small test jig to interface the tether system (balanced 100 ohm impedance) with standard test instrumentation like a spectrum analyzer (single-ended 50 ohm impedance):
I used a couple of BNC jacks, mated to Mini-Circuits T2-1T transformers to convert from 50 ohm single-ended to 100-ohm balanced. The transformer secondaries were connected to short lengths of Cat-5e cable, which were plugged into Cat-5e couplers. 100-ohm resistors were used to terminate both ends of the cable pair not under test. Some of the hookups were done with little pins and jacks so that the configuration could be changed between testing transmission loss and measuring crosstalk.
Here's a shot of the test jig, with the two ETS baluns plugged into the couplers, and a short piece of Belden cable connecting the two baluns. The BNC jacks are connected to a spectrum analyzer- the tracking generator feeds signals to the test jig, and the jig output feeds back to the analyzer:
I ran two sets of tests on this configuration, with the baluns connected by only a short piece of cable. The purpose was to measure the performance of the baluns alone, to serve as a baseline for future tests that add a large length of tether. The first set of tests measured between 0 and 30 MHz, running out to the third harmonic of the 10Base-T baud rate. The second set of tests was between 0 and 300 MHz, to see what the performance on 100Base-T would be. I'll plot two pictures below of the 10Base-T performance.
I first setup the jig without the baluns, running a small Cat-5E patch cord between the two sides of the jig. This was used to get a snapshot of the frequency response of the test cables and the test jig, which was then subtracted off of subsequent tests to provide a normalized measurement of the balun performance. The patch cord was then removed and the baluns attached as seen in the photo above.
Here's the frequency response of the baluns:
This shows pretty good performance. The insertion loss of the baluns is listed by ETS as <0.5db -- but note that this is in addition to the loss of an ideal transformer hybrid, which is 3dB (a factor of two in power) in each balun. The marker I've set at 10 MHz shows a loss of 6.39 dB, so each balun has a 3 dB theoretical loss plus an additional 0.2 dB of insertion loss. So far so good.
Here's the crosstalk of a single balun:
The marker at 10MHz shows a crosstalk of -32.1 dB. This meets the ETS spec of >30 dB.
I'm going to go get some sleep right now, and will post my thoughts of what it all means later.