Re: Fine Tuning Shifting

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:
 

On 8/14/2024 3:35 PM, cyclintom wrote:

>
>
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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