Re: My TDR Evaluation of the SG503 012-0482-00 Cable has been uploaded


 

Hi Albert,
I think my derivation is valid and I know the VF can't be that high. Did I leave something out in the way I calculated it?

My purpose in calculating it (regardless of whether I got a perfect answer) was to show how to do it so everyone else with a TDR could do it. I wanted to illustrate how Velocity Factor was calculated from a TDR display. Instead of looking up a standard Velocity Factor, I was attempting to determine the velocity factor of the Tek cable based on its physical length and the TDR's "time distance" electrical length I measured on the screen. So I measured the time between the discontinuity at the start and at the end of the trace on the screen. Take a look at Figure 2. I marked off the starting point at "A" and the ending point of the cable at "B" and measured 9.0nSec between points A and B on the screen. Then I calculated the velocity factor based on that measurement of the transit time of the pulse through a 36.0" cable.

The result I got, VF = 0.9 was very high and I commented on that in my results. The only cables that have such a high velocity factor are very expensive and usually very fat (large diameter). I know the velocity factor can't be 0.9 and that figure is only a guess because my TDR has not been calibrated in over 20 years. The VF is probably more likely around 0.8 which is still a general indication of a high quality cable.

I also converted the mRho measurements to VSWR figures in a few places because that is also a consideration for some applications. For instance, the VSWR is listed in the specs for the 7A29 plugin as 1.4:1 at 10mV/Div or 1.2:1 on all other settings. The VSWR of the 012-0482-00 cable may have been a consideration of the engineering staff that determined what properties it had to have for the SG503.

This raises another question which none of us may ever have the answer to: What are the unique qualities the Tek SG503 designer wanted this cable to have to be the ideal match to the SG503. It may be something as silly, in retrospect, as the stiffness of the cable. Maybe he/she like stiff cables. Why is it exactly 36in? Maybe he/she had a scrap piece of stiff coax exactly that length in his drawer and he/she said "Use this" to the production people when they asked what kind of cable should ship with the SG503. Meanwhile we are all guessing at whether using this cable with an SG503 is really important.

Dennis Tillman W7PF

-----Original Message-----
From: Albert Otten
Sent: Monday, April 29, 2019 4:54 AM
Subject: Re: [TekScopes] My TDR Evaluation of the SG503 012-0482-00
Cable has been uploaded

On Mon, Apr 29, 2019 at 10:50 AM, Richard Knoppow wrote:

Where does it show the speed of light in a vacuum? The TDR should
be calibrated to the speed of light (electricity) in the cable. For 50
Ohm cable its about 66% of c. Varies with the impedance of the cable.
If you know the physical length of the cable you can determine both
its impedance and the velocity from it.

On 4/29/2019 1:26 AM, Albert Otten wrote:
Interesting stuff Dennis! I have to look at it in more detail. I
consider repeating these tests since I have both Tek cables, though
I don't have quality SMA-BNC adapters.
Most remarkable is the slow speed of light in vacuum in the USA ;-)
Albert
--
Richard Knoppow
dickburk@ix.netcom.com
WB6KBL
The document says: "The speed of light in a vacuum is 224844343.5 m/Sec=
0.2248m/nSec. "
I guess Dennis already used some kind of velocity factor here. But this
makes his following derivation invalid:
"The Velocity Factor (the speed the signal travels in the dielectric of
this cable) is VF = 2* 36.0in * 0.0254m/in * 4.4475nSec/m/9.0nSec ~
0.90. "

Albert


--
Dennis Tillman W7PF
TekScopes Moderator

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