Re: RE: Re: Fine Tuning Shifting

On 8/15/2024 3:55 PM, cyclintom wrote:

On Thu Aug 15 14:31:03 2024 Zen Cycle  wrote:

On 8/15/2024 12:41 PM, Jeff Liebermann wrote:

On Wed, 14 Aug 2024 19:22:06 -0400, zen cycle
<funkmasterxx@hotmail.com> wrote:
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On 8/14/2024 3:35 PM, cyclintom wrote:

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Flunky just talks ahbout crap that he doesn't know about. Notice how
he could not answer how you measure the length of a wire
electronically? That's because he personally doesn't know how to use
an oscilloscope. Which comes as no

>
Or, It could be because Jeff answered it as well as I could so I didn't
feel it was worth commenting, but since you insist, here's how you test
cable length with _out_ using dedicated TDR equipment:
>
- Connect a pulse generator, an oscilloscope, and the cable under test
together with a T connector (Typically BNC, but adaptors may be required
depending on the equipment.
- set up a pulse generator to single pulse, starting with a 1 nS width,
5 V P-P.
- Set up the scope to trigger on the single pulse from the generator,
with the pulse from the generator being displayed at the extreme left of
the display.
- Adjust the time base of the oscilloscope until the reflected pulse
appears on the right side of the display while manually initiating the
pulse on the generator.*
- Longer cable lengths may make the returned pulse not quite
discernible. When the trigger pulse is no longer visible, increase the
pulse width until it is visible, then begin adjusting the time base
again until the reflected pules is visible (It may also be necessary to
increase the pulse amplitude).
- Once the reflected pulse is visible, measure the distance between the
trigger pulse and the reflected pulse on the scope as time delay (Hence
the term Time Domain Reflectometry).
- PLug this delay into the propagation delay value for the cable under
test, and do the math.
>
* setting up the generator to a repetitive pulse could prove problematic
in that the trigger pulse period my be shorter than the propagation
delay of the cable under test giving a confusing display. A competent
technician could manage it, but given the bizarre and completely
non-nonsensical method you described, it's well beyond your comprehension.

>
Yep.  That's how a TDR works.  However, I have a few quibbles.  Using
a 1 nsec wide pulse is difficult to see on the oscilloscope.  The rise
time on a commodity 100 MHz bandwidth oscilloscope is about 3.5 nsec,
which makes seeing a 1 nsec pulse difficult.  Minimum would be a 350
MHz oscilloscope.

>
That's true of low-end scopes. Most modern production* models can latch
onto it with no problem, with the added benefit of calculating the
period between the trigger and the reflected pulse as a measurement
function even if the pulse is too narrow to visibly discern, provided
the amplitude of the reflection is high enough for the scope to see of
course. Sampling rate is just as important here.
>

>
It's much easier to produce and see a 1 nsec rise time step waveform
(either rising of falling).  That's what the cheap square wave TDR
devices use:
<https://www.youtube.com/results?search_query=tdr+74ac14>
Even if the scope does not have adequate rise time to be able to see a
1 nsec pulse, it can see something with a 1 nsec step.  The
measurement point will need to be visually approximated but at least
it will be visible.

>
And this is exactly why a dedicated TDR tester is of much more use. I
presented this method to show tommy-can't-do how TDR fundamentals work -
something he is woefully mis/underinformed about.
>

>
It is possible to create nsec (and picosec) pulse widths using step
recovery diodes:
<https://en.wikipedia.org/wiki/Step_recovery_diode>
<http://www.hp.woodshot.com/hprfhelp/5_downld/lit/diodelit/an918.pdf>
<https://arxiv.org/pdf/1610.07115>
However, that's overkill for measuring cable lengths, where a cheap
74AC14 oscillator and Schmitt trigger squaring amp are sufficient.
>

>
I saw a demo sometime ago of a guy using a 9V battery with a momentary
switch and an in-line resistor as the pulse stimulus for measuring a
long cable spool, it actually worked pretty well.
>
*by production model I mean something intended for use in a production
facility as GP test equipment. 500MHz/1Gs models were a bit expensive
back then, but not too bad on the 2nd hand market these days
https://alltest.net/categories/products/54825A-Agilent-Keysight
>
>
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>

Pretending to explain something to Liebermann who knows nothing is pretty silly. Why did you say that you didn't need a repetitive trigger and then say that "Sampling rate is just as important here."?

I didn't write that you don't need a repetitive trigger. I wrote that you don't need a repetitive pulse from the generator.
Besides that, there's no such thing as a repetitive trigger. For normal waveform analysis Oscilloscopes have Auto, Normal, and Single Sweep trigger settings. For TDR you can't use Auto, it must be either Normal or Single. If you _are_ using a repetitive pulse using amything but Normal would be pointless (can you tell me why?)
There's no relationship between any trigger setting and sampling rate of the scope other then higher rates give less less aliasing and better capture of high transient/slew signals

 Did I misunderstand you or did you say that you were triggering an o-scope on the echo?

yes, I didn't write anything even remotely similar to that.
One tgriggers on the starting pulse and measure TO the echo. But perhaps I misunderstood you.
"Set up the scope to trigger on the single pulse from the generator with the pulse from the generator being displayed at the extreme left of
the display"
"Adjust the time base of the oscilloscope until the reflected pulse appears on the right side of the display while manually initiating the pulse on the generator"
"measure the distance between the trigger pulse and the reflected pulse on the scope as time delay"

I gave you real numbers because I used a PWM signal in the real world.

No, you didn't. PWM is used mainly for motor speed control and lighting intensity control as well as a few other applications where precise variable control is needed. It is not, and never has been, used for testing cable length. A PWM signal is a continuous pulse stream. It would be nearly impossible to see a reflected pulse with a continuous pulse stream from the source.
  I was partner in a telephone installation firm and we paid real money for real rolls of interconnect wire and the larger the roll the more you paid for it. No sense in throwing money away. So you bought rolls of wire as close to the correct size as possible and that meant from the interconnect board in the basement of skyscrapers to whereever the interconnect room was for the company employing our services might be.

 And we were WELL paid. I was making twice as much working as a telephome installation technician as I was designing and programming medical instruments. I was offered a job working on a Respiratory Gas Analyzer. So I trained a replacement that my partner only had to pay wages. He was disappointed that I dissolved the partnership, but it was just as well as he had a heart attack and died a year or so later. I had no idea of how to install the upper floor actual telephone systems. My job was interconnecting to Bell. And the Bell technicians were TERRIBLE at that. Their interconnect panels looked like loose spaghetti.

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