Cable Delay Compensation & Time Error Measurement Accuracy

Little things can make a big difference. While performing PTP Timing and Time Error measurements, we often ignore or downplay the effects cable delay compensation errors have in the accuracy of such measurements. Test cables need to be calibrated.

While there are plenty of papers, webinars and workshops that talk about TE, cTE, dTE, "2Way TE", limits, instrument accuracy, etc., there is very little mention of the effects cables have in the final results and accuracy of those measurements. Poorly compensated antenna, test and/or link cables can have significant effects in the accuracy of Time Error measurements. Specially when the topic is not mentioned in trainings, guides or test methods and procedures.

While can assume that a 1PPS signal at the output of a PRTC or GM is accurate at the port, once we connect a patch cord or distribution cables, we 100% sure the pulse is inaccurate at the other end of the cable. We certainly know cables introduce delays, but can not assume everyone knowns that or have it top-of-mind at all times. When we transition from lab to deployment, we have field technicians and field engineers being exposed to precision timing and synchronization for the first time. It is important to clearly document the need for accurate cable delay measurement and compensation.

In the past, short cables' delay may have been considered negligible, but these days the PASS/FAIL limits and masks are in the nanoseconds, so cable delay compensation errors can be significant, when you are trying to measure 5 or 30 ns.

We recommend measuring (a.k.a. calibrate) and labeling all of our cables, with their respective end-to-end delays.

Here are some of the cable delay correction error sources I have found.

Things to Watch Out For

Is it 2m or 6ft?

Do you know where that coax cable or patch cord came from? We tend to look at a cable and intuitively say "that's a 2m cable", or is it 6 feet? As a result some people use 2m and 6 ft interchangeably. In fact 3 ft = 0.9144m, which is equivalent to 8.5% error. Longer cables can introduce significant amount of error to TE measurements. 

The "Rules of thumb" and Approximations

During presentations, webinars and training, we often tend to oversimplify things to go faster, present examples the audience can grasp quickly and get a message across. One significant example is the use of 5 ns/m or 1.5 ns/ft cable delay ratios. But we forget that people may take it literally when they see a 2m coaxial or fiber cable and conveniently "label" it 10 ns. After all, it is just simple and quick math we can do in our heads.

The Speed of Light and VP% Approximations

While approximating the speed of light to 300,000,000 m/s (299,792,458 m/s) or 1,000,000,000 ft/s (983,571,056 ft/s) can introduce errors of 0.069% and 1.67% respectively. This cumulative error can be compounded by the use Typical Velocity of Propagation (VP) from cables. Typical and Actual VP% can have significant differences, even when the exact cable type is known and its datasheet is available.

We often see people using unmarked cables that came with network elements (NE), instruments or other devices. One could guess construction and composition of such cable, look up the typical VP% on a generic table, measure its mechanical length with a tape measure and think that we are good to go. But that could mean compounding errors in the ns/m range. The longer the cable, the bigger the error.

The use of Regular TDRs (or OTDRs) and Double Estimation

Ironically, Time Domain Reflectometry measures time of flight, but most TDRs keep that information to themselves and instead provide length (or distance) estimates based on:

• Internal table of generic VP%, for different cable types (high inaccuracy)
• The VP% entered by its user, (hopefully) based on the cable datasheet

Then users have to take the length reading and estimate (once again) the delay, based on a VP%. Each conversion can introduce significant error to the final value. It is also assumed that the cable is homogeneous, end-to-end.

Use of Different Types of Cables

In some cases, we may not have full access to the cable, because it is already installed. Roof antenna cables are a good example. Not only is more difficult to identify the type of cable, but there may be different types of cables: Distribution, Riser and Outdoors. Which VP% do you use for that? How do you accurately measure its length?

Proposed Solutions

In principle, the solution is simple: Measure the Actual Cable Delay and label it. In practice, that may come with a few caveats.

Using the Oscilloscope Technique 

If one has access to both ends of the cable, the delay can be measured by using a 1PPS signal and an a common dual channel oscilloscope. This can also be used to calculate and document the actual VP% of a cable before cutting and installing it.

Basically, Channel 1 (in Hi-Z) marks the time the pulse enters the cable and Channel 2 (50Ω termination) measures the exit of the pulse. The difference between the two rise times is the actual cable delay.

The source of the 1PPS can be a PRTC, GNSS receiver, an arbitrary signal generator or in fact any other low frequency square signal source. It doesn't need to be accurate, because it is a relative measurement. There is no need to know the actual mechanical length or VP% of the cable either.

Using the 1PPS Phase Error Measurement Function

In the case of VeEX's TX340s test set, user can apply a setup to the oscilloscope technique, by using the Clock Wander & Phase Measurement function (in 1PPS Phase Error Measurement mode), connecting the red T to the Reference Clock (B) input and the other end of the cable to the RX2 measurement port.

Using the multi-test capability of the test set, Port 1 can be configured in SyncE mode and set it to output a 1PPS pulse.

The test set will measure the phase difference between the two ends of the cable. (For more details click on this link.)

Using a Timing-oriented TDRs

The most practical way to measure cable delay is to to use a TDR that can provide direct time of flight measurements (no conversion). They perform single-ended cable delay measurements, even in mixed cable conditions. It should work in most conditions, including multiple cable types and cables that are already installed. The only restriction is that there are no active components in the cable (amplifies, power injectors, etc.)

TDRs are relatively inexpensive toots, especially when compared against the potential benefits of performing measurements with more accurate cable delay compensation. 

Conclusion

Always look for ways to minimize sources of additional error. One may invest in expensive highly-accurate instruments and reference clocks, but all that expected accuracy and repeatability can be easily be overshadowed by a couple of procedural errors. Little things, like cable delay compensation, can have a big impact in measurement accuracy.