Usefulness of different TDR designs? - Page 1
Forum section: electronics / projects

author: Per Hansson
author_rank: Unknown
timestamp: March 11, 2019, 06:57:27 pm
content: 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 componentsFor 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 tooHere is a rundown of what I have found so far:Avalanche breakdown based designs:Mr Carlson's Lab TDRgoo.gl/JWQzFyhttps://www.eevblog.com/forum/projects/tdr-power-supply/Picosecond Pulserhttp://www.siliconvalleygarage.com/projects/picosecond-pulser.htmlhttps://www.eevblog.com/forum/blog/eevblog-306-jim-williams-pulse-generator/Leo Bodnar Fast risetime pulse generatorhttp://www.leobodnar.com/shop/index.php?main_page=product_info&cPath=124&products_id=295https://www.eevblog.com/forum/projects/yet-another-fast-edge-pulse-generator/Schmitt trigger based designs:Tomi Engdahl TDR kithttp://www.epanorama.net/circuits/tdr.htmlhttp://www.epanorama.net/newepa/2010/09/29/tdr-kit-built/Alan W2AEWhttp://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 TDRhttps://hackaday.io/project/164165-tiny-tdr/Electroresales eBayhttps://www.ebay.com/itm/TDR-Time-Domain-Reflectometer-Fast-Clock-Version-Detect-cable-faults-More-/152587007700

author: Per Hansson
author_rank: Unknown
timestamp: April 17, 2019, 07:29:27 pm
content: As there was not a huge number of responses I decided to try and put some light on this myselfI 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 = 1028mmHopefully this is useful for someone else

author: iMo
author_rank: Super Contributor
timestamp: April 18, 2019, 11:11:11 am
content: It looks your 20m long cable is an RG58, isn't it?

author: Per Hansson
author_rank: Unknown
timestamp: April 18, 2019, 08:22:40 pm
content: 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

author: rhb
author_rank: Super Contributor
timestamp: April 19, 2019, 02:56:22 am
content: 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/#msg2336586I've posted examples using both the 11801 & SD-26 and one of Leo Bodnar's generators and a 200 MHz Instek DSO.

author: David Hess
author_rank: Super Contributor
timestamp: April 19, 2019, 10:06:47 am
content: 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.

author: Per Hansson
author_rank: Unknown
timestamp: April 19, 2019, 09:41:14 pm
content: 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

author: rhb
author_rank: Super Contributor
timestamp: April 19, 2019, 11:06:46 pm
content: What are the typical faults in the Tek 1502B? There are a slew of parts units on eBay for $75 and $43 shipping.

author: texaspyro
author_rank: Super Contributor
timestamp: April 20, 2019, 01:58:50 am
content: Quote from: rhb on April 19, 2019, 11:06:46 pmAlmost 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.

author: bson
author_rank: Unknown
timestamp: April 20, 2019, 07:44:05 am
content: 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.

author: rhb
author_rank: Super Contributor
timestamp: April 20, 2019, 12:43:37 pm
content: 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.

author: texaspyro
author_rank: Super Contributor
timestamp: April 20, 2019, 05:27:57 pm
content: Quote from: rhb on April 20, 2019, 12:43:37 pmWhy not? I have had over 100 of them and they seem to work quite well.

author: rhb
author_rank: Super Contributor
timestamp: April 20, 2019, 05:47:20 pm
content: Dead ones are rather unrepairable.Why have you had over 100?

author: texaspyro
author_rank: Super Contributor
timestamp: April 20, 2019, 06:35:16 pm
content: Quote from: rhb on April 20, 2019, 05:47:20 pmThe 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.

author: rhb
author_rank: Super Contributor
timestamp: April 20, 2019, 06:46:07 pm
content: 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.

author: Per Hansson
author_rank: Unknown
timestamp: April 20, 2019, 09:40:15 pm
content: Maybe I can ask a bit off topic question in my own thread thenThe 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-dueBattery 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–7Ptexaspyro 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

author: rhb
author_rank: Super Contributor
timestamp: April 20, 2019, 09:51:41 pm
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: April 20, 2019, 11:01:42 pm
content: Quote from: Per Hansson on April 19, 2019, 09:41:14 pmIf 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.

author: rhb
author_rank: Super Contributor
timestamp: April 21, 2019, 12:53:57 am
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: April 21, 2019, 08:09:41 pm
content: 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.

author: rhb
author_rank: Super Contributor
timestamp: April 21, 2019, 08:54:00 pm
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: April 22, 2019, 08:34:35 pm
content: Quote from: rhb on April 21, 2019, 08:54:00 pmI 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.

author: rhb
author_rank: Super Contributor
timestamp: April 22, 2019, 09:28:33 pm
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: April 22, 2019, 10:41:59 pm
content: Quote from: rhb on April 22, 2019, 09:28:33 pmSorry, 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.QuoteI 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.QuoteI 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.

author: rhb
author_rank: Super Contributor
timestamp: April 22, 2019, 11:23:15 pm
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: April 22, 2019, 11:57:01 pm
content: Quote from: rhb on April 22, 2019, 11:23:15 pmTheRIS described here by LeCroyis just what Tektronix and others would call equivalent time sampling or random equivalent time sampling. (1) I remember finding this last time I searched on this subject.QuoteI may have been thinking of whatHP did in the HP 54645A/D(2) in 1997 which is about the time that I remember although this is not the application note I remember.This is not the documentI remember either. I think HP either sent me a brochure about it at the time or I read about it in a trade magazine.(1) It is really annoying when trying to use this if the waveform synchronizes with the sample clock; you might think that impossible but I have had it happen. Universal counters face the same problem and may deliberately dither their timebase with noise to avoid it.(2) Alsohere.

author: rhb
author_rank: Super Contributor
timestamp: April 23, 2019, 01:07:47 am
content: Thanks for the LeCroy link. That's a much better description than the LC648DLX manual. It also makes clear why it doesn't work very well aside from not having a relay to remove the anti-alias filter from the signal path.What LeCroy is doing is absolutely useless so far as I have been able to tell. My square wave pulser from Leo has a 36 ps or better rise time. There is *no* difference in the rise time measured using RIS or regular sampling. It's ~250 ps either way. I was quite disappointed. However, it may also be related to clock granularity. The LeCroy is my fastest scope other than the 11801 which is a completely different sort of beast.Donoho did the mathematical proof of compressive sensing in 2004. I was quite stunned when I discovered his work in 2013. I was solving massively underdetermined inverse problems following Mallat's discussion of basis pursuit when it sank in that I'd been taught you can't do that. So I had to find out how this could be happening. That led me to Foucart and Rauhut and the most difficult math I've ever encountered. One of Donoho's proofs is 15 pages! Fortunately the other two theorems in the paper were only 2-3 sentences. Donoho has a great sense of humor, so he commented on it at the end of the first proof.In the case of compressive sampling the wave form does not need to be repetitive. As I understand it, the sole requirement is that the sample intervals be completely uncorrelated. I'd love to talk to someone with the fortitude to read Foucart and Rauhut. With Mallat as an essential prerequisite, it's a lot of work.

author: David Hess
author_rank: Super Contributor
timestamp: April 23, 2019, 02:19:12 am
content: Quote from: rhb on April 23, 2019, 01:07:47 amThat is the part I remember. Randomizing the sample time was suppose to suppresses aliasing on single shot captures and that is sort of what the HP notes I linked claim. The document from HP that I remember however went into a lot more detail.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 23, 2019, 11:23:03 am
content: Quote from: bson on April 20, 2019, 07:44:05 amAlternatively, you can use a CW signal from a signal generator, monitoring the outgoing signal to the cable with an Oscilloscope.(Or a swept signal if you want to be fancy).At 1/4 wavelength, an o/c far end will give you a (sharp) null in the display, and a s/c, a peak.I tested my reel of RG58 several times this way, & the cable length always came out correct for 0.66.Now I just use the tape measure (for that particular reel).

author: rhb
author_rank: Super Contributor
timestamp: April 23, 2019, 12:47:31 pm
content: Those provide a better idea of what RIS/ETS was about. I was taught that ETS implied that you had to provide an external band pass filter to limit the BW to the Shannon-Nyquist limit. I was a bit disturbed when I discovered that the EE community were not being very scrupulous about the mathematical niceties. My LeCroy samples at 2 GSa/s with a 1.5 GHz BW. So everything above 500 MHz is aliased. Gad!!!The attached figure is from "A Mathematical Introduction to Compressive Sensing" by Foucart and Rauhut.The top left shows the amplitude spectrum of the signal which has 5 non-zero spectral components. The lower left shows the time domain signal corresponding to the spectrum above it. The dots in the lower left show the 16 sample points selected randomly.The upper right is the result from attempting to compute the amplitude spectrum using a standard L2 norm discrete Fourier transform. The lower right shows the spectrum *exactly* recovered using an L1 discrete Fourier transform.When I was working on Bin Liu's minimum weighted norm regularization (Sacchi's student) I attempted to regularize data by means of an irregularly sampled discrete Fourier transform. In my case the average sampling met the Nyquist criterion, it was simply that the samples were not regular. The L2 norm inherent in the DFT produced a broad nasty mess for a single sine wave. That and the realization that the process appeared to be severely dip limited led me to abandon the work. It was a complex algorithm and would have been at least 6-9 months of full time work to implement.To put things in slightly different terms. The upper right figure is the results of attempting to solve Ax=y where x is a vector of Fourier coefficients using the traditional L2 (least squares) norm. The lower right is solving the exact same problem except using an L1 (least summed absolute error) norm.When I was at Austin in 85-89 we were so constrained by the 125K multiply-add rate of the 11/780 that serious work using an L1 norm was unthinkable. We did occasionally use SVD and truncate the eigenvalues, but without understanding that exact reconstruction possible in the noise free case.The operations research people were solving L1 norm problems via linear programming, but there was no interaction between OR and seismic that I am aware of. And in any case, the sheer volume of data we had to contend with would have made serious work impractical. We were already contending with 5-8 day runtimes for fairly simple stuff.Compressive sensing arose out of Donoho's recognition that if you could acquire data in the traditional manner and compress it to 1% of the initial volume, you could skip the traditional acquisition step. Candes had already proved that exact reconstruction was possible in the noise free case with fewer samples. The two of them were firing papers back and forth over the course of 2004. Donoho was at Stanford and Candes was at CalTech. It's really fun to watch the ideas fly back and forth. Both are excellent writers and just reading the abstracts and introductions will give a strong sense of how things developed without delving into the complexities of the mathematics.Have Fun!Reg

author: Per Hansson
author_rank: Unknown
timestamp: April 24, 2019, 02:32:30 pm
content: Quote from: David Hess on April 22, 2019, 10:41:59 pmHmm, I don't think I'm as deeply versed in Doctor Who as you areA time machine would be useful though, will put it on my to-do listQuote from: rhb on April 21, 2019, 12:53:57 amYes having Leo's and maybe also Mr Carlson's TDR for a comparison would be nice.Would be interesting to see how they stack up for TDR use.

author: rhb
author_rank: Super Contributor
timestamp: April 24, 2019, 04:29:02 pm
content: I'm planning to play with fast edge circuits when my SD-32 arrives.

author: David Hess
author_rank: Super Contributor
timestamp: April 24, 2019, 08:28:58 pm
content: Quote from: rhb on April 23, 2019, 12:47:31 pmThat sounds more like an under-sampling application where the bandwidth of the input signal must be limited to below the Nyquist frequency as oppose to a baseband application like an oscilloscope where the input bandwidth must extend to DC.IF sampling receivers do this to extract say a 15 kHz passband from a 15 kHz IF stage using a 100 kSample/second ADC at 10.7 MHz or whatever the IF frequency is. The right kind of sampling oscilloscope can do the same thing using an external clock. Oddly enough, some old DSOs can do it also because they support an external clock input. This has the effect of using the ADC as the final mixer stage.With a Costas loop to recover the carrier, such an oscilloscope can directly demodulate AM from an IF within its bandwidth. I think Linear Technology published an application note doing this using one of their sampling ADCs.QuoteThat sure does not seem right. Exactly what LeCroy model DSO is it? Typically a DSO of that caliber would deliver a real time sample rate of 2 GSamples/second but deliver 20 Gsamples/second or higher using ETS or what LeCroy called RIS in the application note I linked.My Tektronix 2440 is like that. The full 300 MHz bandwidth is not supported by its real time sample rate of 500 MSamples/second but the sample rate in ETS mode is 25 GSamples/second.Quote from: Per Hansson on April 24, 2019, 02:32:30 pmOne of the Tektronix 7B92A timebases I bought produced a sweep which went backwards in time. Suggestively, it was prominently marked "Physics Department" in red ink on the side. Unfortunately, I fixed it before considering what I had carefully enough.

author: rhb
author_rank: Super Contributor
timestamp: April 25, 2019, 12:19:57 am
content: The Shannon-Nyquist limit is the BW of the signal, not the maximum frequency. I'll leave Shannon's paper to the interested reader.The LeCroy in question is a DDA-125/LC684DLX. It's 2 GSa/s on 4 channels, 4 GSa/s on 2 channels and 8 GSa/S on one channel with an external adapter to combine 2 & 3. I don't have the adapter unfortunately.But it has a <250 ps rise time whether it is in 2 channel or 4 channel mode. It also has 20% overshoot which is actually stated in the datasheet. At least they are honest enough to include it. The rise time is the same in RIS mode.The DDA-120 doesn't need the external adapter to merge 2&3. So it looks like a good candidate for a set of switched low pass filters with 750, 1500 and 3000 MHz corner frequencies. And a VGA LCD to replace the CRT and provide room for the filters and SMA relays.The DDA-125 will dither the clock to give 40 ps sample intervals and claims a measurement resolution of 5 ps. However, I've not observed it to be useful for anything. It certainly does not give you a faster rise time measurement unless there is some menu toggle I've not found. I had great hopes of 11801 level performance when I bought it. But at least I can do hysteresis plots without requiring an external analog integrator.As far as I'm concerned, the EE sampling jargon is an egregious abuse of the English language and the mathematics put forth by Wiener and Shannon.Sampling scopes do *not* evade the Shannon-Nyquist constraints any more than wavelets evade the time-frequency resolution limits of the Fourier transform. A sampling scope is only sampling a very narrow range of frequencies. One need only consider the integral transform of the sampling window to see this.Or as I used to put it when people would breathlessly tell me that wavelets had better resolution than short window Fourier transforms, "If a statement is true in French, then it is still true correctly translated into any other language."

author: ocw
author_rank: Regular Contributor
timestamp: April 25, 2019, 04:46:37 pm
content: There are a number of factors influencing the use of a TDR or Frequency Domain Reflectometry (FDR) measurements. While I have used a Tektronix 1503B and various Riser Bond TDR's to locate and then correct many cable problems, FDR's are generally replacing TDR's since they do a better job of locating most faults. See:https://www.anritsu.com/en-US/test-measurement/solutions/en-us/distance-to-faultExamples of this are shown in my attachments. I am comparing a Tektronix 1503B to a Agilent E7495A. The best example is probably comparing an inexpensive Chinese 2M BNC to BNC RG58 cable. The end of the cable is connected to a 6 GHz 50 ohm load. The Tektronix TDR display indicates that it is perhaps not the best cable in the world, while the Agilent FDR display clearly demonstrates that besides the connectors being marginal, the cable is not the best quality either.The RG214 cable comparison shows the differences when measuring a better quality cable.Additional cables are shown on just the Agilent FDR since the differences are more easily seen on that instrument.

author: rhb
author_rank: Super Contributor
timestamp: April 25, 2019, 07:50:11 pm
content: While most TDRs seem to use a square wave, if an impulse is used the energy is distributed across a very broad spectrum.I have one of Leo's 100 ps impulse units. The output spectrum of that is approximately a sinc(f)**2 with the first zero at 50 GHz. Unfortunately, a DSO with that BW is rather more expensive than an FDR.I plan to build some pulsers in an attempt to have something capable of testing my SD-32 which at 7 ps is quite a challenge.

author: FriedLogic
author_rank: Regular Contributor
timestamp: April 25, 2019, 11:11:50 pm
content: Quote from: rhb on April 25, 2019, 12:19:57 amHi,The point of RIS etc. is more to fill in the trace with actual measurements rather than a calculated interpolation, and so get around the artefacts and errors that come with it - and in the case of older scopes like that it's needed even more because the ADC's used could not sample fast enough, which was common at the time. I wouldn't have expected it to change the rise time much, except to the extent that the interpolation was wrong.I have an old 100MHz Metrix that samples at 50MS/s, and that can get interesting....

author: rhb
author_rank: Super Contributor
timestamp: April 26, 2019, 01:39:34 am
content: I have an SD-32 sampling head in transit which should arrive on Saturday. I am very interested in suggestions of how to produce an edge which is faster than the 7 ps rise time of the SD-32.

author: David Hess
author_rank: Super Contributor
timestamp: April 26, 2019, 03:01:45 am
content: Quote from: rhb on April 25, 2019, 12:19:57 amFrom the manual:Repetitive signals can be acquired and stored at a Random Interleaved Sampling (RIS) rate of 10 GS/s for all models except LC564, LC584, AND LC684 SERIES, whose RIS rate is 25 GS/s.QuoteThe 40 picosecond sampling resolution in RIS mode is consistent with the given 25 GS/s sampling rate in RIS mode. 5 picoseconds probably represents the interpolated measurement resolution which is not unrealistic.QuoteWhy would it? The sampling method has nothing to do with input bandwidth.QuoteSampling oscilloscopes do not act or operate as you describe. Their bandwidth extends from DC to whatever their -3 dB point is. The -3 dB point exists for a sampling input however it does not really have the same meaning because the frequency response curve is non-linear with a null at the reciprocal of the sampling gate time and every harmonic past that. For this reason they do not obey the 0.35 rule but their transition time can definitely be measured or calculated based on the sampling gate width. This actually provides an easy way to measure sampling input bandwidth; find the first null and work backwards using the sin(x)/x response; a leveled signal source is not required. (1)One thing not commonly realized is that sampling oscilloscopes often have trigger bandwidths considerably in excess of their sampler bandwidth and their trigger bandwidth can be extended using injection locking. Injection locking a sweep used to be pretty common. Tektronix made normal oscilloscopes which included the same type of triggering (HF SYNC) through the 1980s for special applications.(1) This breaks down somewhere above 20 GHz where samplers get weird.

author: rhb
author_rank: Super Contributor
timestamp: April 26, 2019, 11:27:53 am
content: Write out the acquisition equation in the time domain and then apply an analytic Fourier transform.

author: David Hess
author_rank: Super Contributor
timestamp: April 26, 2019, 01:50:07 pm
content: It is irrelevant; at this point I am not convinced that you understand how sequential and random sampling work and what equivalent time sampling means. Your LeCroy supports random equivalent time sampling (RIS in LeCroy speak) of up to 25 GS/s using real time sample rates of 2, 4, or 8 GS/s depending on the number of active channels. This does not conflict with its 1.5 GHz of bandwidth at all except of course that it cannot capture 1.5 GHz of bandwidth with all 4 channels operating at the same time in a single shot without aliasing. That is what RIS is for.I am still puzzling over chapter 7 of the manual which discusses 5 picosecond timing resolution in connection with 40 picosecond sampling resolution. I wonder if that is based on 5 picosecond resolution in the time delay counter which measures the difference between the trigger and sample clock. Usually that is irrelevant because samples are only saved with up to 40 picosecond resolution in this case in the acquisition record and any extra resolution from the time delay counter is lost. Maybe LeCroy is retaining the 5 picosecond timing resolution somehow outside of the acquisition record.With the same design, Tektronix discards any extra resolution from the time delay counter. (1) But timing resolution greater than the acquisition record resolution is still possible when measurements take the vertical signal levels into account. That is how digital time delay counters produce timing resolution much greater than their sampling rate. Tektronix uses something like interval arithmetic to calculate and display measurement results with the proper number of significant digits and I wish other DSO manufacturers did the same thing.(1) Going by memory, the Tektronix 2440 TDC has a timing resolution of 0.5 picoseconds (or maybe 2.0 picoseconds) but this is obviously meaningless. The extra resolution supports gain, offset, and perhaps linearity calibration of the TDC and I suspect LeCroy does the same thing.

author: rhb
author_rank: Super Contributor
timestamp: April 26, 2019, 05:49:42 pm
content: If you want to discuss pulse generation circuits, I'm fine with that. But I have no desire to rehash 3 semesters of grad school mathematics. Proving the orthogonality of the Fourier basis once was quite enough for me.If EEs want to have their own special meaning for ETS, they may do so. But I am not interested in such abuses of language and mathematics. Wiener, Shannon and Nyquist got it right and stated it properly.

author: ocw
author_rank: Regular Contributor
timestamp: April 26, 2019, 08:24:49 pm
content: My prior post showed the comparison evaluations of short transmission line jumpers as measured by my Tektronix 1503B TDR and Agilent E7495A FDR/Station Test Set. I am more commonly evaluating lines from 100 to 2,000 feet long. That first attached picture shows the inner and outer conductors of some slightly overheated 3 1/8" rigid line that I replaced. There wasn't much left of the Teflon insulators near those points.With my being more concerned about longer lines I thought that I would include views of about a 175 foot long line to a receive antenna. It is mostly LMR400 cable although there are some shorter pieces of other cable also included. You can see those connections much better on the E7495A FDR. With saved measurements, you can see deterioration of connections or in the line itself. You can also make measurements on the specific frequency or range of frequencies primarily used. For comparison, I also included a Riser Bond 1205CX measurement as well.

author: rhb
author_rank: Super Contributor
timestamp: April 26, 2019, 09:26:53 pm
content: Why is there a reflection at around 140 ft on the Tek and RB displays and not on the Agilent?Also what is the frequency step on the Agilent? Does the Agilent correct for transmission loss? It certainly looks as if it is and it would be much easier to do in the frequency domain than in the time domain.Thanks for posting these.

author: ocw
author_rank: Regular Contributor
timestamp: April 26, 2019, 10:26:44 pm
content: QuoteYou are missing the obvious. Note what the Agilent reports at 152 feet vs. the end of the line at 176 feet. That is what the Tek and RB report at a slightly closer length. That is a connection point between the end of the LMR400 and the more flexible line used in the rotator loop. Note the "noisy" return loss of that rotator loop cable. You only see that on the Agilent.QuoteAs it shows on the Agilent display, I left it in the auto-frequency mode to automatically chose the best frequency range for the best length resolution. It chose 375 - 672.485 MHz. As the Agilent shows I selected the mostly proper LMR400 cable. The Agilent takes the velocity factor and attenuation of the cable into account when preparing its measurements.The receive antenna is a special one which I made for TV channel 6 through the lower half of the FM broadcast band (80 - 98 MHz). It is a ten wide spaced element Yagi antenna almost 100 feet high. That's why the return loss of the antenna was so high--it wasn't made for those higher measurement frequencies. My concern was line evaluation at a higher frequency where its limitations are more obvious with greater location resolution.

author: rhb
author_rank: Super Contributor
timestamp: April 26, 2019, 11:50:52 pm
content: Without readable scales it's a bit hard to compare things. Does the Agilent actually use a continuous sweep?I was actually asking about what Joel Dunsmore calls "reflection masking" rather than intrinsic cable attenuation. In geophysics we call the portion of the incident wave reflected at an interface transmission loss. Physics uses the same terminology for EM.

author: ocw
author_rank: Regular Contributor
timestamp: April 27, 2019, 01:30:34 am
content: The Agilent automatically places markers on the four distances with the least return loss and shows those distances and RL figures. That obviously makes it easy to see any change in future measurements. For the two TDR's I moved their one marker to the end of the coax.QuoteMy spectrum analyzer shows that the frequency range chosen (either automatically or manually chosen) is swept continuously in frequency.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 27, 2019, 02:48:30 am
content: Quote from: ocw on April 25, 2019, 04:46:37 pmI was confused by the distances in feet, not matching what I thought meant "2metre long" cable, not realising "2M"in this context was a part number.I lost interest, but now I need to revisit your interesting postings.

author: ocw
author_rank: Regular Contributor
timestamp: April 27, 2019, 05:08:17 am
content: QuoteIt was a 2 meter long cable being measured. It is just that the cheap cable did not meets its velocity factor specification.

author: tomato
author_rank: Regular Contributor
timestamp: April 27, 2019, 01:02:51 pm
content: Quote from: rhb on April 26, 2019, 05:49:42 pmYou want to discuss instruments designed by EEs on an EE forum, but you reject the conventions used by EEs with a condescending attitude? Good luck with that approach.