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Usefulness of different TDR designs?
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Per Hansson:
Hi, just curious if someone could help to explain the main differences in different TDR implementations.
There is the original Jim Williams design that uses a transistor in reverse breakdown mode.
And the newer design with a Schmitt trigger IC.
The obvious difference is of course in voltage sent over the wire:
Just 5v with the IC but could be over a hundred volts with the transistor version.

But what practical differences are there, is one better than the other?
Yesterday I spent all day on this and in the end had over half a dozen PCB's loaded on OSH Park.
Then I figured maybe it's better to ask this question before I spent a month looking for unobtanium components ;)

For my own uses I'd mainly be testing coax cables with BNC connectors already on them.
Lengths of 1.2m and 15m would be common, this to look for bad connectors and cable faults in machines.
If something like a Tektronix 1502 would be better suited or maybe anything completely different please don't be shy to tell so too :)
Here is a rundown of what I have found so far:


Avalanche breakdown based designs:

Mr Carlson's Lab TDR
goo.gl/JWQzFy
https://www.eevblog.com/forum/projects/tdr-power-supply/

Picosecond Pulser
http://www.siliconvalleygarage.com/projects/picosecond-pulser.html
https://www.eevblog.com/forum/blog/eevblog-306-jim-williams-pulse-generator/

Leo Bodnar Fast risetime pulse generator
http://www.leobodnar.com/shop/index.php?main_page=product_info&cPath=124&products_id=295
https://www.eevblog.com/forum/projects/yet-another-fast-edge-pulse-generator/



Schmitt trigger based designs:

Tomi Engdahl TDR kit
http://www.epanorama.net/circuits/tdr.html
http://www.epanorama.net/newepa/2010/09/29/tdr-kit-built/

Alan W2AEW
http://marcusjenkins.com/tdr-pulse-generator-for-testing-coax/
https://www.eevblog.com/forum/projects/super-simple-_fast-edge_-pulse-generator-for-scope-based-basic-tdr/

Tiny TDR
https://hackaday.io/project/164165-tiny-tdr/

Electroresales eBay
https://www.ebay.com/itm/TDR-Time-Domain-Reflectometer-Fast-Clock-Version-Detect-cable-faults-More-/152587007700
Per Hansson:
As there was not a huge number of responses I decided to try and put some light on this myself ;)
I bought a "for parts" Tektronix 1502B on eBay, it just had a broken LCD temperature sensor that I have bypassed.
Secondly I bought the Electroresales TDR on eBay as well.
In the screens below I compare them, the Tektronix easily wins in the rise time:
As you can see the 1m cable is pretty much impossible to get any decent readout from with the schmitt trigger based design.
However with the Tektronix it is no problem to see it in high resolution.
I have already used the Tek in the field and it worked admirably: showing a clear disturbance in a cable as it was moved by a machine in a cable chain...
Sadly I forgot the power adapter for the Electroresales TDR so I could not compare it live then.
That said even if the rise time can't be compared the schmitt trigger based design seems very accurate:
The 1m cable I measured in at 10.4ns with my Fluke 225C scopemeter, which translates to: 299.792458 x 0.66 x 10.4 / 2 = 1028mm
Hopefully this is useful for someone else :)
iMo:
It looks your 20m long cable is an RG58, isn't it?
Per Hansson:
Yes it's a RG58C/U for both the 1m and 20m cables.
This can also be indirectly deduced from the velocity factor setting on the Tek TDR ;)
rhb:
I did not see this earlier.  I plan to implement VNA via TDR using one of Leo Bodnar's excellent <40 ps square wave generators and any DSO.

I've been doing a good  bit of TDR using an 11801 calibrator output and an SD-26 20 GHz sampling head.  I hope to find an SD-24 at a reasonable price.  You can see some examples here:

https://www.eevblog.com/forum/beginners/oscilloskope-50ohm-input/msg2336586/#msg2336586

I've posted examples using both the 11801 & SD-26 and one of Leo Bodnar's generators and a 200 MHz Instek DSO.
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