Usefulness of different TDR designs?

[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.

[David Hess]

The 1502B has better than a 200 picosecond pulse and a sampling bridge which is even better so its high bandwidth of about 2 GHz is going to be difficult to beat with something that you construct.

Some logic gate based pulse generators are faster than others but it is difficult to get good pulse fidelity with a general purpose part simply because of mechanical considerations which make it difficult to control parasitics.

Slower but higher voltage pulse generators are useful for "long line" TDR applications like the Tektronix 1503 is designed for.

[Per Hansson]

Thanks for the insight David, yea the 1502B seems to be a good fit for my uses.
It provides almost an order of magnitude better rise time than a traditional schmitt trigger.
And it does not require a scope with a very high bandwidth, that ideally should offer a 50Ω input.
I think I have a keeper

[rhb]

What are the typical faults in the Tek 1502B?  There are a slew of parts units on eBay for $75 and $43 shipping.

[texaspyro]

What are the typical faults in the Tek 1502B?

Almost always a bad sampler board (unfixable sealed hybrid module)
Bad LCD display (very hard to find).

I'd avoid non-working units like the plague...

At one time somebody was working on doing a replacement module, but nothing seems to have come of it.

The non A/B units with the CRT are much more likely to be fixable.   Main faults are caps/transisors on the low voltage power supply board and failed high-voltage caps on the CRT power supply board... all farily easily fixable... schematics and excellent theory of operation are available.

I have seen a couple of 1502's with bad 20 mA tunnel diodes.  Only replacement is some Russian TD's.  But they are GaAs  and put out a higher voltage pulse.  You need to modify the detection amplifier gain.

[bson]

If it really matters you shouldn't rely on a 0.66 vop; instead, determine it from a known length of the same cable (using the TDR), preferably off the same spool or batch.

[rhb]

Thanks.  I also queried TekScopes and it's not worth messing with a 1502. 

I've ordered a custom 1 MHz version of Leo Bodnar's 10 MHz square wave generator specifically for TDR with a DSO.  I'll stick to that.

[texaspyro]

Thanks.  I also queried TekScopes and it's not worth messing with a 1502. 

Why not?   I have had over 100 of them and they seem to work quite well.

[rhb]

Dead ones are rather unrepairable.

Why have you had over 100?

[texaspyro]

Dead ones are rather unrepairable.

Why have you had over 100?

The CRT based TDR's are quite repairable,  the only ones I could not fix had broken CRTs.

I have a TDR fetish...    Actually, at one time, I got a reputation for being able to repair them and they just wandered my way.  Also bought a couple of large-ish lots of them.  I still have over 50 Tek TDRs (1502, 1503, 1503A, 1503B... alas no 1502A/B's)... all calibrated and working.   

The 1502A/B units are VERY expensive.  Apparently they are used in airplane maintenence and are the only approved device for lots of applications.

[rhb]

Yeah, the FAA is a bit weird about airplane servicing procedures and doubtless not enthused about qualifying a new device.  The HP 34401A stayed in production for many years because it was specified as a requirement for a lot of work.

[Per Hansson]

Maybe I can ask a bit off topic question in my own thread then
The 1502B contains a Lithium battery, the service manual mentions nothing about what is lost if that fails.
Only that it has an expected lifetime of 7 years, well, the 1502B I bought has a build date of 1994 so I think it's a bit past-due
Battery still measures 3.7v though, it is a "Keeper" cell called BT1010 in the schematics, with Tek partno: 146–0049–00 and manufacturer partno: LTC–7P

texaspyro also mentions the bad LCD's, I also saw that you got the same advice that I have seen here on tekscopes rhb and that is that an updated LCD exists.
Does anyone know about part numbers or any other details to help locate such a panel?
It has been discussed here in the past: https://www.eevblog.com/forum/repair/tek-1502b-tdr-restoration-project-and-more/msg1305389/#msg1305389

[rhb]

FWIW the lithium cells in my 30 year old 11801 are still at 3.1 V. At the time no one knew the shelf life.

[David Hess]

Thanks for the insight David, yea the 1502B seems to be a good fit for my uses.
It provides almost an order of magnitude better rise time than a traditional schmitt trigger.
And it does not require a scope with a very high bandwidth, that ideally should offer a 50Ω input.
I think I have a keeper

If I wanted something which is higher performance, I would look for a Tektronix 7S12 with S-6 loop-through sampling head (and a 7000 mainframe for it) or a mainframe for a SD-24 sampling/TDR head.  There are a bunch of HP alternatives but I am not familiar with them.

Making something better than a 1502 would be a challenge but could be done.  200 picoseconds is feasible with a good avalanche pulser but there are some suitable integrated parts which can be faster.  I think you can get to 200 picoseconds with SMD transistors as well.

[rhb]

Leo Bodnar uses a Maxim 3949 laser diode driver with a 22 ps rise and fall time in his excellent pulsers.  My BNC version has a 36 ps rise time according to the CSA803 printout provided.

Unless you just want to see if you can beat Leo, I recommend buying one.  The 3.5 mm and 2.4 mm versions are faster.  But more costly as the connectors cost as much as all the rest.

Leo will provide 1 MHz square wave units on request for an extra 10 pounds.  I have ordered two for TDR with a DSO.

The Tek 11801 calibrator is ~20 ps rise time.  The SD-24 heads have a pair of the same hybrid module.  It's only a 250 mV step, so it won't drive a long cable.

[David Hess]

I did not suggest using a part like the Maxim 3949 laser diode driver because without an accompanying sampling gate, extra performance from the faster edge will no be available.  Analog Devices has some fast comparators with similar performance.  A 4 GHz sampling gate is feasible with a surface mount design and does not require an edge that fast.

[rhb]

I'm afraid I don't understand what you mean by  "sampling gate".  Would you mind providing a more complete description?  Both of my current units are covered with heat shrink tubing for mechanical and ESD protection.  IIRC it's a clock chip, MCU w/ USB interface and the laser driver plus the obligatory passives.

My understanding of the circuit is the 10 MHz clock is driving the the modulation input of the laser driver which dumps the output straight into the connector via a blocking capacitor. The Maxim part provides the slew rate for the sharp edge.  The MCU varies a digital pot to set the output voltage.  But that's only a rough guess.  I've never spent any time analyzing it.  It is so much faster than the 1.5 ns unit I built 30 years ago it's not even funny.

I've got a pair with 1 MHz clocks in place of the 10 MHz on order which makes them more useful for general TDR work.

[David Hess]

I'm afraid I don't understand what you mean by  "sampling gate".  Would you mind providing a more complete description?

I mean the input side of the TDR which is typically a sampling bridge or other sampling structure to provide the highest possible bandwidth.  The clever part is that since sequential sampling can be used, there is no need to adjust sampling gain for random sampling which considerably simplifies things. (1)

Sacrificing amplitude for a faster edge does not increase performance unless the input bandwidth is sufficient to take advantage of it.  Of course an integrated laser driver or comparator with a CML output might still be preferable because of simplicity.  Just do not use it for the extra edge rate which will be irrelevant unless you can design and fabricate a sampler to take advantage of it.

(1) Offhand I cannot think of any advantage to making a random sampling TDR and there are several reasons not to.  A Tektronix 7T11 can be used as one and I have done it just to see how well it works but only for comparison purposes.

[rhb]

OK, you're just describing my 11801.  BTW I just bought an SD-32 for $500 :-)  Speed limit?  What speed limit?

Interestingly, random sampling has a major advantage though I've not seen it exploited properly.  My LeCroy claims to have it, but I've not observed its implemented in a useful fashion.  However, I don't have a way to get machine readable data out of the DDA-125 yet.

Aliasing arises because the Fourier transform of a periodic spike series is a periodic spike series.  The Fourier transform of a random spike series is a single spike, so aliasing does not take place.  I first encountered the concept in a dissertation by one of Mauricio Sacchi's students at Alberta, but was completely unable to understand what was going on.  The focus of the work was regularization of irregularly acquired data via the minimum weighted norm.  It enjoyed a brief period of interest in the seismic processing community, but then faded away because of some mathematical issues which had made me drop work on an implementation.

However, David Donoho and Emmanuel Candes showed that random sampling was a major advantage.  Donoho developed the idea and coined the name "compressive sensing" but it was part of a staggering burst of work he did in conjunction with Candes.  There is a catch  though, sparse L1 pursuits, the term I use because the application of the mathematics is so much broader than just data acquisition, is computationally intensive.  When they first started doing it for MRIs at the Stanford pediatric hospital it took many hours to process the data. I know that significant speed ups were developed to replace the original linear programming solution, but I don't know what the state of the art is now.  I've not read any of the professional literature on the subject since 2016 which is almost an eternity in a field evolving as fast as sparse L1 pursuits.

Leo's pulser with a 1 MHz clock will do a very good job as a pulse source for TDR using a DSO.  Once I get mine and the resistive splitters I ordered from China I plan to do a long thread on using TDR to solve vector network analysis problems. the major problem with the typical DSO is the rectangular passband results in severe ringing after the step.  So you need about 3 m of cable for a delay line.  But once you can window off the reflection of interest, it's trivial to get magnitude and phase over the BW of the DSO with 100 dB dynamic range.  So adding VNA capability to a $400 DSO with an extra $100 in accessories should be a real boon to people interested in HF radio construction.

I'm also going to consider trying to beat Leo's rise time, but I'm not really likely to succeed.  I'm hoping to find a dead SD-24 cheap enough I can disassemble it and study the Tek pulse generator.

[David Hess]

OK, you're just describing my 11801.  BTW I just bought an SD-32 for $500 :-)  Speed limit?  What speed limit?

Interestingly, random sampling has a major advantage though I've not seen it exploited properly.  My LeCroy claims to have it, but I've not observed its implemented in a useful fashion.  However, I don't have a way to get machine readable data out of the DDA-125 yet.

Sorry, I mean random sampling versus sequential sampling as it applies to a sampling oscilloscope.  Tektronix made two sampling oscilloscopes which supported random sampling.  The advantage is that no delay line or pretrigger is required.  Without a delay line, bandwidth of the sampler is maximized because the delay line needed to generate a pre-trigger is not needed.

To give a solid example, your SD-32 has a bandwidth of 50 GHz but if you must use the DL-11 delay line to generate a pre-trigger signal, then its bandwidth is limited to the DL-11's 5 GHz of bandwidth.  My older 7T11 with S-4 sampling head can operate in random sampling mode to make full use of the S-4s 14 GHz bandwidth without a delay line under conditions where no pre-trigger is available.

In a TDR, random sampling would allow the TDR to measure the reflected pulse immediately after or even before it is sent (1) but since a TDR can always generate its own pretrigger signal of whatever duration is required, random sampling is never needed.

Aliasing arises because the Fourier transform of a periodic spike series is a periodic spike series.  The Fourier transform of a random spike series is a single spike, so aliasing does not take place.  I first encountered the concept in a dissertation by one of Mauricio Sacchi's students at Alberta, but was completely unable to understand what was going on.  The focus of the work was regularization of irregularly acquired data via the minimum weighted norm.  It enjoyed a brief period of interest in the seismic processing community, but then faded away because of some mathematical issues which had made me drop work on an implementation.

However, David Donoho and Emmanuel Candes showed that random sampling was a major advantage.  Donoho developed the idea and coined the name "compressive sensing" but it was part of a staggering burst of work he did in conjunction with Candes.  There is a catch  though, sparse L1 pursuits, the term I use because the application of the mathematics is so much broader than just data acquisition, is computationally intensive.  When they first started doing it for MRIs at the Stanford pediatric hospital it took many hours to process the data. I know that significant speed ups were developed to replace the original linear programming solution, but I don't know what the state of the art is now.  I've not read any of the professional literature on the subject since 2016 which is almost an eternity in a field evolving as fast as sparse L1 pursuits.

I remember when this technique was advertised in DSOs but I thought it was HP.  Either it did not remain in use for long or they stopped advertising it.

But once you can window off the reflection of interest, it's trivial to get magnitude and phase over the BW of the DSO with 100 dB dynamic range.  So adding VNA capability to a $400 DSO with an extra $100 in accessories should be a real boon to people interested in HF radio construction.

I have been arguing for this capability in low end DSOs for years.  All it takes is adding differentiation and returning instead of discarding the FFT phase results although the proper averaging and noise marker functionality would make it even more useful.

I have been very tempted to get an old Tektronix TDS series DSO or even an 11k series because they are capable of this.

(1) A must when doing maintenance on a TARDIS.  This explains the rarity and high price of 7T11 and 7T11A plug-ins; those who still do TARDIS maintenance can pay unearthly (Gallifreyan) prices on the Terran market and can rebid on lost auctions if they choose.

[rhb]

LeCroy offers a RIS function.  I cannot discern any utility to it, but in all fairness I've not spent a lot of time trying to devise a test case.  I've not played with a Tek sampler that had the feature.

Most certainly no one has implemented what Donoho and Candes described in a commercial product.  You cannot get a PhD by repeating someone else's work.  And failure is not an option.  If you fail you have to start over on a new project. At least that's the case at reputable schools.