TDR Calibration

[veedub565]

Hi, I've got a couple of queries here, and both relate to the calibration of TDR (Time Domain Reflectometer) or more specifically, verifying the accuracy of them. A performance verification if you like rather than an actual calibration.

Firstly, what would be the best method of verifying the accuracy of a TDR ?
Up until now I have been using fixed coax cable lengths (40m, 85m, 350m) I've used these cable lengths as a reference to test the TDR against. How do I know exactly what length the cables are? well...

I verify the cable lengths using the following method.
1. I physically measure a short 1m length of the same type of cable.
2. I then measure it's electrical length using a Fluke DTX cable analyser, in this case velocity propagation is 0.67
3. I then note the error between what I physically measured, and what I measured electrically.
4. I then measure my reference lengths (40m, 85m, 350m) using the Fluke DTX and apply the error correction.
5. That then gives me the length of my reference cables

The problem I can see with this is that Physical length and Electrical length are two different things. it might physically measure exactly 1m, but electrically could read differently. It's no good knowing exactly how long my reference cables are (Physically) if I am using them for a (electrical) length test. I need to know the precise electrical length of the reference cables. I guess that's independent of any physical measurement or reference. 

That leads me to my next question,
Assuming the reference cable length is best method to use. What is the best method of accurately measuring the length of my reference cables ?


Would I be better off using simple maths along with a pulse generator and counter to simulate a physical cable ?  The outgoing pulse triggers the pulse generator, and it sends a delayed pulse back. Pulse delay is calculated according to the length and propagation factor you want to simulate. Counter verifies accuracy of the pulse delay.

I've heard a Vector Network Analyser can measure cable length far more accurately than a TDR ?

[Jay_Diddy_B]

Hi,

If you sweep an open circuit cable with a VNA and measure impedance. The first short will appear when the electrical length of the cable is 1/4 of wavelength. The first high impedance point is when the electrical length of the cable is 1/2 wavelength.

Let us say that you sweep a cable and the first short appears at 1MHz. The period of the waveform is 1000ns. The electrical length of the cable is 250ns.

Electromagnetic Radiation in free space travel at 3 x 10⁸ m/s

so 250ns = 75m.

If you measure the physical length of the cable. If the physical length of the cable is 50m

Then the measured velocity factor is 50/75 = 0.66667

So now you can measure the length of other cables.

Since velocity factor is the reciprocal of the square root of the dielectric constant (relative permittivity) the VF is essential a constant depending on the dielectric in the cable.

It is the velocity factor that determines the relationship between the electrical length, measured with a TDR or a VNA and the physical length.

If you take your 350m cable and sweep it. If you assume that the VF is 0.67 (solid polyethylene) the electrical length would be 350 / 0.67 = 522m. This is 1/4 wavelength. Wavelength = 522 x 4 = 1088m

The frequency of the first low impedance point for the cable if it terminated in an open should be

3 x 10⁸ / 1088 = 275kHz



Jay_Diddy_B

[David Hess]

Where absolute accuracy is required, air lines which have a predictable impedance and velocity factor are used.  But TDR accuracy is limited by cable being measured so it is less critical.

[tggzzz]

Where absolute accuracy is required, air lines which have a predictable impedance and velocity factor are used.  But TDR accuracy is limited by cable being measured so it is less critical.

The alternative is to use a calibration cable, e.g. the Tek 012-0482-00 https://groups.io/g/TekScopes/topic/my_tdr_evaluation_of_the/31377831

There are, of course, two aspects to calibrating a TDR.

The distance, which is dependent on matching the TDR's velocity factor twiddle factor to that of the cable.

The amplitude/impedance variations, which are complicated by many 50ohm cables being 48-52ohms by design.

[notfaded1]

This make me think about all the different specs for different coax cable types on the connection to my GPS antenna on the roof... really wanting to get the delay correct to account for the cable length.

Bill