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03/12/2007 Entry: "GPS cables"

I made up six nominally 10 foot cables to feed from the 8-way antenna splitter to the various receivers. All are LMR-240 and have an N connector on the splitter end. The other ends are two each N, BNC, and TNC.

I tested using the HP 5370, feeding a 1ms pulse to a tee at the start input, then through a 4 foot cable with appropriate adapters to the cable under test, and another 4 foot cable to the stop input. I measured 100K samples and used the mean value.

The baseline (4 foot cables with BNCs, each with a BNC female to N female adapter, and an N female barrel) measured 12.759 ns. The N barrel was about 1 inch long, so let's subtract 0.1ns to get to 12.659 ns (our first note of imprecision!).

The two N cables should have the best accuracy, since they simply substituted for the barrel. Cable 1 measured 25.204 ns and cable 2 was 25.219 ns. Subtracting the baseline:

#1 is 12.545 ns
#2 is 12.560 ns

The two TNC cables were the toughest because I didn't have a BNC female to TNC female tweenie. Instead, I had to use a BNC female to TNC male, plus the ends of a TNC tee. As a result, the test fixture was about 1 inch longer than for the N cables. So, we need to subtract about 0.1 ns to compensate for that. Cable 3 measured 25.348 ns and cable 4 measured 25.339 ns. After subtracting 0.1ns and subtracting the baseline:.

#3 is 12.589 ns
#4 is 12.580 ns

For the final BNC cables, the BNC-N adapter was replaced with a BNC female barrel. The fixture length is about the same, so I'll make no correction for it. Cable 5 measured 25.187 ns and cable 6 was 25.207 ns. Subtracting the baseline:

#5 is 12.528 ns
#6 is 12.548 ns

So, across 6 cables the spread is 12.528 to 12.589 ns -- 61 picoseconds. The fact that the two longest cables are the TNC lead me to suspect that my guess of the fixture length is a bit off there, and that the spread is thus somewhat less, but I'm not going to worry about that. Officially, all six cables will be tagged as 12.55 nanoseconds.

Thanks to spousal unit Jody (KC8KDC) for help in getting the cables measured so closely!