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Measuring signal propagation speed

boreas_eos

Hi,

 

I am interested in measuring the time delay of signal propagation via two conductors (please see attached image below).

 

The aim is to measure time delay in signal propagation delay from point A to point B of two conductors when they have a different shape.

 

Assumptions:

  1. Both conductors are made of identical wire (same gauge, conductor, insulation)
  2. Both conductors have the same length (from point A to point B).
    • Distance between point A and B is 10 meters (+- 1 millimeter error tolerance)
    • Conductor 1 has a hollow sphere of the diameter of 1 meter in the middle. The length of sphere diameter (1 meter) plus the length of Conductor 1 wire (9 meters) adds up to 10 meters long which is equal to a length of Conductor 2
  3. We can assume that the capacitance and inductivity effects of cables can be neglected. Sphere acts as the capacitance, however, it should be negligible in terms of introducing signal propagation delay (please correct me if I am wrong)

 

Measurement:

  1. We wish to determine if a curved shape of the sphere delays the signal propagation. For example:
    • The signal propagates via Conductor 2 in 3.3356 E-8 seconds (time calculated by dividing 10 meters (Conductor 1 length) with speed of light in vacuum)
    • The signal propagates via Conductor 1 in 3.5257 E-8 seconds (as effective 'conductor' length is 10.75 meters as the signal travels 9 m through the wire + half sphere length which is 1.57 meters = 10.75)

 

What it all boils down to:

  • How can we measure time delays of the order of E-8 seconds?

 

I am a novice to a field and am not aware of such a method. Setup of the experiment is arbitrary, the image below is just an example. For example, We may connect Conductors 1 & 2 to the same current source at point A. At point B, we could connect Conductors 1 & 2 to piece of electronic (transistor, ALU operational logic unit), anything which can tell us via which conductor signal has arrived first, maybe we could use optical elements to do so? Means aren't important, what is important is to prove the existence of time delay.

 

Once existence of time delay is proven we will shorten one of the conductors until we make time delay minimal, meaning that signal will take the same time to propagate via both conductors (note: 'same time' isn't achievable, however, tolerance of few millimeters propagation difference [resulting in time delay of the order of E-11] can be ignored)

 

If you have ideas on the subject I would like to hear from you mladen.golubovic@gmail.com / +1-201-665-8754

 

Thanks,

Mladen

image

Attachments:
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  • To calculate propagation velocity you probably have to treat the conductor systems as transmission lines and figure out the dielectric "constant".

    https://en.wikipedia.org/wiki/Velocity_factor

    You can get ultra fast comparators that operate in excess of 10 GHz that may be able to tell you which signal arrived first and consequently figure out the geometry that forces them to arrive at the same time. But of course it is tough to position the comparator in a location where it doesn't need extra length of cable.

  • An oscilloscope with TDR module (used as TDT) or a Vector Network Analyzer can be used to make such a measurement.

     

    They both inject a signal and measure what comes through the cable. In the case of TDR/TDT, a differential source will pulse both cables at the same time so you can see the skew on the other side of the cable. If you don't have a TDR, you need a device with a sufficiently fast edge. You can use something like a high-speed pattern generator, or some companies make small TDR modules that are just an avalanche diode. Use the pulse source to deskew the oscilloscope channels. Then send the pulse through the cable and measure the skew.

     

    On a VNA, it is probably available as an S21 measurement. Measure the delay through the cable and divide that by its length. Repeat for both cables. (Or if you are confident they are the same length, skip the math.)

  • Many thanks to James & Douglas for the reply. If anybody is aware individual and/or a company which may help me out with experiment please let me know. Once again, many thanks for taking the time and effort!

     

    Regards,

    Mladen

    mladen.golubovic@gmail.com