Author Topic: Oscilloscope training class (long)  (Read 158418 times)

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Offline WBB

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Re: Oscilloscope training class (long)
« Reply #50 on: March 14, 2012, 09:24:17 am »
Thanks for the response!. Somehow I figured there would be an "it depends" in there, such is life! It does sound promising though.

Some of the stuff is easy to find. Shields like to break near connectors, or if the cable passes through trees I can pretty much bet good money a squirrel has gotten my coax mixed up with his acorns. Never knew aluminum was so tasty.

The worst for me inevitably turns out to be a couple shotgun pellets shorting out the cable. Not exactly the easiest thing to find and they NEVER radiate when you're looking for them!

What would be the maximin rise time that would be preferred in a function generator for this?

(I don't own a fc or I would have tried to answer those questions myself!)
 

Offline w2aew

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Re: Oscilloscope training class (long)
« Reply #51 on: March 14, 2012, 11:02:40 am »
Thanks for the response!. Somehow I figured there would be an "it depends" in there, such is life! It does sound promising though.

Some of the stuff is easy to find. Shields like to break near connectors, or if the cable passes through trees I can pretty much bet good money a squirrel has gotten my coax mixed up with his acorns. Never knew aluminum was so tasty.

The worst for me inevitably turns out to be a couple shotgun pellets shorting out the cable. Not exactly the easiest thing to find and they NEVER radiate when you're looking for them!

What would be the maximin rise time that would be preferred in a function generator for this?

(I don't own a fc or I would have tried to answer those questions myself!)

Ideally, you'd want the risetime to be a few times faster than the propagation delay through the cable of interest, so that you get a clean "plateau" between the incident edge and the reflected edge.  For typical coax with a 0.66 velocity factor, the speed of a signal is 7.79 inches per nanosecond.  Thus, a 10ns rising edge of the pulse will be spread out over about 6.5 feet of cable (i.e. the voltage 6.5 feet from the generator is just starting to move at the same time that the rising edge has finished moving at the generator).  If the pulse's risetime is slower than the roundtrip delay, then the reflection will be seen during the edge, making it much harder to accurately measure the delay.

For basic coaxial work like was shown in the video - for 10, 20, 30 feet or more of cable, a 10ns risetime is good enough. 

You might check the risetime of calibrator signal on your scope - it might be fast enough to use for this application.
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Offline vk6zgo

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Re: Oscilloscope training class (long)
« Reply #52 on: March 14, 2012, 12:00:10 pm »
I remember using a TU-5 Pulser for this purpose,which used a tunnel diode.
In use,you connected it to the 100 volt calibrator on a 545B or similar,& the TU-5 produced a very much lower level,(but very much faster rise time) pulse which was excellent for checking the 'scope,& for TDR purposes.

I had one hanging around for years,but I don't have a 545B,or anything similar,so no high voltage calibrator.
I thought of making a vacuum tube multivibrator to produce a 100volt square wave,but "put it in the too hard basket".
Eventually,I gave it to a friend who collected 545s.
 

Offline BravoV

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Re: Oscilloscope training class (long)
« Reply #53 on: March 14, 2012, 12:14:13 pm »
- Yes, you can determine the distance to a short (only the first short!).  You can see this at the end of the video where I dialed the pot down to zero.  You can measure the width of the pulse that's shown, and that will be the roundtrip delay from the scope to the short and back.  Calculate the distance the same way as I did for the open.

For those that missed it, here is the video that WBB is referring to:

Use a scope to measure the length and impedance of coax

Great tutorial video as always W2aew, thank you Sir !

This video inspires me to learn & tinker with this poor man TDR thingy, and since you did mention high speed rise signal as a reference in that video, I assume I can also use my vintage Tek 2901 time mark generator that generates those really sharp spikes for this purpose too right ?

Example of the time mark signals generated by that 2901 at various speed at my Tek 2465B.



« Last Edit: March 14, 2012, 12:24:11 pm by BravoV »
 

Offline WBB

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Re: Re: Oscilloscope training class (long)
« Reply #54 on: March 14, 2012, 12:24:18 pm »

Ideally, you'd want the risetime to be a few times faster than the propagation delay through the cable of interest, so that you get a clean "plateau" between the incident edge and the reflected edge.  For typical coax with a 0.66 velocity factor, the speed of a signal is 7.79 inches per nanosecond.  Thus, a 10ns rising edge of the pulse will be spread out over about 6.5 feet of cable (i.e. the voltage 6.5 feet from the generator is just starting to move at the same time that the rising edge has finished moving at the generator).  If the pulse's risetime is slower than the roundtrip delay, then the reflection will be seen during the edge, making it much harder to accurately measure the delay.

For basic coaxial work like was shown in the video - for 10, 20, 30 feet or more of cable, a 10ns risetime is good enough. 

You might check the risetime of calibrator signal on your scope - it might be fast enough to use for this application.

Thanks again! Normally if the problem is in the 1st 30' or so I'll find it by other means, usually when I'm not really looking!  It's those few hundred feet that follow that can be a royal pain.  Thanks for the tip about the Cal signal as well. A function gen was the next thing that I wanted to get anyway, but the Cal signal should give me a bit of a starting point to help determine what I should get. Again, much appreciated!
 

Offline sdscotto

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Re: Oscilloscope training class (long)
« Reply #55 on: March 14, 2012, 12:33:34 pm »
While I can't find it at the moment, I remember one of Jim Williams Application Notes on building a ~400ps pulse generator for testing oscilloscopes and probes.

IIRC is was AN47, but doesn't appear to be on the Linear web site anymore. 

I'm dumb as a stump compared to most of you real engineers, but still got a kick out of reading his stuff, much like Alan's recent tutorials.
 

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Re: Oscilloscope training class (long)
« Reply #56 on: March 14, 2012, 12:39:45 pm »
IIRC is was AN47, but doesn't appear to be on the Linear web site anymore. 
If I go to linear.com and search for AN47, the first link is a PDF of the correct appnote. The later circuit with a charge line, as described in LT AN79, 94 and 122, might be better, since it generates something approaching a step, as opposed to a semi-Dirac pulse.
 

Offline sdscotto

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Re: Oscilloscope training class (long)
« Reply #57 on: March 14, 2012, 12:40:26 pm »
I played with some old coax after watching Alan's latest video and used my 2901, too.   Here are the rise time measurements on mine:

 

Offline sdscotto

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Re: Oscilloscope training class (long)
« Reply #58 on: March 14, 2012, 12:42:44 pm »
IIRC is was AN47, but doesn't appear to be on the Linear web site anymore. 
If I go to linear.com and search for AN47, the first link is a PDF of the correct appnote. The later circuit with a charge line, as described in LT AN79, 94 and 122, might be better, since it generates something approaching a step, as opposed to a semi-Dirac pulse.

Yeah, my bad, I just went to this link and saw it missing in the first section. 
http://www.linear.com/designtools/app_notes.php
 

Offline w2aew

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Re: Oscilloscope training class (long)
« Reply #59 on: March 14, 2012, 01:51:06 pm »
Those time mark generators are cool.  Wish I had one myself!  You can give one of them a try.  Since it isn't putting out a pulse, but rather an "impulse", you can measure the delay between the peak of the incident impulse to the reflected impulse (which might appear on the trailing edge of the incident impulse).  Just be sure to make the repetition interval of your marks much longer than the expected round trip delay so that you don't mix up a reflected impulse for a repeated incident one.
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Offline w2aew

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Re: Oscilloscope training class (long)
« Reply #60 on: March 14, 2012, 01:58:00 pm »

Great tutorial video as always W2aew, thank you Sir !

This video inspires me to learn & tinker with this poor man TDR thingy, and since you did mention high speed rise signal as a reference in that video, I assume I can also use my vintage Tek 2901 time mark generator that generates those really sharp spikes for this purpose too right ?

Example of the time mark signals generated by that 2901 at various speed at my Tek 2465B.


Wow - impressive photos!  You'll have to tell me how you get such nice photos of analog scope screens like that!  No reflections of the camera, etc.  Nice.  Are you using some sort of a hood like the old polaroid style scope cameras?

Now, go grab some coax and let's see what those TDR reflections look like!
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Offline sdscotto

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Re: Oscilloscope training class (long)
« Reply #61 on: March 14, 2012, 03:01:32 pm »
Here's a little test with some cheap thin CCTV coax, probably 75?, both with open end, and 50? termination.
 

Offline w2aew

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Re: Oscilloscope training class (long)
« Reply #62 on: March 15, 2012, 01:01:49 am »
That looks like a long length of coax!
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Offline sdscotto

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Re: Oscilloscope training class (long)
« Reply #63 on: March 15, 2012, 02:31:29 am »
That looks like a long length of coax!

I'm guessing it's about 50'... 400ns / 7.79ns per foot = 51.3

 

Offline WBB

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Re: Oscilloscope training class (long)
« Reply #64 on: March 15, 2012, 02:57:54 am »
Not nit picking here, just trying to be sure I understand the principle. In the above examples, using the really fast & small pulses. The first pulse is transmitted from the generator, and the second pulse is the reflection? So at 240ns between pulses the length would be about 80 feet?
 

Offline sdscotto

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Re: Oscilloscope training class (long)
« Reply #65 on: March 15, 2012, 03:12:31 am »
Aw heck!   I wrote down the 400ns in the above post from notes on another piece of Coax.

You're absolutely right, the return pulse is coming at 240ns.
« Last Edit: March 15, 2012, 03:19:36 am by sdscotto »
 

Offline WBB

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Re: Re: Oscilloscope training class (long)
« Reply #66 on: March 15, 2012, 03:25:21 am »
Aw heck!   I wrote down the 400ns in the above post from notes on another piece of Coax.

You're absolutely right, the return pulse is coming at 240ns.

I was just going by the markers on the scope which look to be reasonably well placed.

I think your error was the 7.79ns/foot. Its more like 7.79"/ns.
 

Offline WBB

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Re: Oscilloscope training class (long)
« Reply #67 on: March 15, 2012, 11:14:53 am »
Another question, not that I haven't already veered this far enough off topic from oscilloscopes to function generators, sorry.  Would it be possible to use the TTL signal for short distances and switch to the normal square wave type signal for longer distances?  I really like the older equipment (both for budget as well as nostalgia) but looking up the specs for the older models isn't the fastest thing in the world.  Better than it used to be though!  Anyway, it seems pretty common to find TTL rise times of <50ns but the square wave rise times seem to hover around the 100ns range.  So I thought the TTL signal would be decent for short distances and the normal square wave would work fine once cable length exceeded rise time delay.  Or am I completely off base?

Thanks again!
 

Offline w2aew

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Re: Oscilloscope training class (long)
« Reply #68 on: March 16, 2012, 12:11:50 am »
That looks like a long length of coax!

I'm guessing it's about 50'... 400ns / 7.79ns per foot = 51.3

Oops - not quite!  For coax with a velocity factor of 0.66, the speed is 7.79 INCHES per nanosecond!   (7.79ns/ft would be faster than the speed of light!).

You measured about 240ns. Thus that would be 240ns * 7.79in/ns * 1ft/12in *1/2 = 78'
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Offline w2aew

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Re: Oscilloscope training class (long)
« Reply #69 on: March 16, 2012, 12:16:21 am »
Another question, not that I haven't already veered this far enough off topic from oscilloscopes to function generators, sorry.  Would it be possible to use the TTL signal for short distances and switch to the normal square wave type signal for longer distances?  I really like the older equipment (both for budget as well as nostalgia) but looking up the specs for the older models isn't the fastest thing in the world.  Better than it used to be though!  Anyway, it seems pretty common to find TTL rise times of <50ns but the square wave rise times seem to hover around the 100ns range.  So I thought the TTL signal would be decent for short distances and the normal square wave would work fine once cable length exceeded rise time delay.  Or am I completely off base?

Thanks again!

Whatever gives you a fast enough risetime to discern the difference between the incident edge and the reflected edge would work.  It all depends on the typical lengths/distances that you need to measure.  10ns edges work pretty well for lengths greater than several feet.  for a 50ns edge, you'd probably have trouble measuring distances less than 30-40 feet or so.
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Offline BravoV

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Re: Oscilloscope training class (long)
« Reply #70 on: March 17, 2012, 06:02:58 am »
Wow - impressive photos!  You'll have to tell me how you get such nice photos of analog scope screens like that!  No reflections of the camera, etc.  Nice.  Are you using some sort of a hood like the old polaroid style scope cameras?

Now, go grab some coax and let's see what those TDR reflections look like!
I will post my results on this poor man TDR once I did it, its just I don't have any long coax with me right now, most are only 1 to 2 meters max.


About those scope screen photos, thanks, actually its quite easy and no fancy hood at all.  ;)

I was using an entry level dslr with just stock lense, the secret is I used a camera remote that made it so easy to shoot in the dark, hence no reflections at all.  ;)

Also the handy part of using remote is, once the scope is ready with the camera placed and set properly like the focus & framing and etc, then all I need is just switch off my bench table light for few seconds while I pushed the remote in my other hand, and then turned the light on again.

About the reflection, usually I turned off any background light when I'm going to shoot scope's screen, and with only my bench table light which was the only light source in the room.




Actually even using a P&S camera is possible too, as long it has a self timer, but using remote is far more convenient, no need to rush in the dark accidentally bumping or knocking out things around.  ???


Now, the other part is altering the photos, for scope's screen shot, I altered it using program to make it better since this is just a scope's screen. And the procedures that I used are not complicated and pretty fast I'd say.

Example of the original photo straight from the camera, only cropping and resizing applied, nothing else.




Using the popular freeware PAINT.NET, you can get it here -> HERE or other graphic editor like Photoshop if you have it, adjust the brightness level & contrast using the simple level menu.

This is the default setting when we opened this Levels Adjusment sub menu.




Just a few adjustments on the those levers, after few trials, you will get used it easily.




The result with just those few drags, no fuss nor complicated tasks, then save it, that's it.  ;)




Hope this helps.

« Last Edit: March 17, 2012, 06:17:58 am by BravoV »
 

Offline w2aew

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Re: Oscilloscope training class (long)
« Reply #71 on: March 17, 2012, 08:49:08 am »
Nice - thanks for the tips.  I'd have to run across the room to hit a lightswitch!
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Offline wolfrum

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Re: Oscilloscope training class (long)
« Reply #72 on: March 24, 2012, 03:09:56 am »
One of the best videos and very good explanations
 

Offline w2aew

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Re: Oscilloscope training class (long)
« Reply #73 on: April 26, 2012, 09:59:39 am »
I've posted a few videos in the recent weeks, the most recent was today where I discuss some of the auxiliary input/output connections on the rear panel of many old analog scopes.  Enjoy!
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Offline Tube_Dude

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Re: Oscilloscope training class (long)
« Reply #74 on: April 26, 2012, 11:46:08 am »
I've posted a few videos in the recent weeks, the most recent was today...

Thank you! Very useful. I love analog scopes too...  8)
Jorge
 


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