Super simple "fast-edge" pulse generator for scope-based basic TDR - Page 1
Forum section: electronics / projects

author: w2aew
author_rank: Super Contributor
timestamp: April 22, 2013, 12:32:47 am
content: Here's a video I posted today that shows a very simple oscillator / pulse-generator that has about 2ns edge speed and the ability to drive 50ohm lines. This makes it a nice, low-cost circuit for use as a very basic TDR (time domain reflectometer) for checking cable length and impedance. Enjoy!

author: Rufus
author_rank: Super Contributor
timestamp: April 22, 2013, 02:01:30 am
content: Heh, photo of my version attached.A dead bug 74FCT octal bus driver (fastest thing in my junk box at the time) and 7 x 330R resistors. The first buffer drives the other 7 and I feed it with a signal generator for more flexibility. You can't see it but there is an 0805 ceramic cap soldered between the ground plane and chip VDD pin.I don't have anything fast enough to properly measure its rise time. Would the spectrum from a SA give you a meaningful indication?One reason I made it was as a source for hf compensation trimming on scope probes.

author: babysitter
author_rank: Frequent Contributor
timestamp: April 22, 2013, 07:22:24 am
content: Kindly allow me to refer to the charged line pulse generator that has some properties making it interesting for the TDR theme.I made a little jig from a ebayed mercury wetted relais, if wanted I can post something later.Hendi

author: ice595
author_rank: Regular Contributor
timestamp: February 04, 2014, 03:18:47 am
content: I just designed and etched a little PCB for this pulse generator circuitusing a laser printer with the toner transfer methodSince I didn't have any SMD resistor or caps at the time so I had to do this the "rural area" wayI also added a high performance oscilloscope probe socket as an after-though to make accurate measurements.With my vintage HP 54502A 400MHz 400MS/s oscilloscope I able to measure <4ns rise time with averaging turned on and equivalent time sampling onsince my Rigol DS1102E doesn't have a 50ohm input option, the measured rise time is WAY out of wack.and no I don't have any 50ohm terminators at the moment so I can't just use a T connector to make the input 50ohmsAnyways, I tried to measure a 75ohm TV coaxial cable and the calculation doesn't match the actual length of my coax cableaccording to my calculations, my coax cable should be around 8 meters but the actual length is about 10 meters.is it because my TV coax cable doesn't have a velocity factor of 0.66?also, should the cable's 75ohm characteristic impedance matter too much? no right?attachment contains EAGLE CAD schematic and board layout if anyone is interested in etching their own PCBnote: click the rats-nest tool to create the ground plane, EAGLE doesn't seem to save my ground plane

author: jwm
author_rank: Regular Contributor
timestamp: February 04, 2014, 03:47:55 am
content: Nice. I made one with a 74ac14 and a repurposed junk pcb not long ago and was pleasantly surprised at how well it worked. I used one of the six inverters as an oscilator to generate the pulses and the other 5 with 220 ohm resistors on the other five inverters in parallel, looks like pretty much the same circuit as you.Experimenting with a TDRJohn

author: ice595
author_rank: Regular Contributor
timestamp: February 04, 2014, 04:03:12 am
content: what was the rise time for your pulse generator?

author: w2aew
author_rank: Super Contributor
timestamp: February 04, 2014, 04:07:26 am
content: Quote from: ice595 on February 04, 2014, 03:18:47 amTypical CATV coax is about 0.85 velocity factor.The board is a nice clean layout. You should note however this is a case where a neat and clean layout might not give you the best results. This design puts several of the gates in parallel to increase the drive capability of the generator. Ideally, the inputs and outputs of each of the gates should be matched in length. The longer delay of the gate outputs on the left of the board compared to the ones on the right is probably contributing to the ringing you're seeing. For the TDR application though, it's not too big of a worry though.

author: jwm
author_rank: Regular Contributor
timestamp: February 04, 2014, 04:45:55 am
content: Quote from: ice595 on February 04, 2014, 04:03:12 amI am not sure, faster than any equipment I had at the time (60mhz scope) could measure, is there a good way to measure rise time other than "find a really fast scope".I have been meaning to build up the williams avalanche pulse generator too and see how it functions as a TDR. I'd like to be able to measure shorter lengths of cable than I was able to with my first version. though, I am probably still scope-limited. (have a 100mhz one now).I'd be interested in any tricks to measure very short lengths of cable, like on the order of a light-nanosecond. Not for any particular reason other than since I built the 74ac14/oscilloscope version it has piqued my curiosity about how it might be done without crazy expensive equipment.John

author: JoeyP
author_rank: Frequent Contributor
timestamp: February 04, 2014, 04:46:54 pm
content: The risetime is probably much faster than your 400MHz scope. I've built simple pulse generators using Fairchild NC7SZ14 (an ultrafast single element version of 74xx14. They cost $0.10 in quantity). No need to parallel multiple units. Just the most basic resistive matching pad to produce a 50 ohm source. Example 80 ohms series with output to 100 ohms parallel. The 80 ohms accounts for ~20 ohm output impedance of the driver. The resulting output has a risetime <1ns and amplitude >1Vpp into 50 ohms, and surprisingly flat pulse top.

author: dannyf
author_rank: Super Contributor
timestamp: February 04, 2014, 07:29:13 pm
content: I thought the edges on an avalanche transistor is pretty fast - sub ns if I may recall.

author: free_electron
author_rank: Super Contributor
timestamp: February 04, 2014, 08:10:06 pm
content: Quote from: dannyf on February 04, 2014, 07:29:13 pmMy pulser reaches 180pS... (Jim williams based design but using a stripline as tank capacitor

author: JBeale
author_rank: Frequent Contributor
timestamp: February 04, 2014, 08:21:08 pm
content: If you only need 3.3V pulse with tr < 2 nsec you can use a $25 Raspberry Pi with no extra circuits at all. The pulse comes from GPIO 18 (pin #12 on P1 header), and this also gives you a programmable pulse width of any length from 4 nsec upward in 2 nsec increments. I measured 1.6 nsec risetime with a 200 MHz scope & 200 MHz probe, so it is probably faster, but I just can't measure it.Here are the details:http://www.raspberrypi.org/phpBB3/viewtopic.php?f=72&t=67741#p495574

author: ice595
author_rank: Regular Contributor
timestamp: February 04, 2014, 09:12:52 pm
content: sorry to be a little bit off topic here.how can you tell the type of dielectric used in a coax cable by visual inspection?I spliced open my CATV coax cable and the dielectric material is white and seems pretty solid.since it has a velocity factor of 0.85, is the dielectric made of foam then?and also, how would Solid Polyethylene (PE) look like?

author: jwm
author_rank: Regular Contributor
timestamp: February 05, 2014, 01:03:23 am
content: Quote from: JBeale on February 04, 2014, 08:21:08 pmSurely you kid. replacing a 12 cent chip with a combined 110ma drive output and schmitt triggering with a $40 computer requiring a boot up, has all the complexity of running a whole OS, uncertain timing due to not having a RTOS, a power consumption in the Watts vs mWatts, an unknown board layout that is most certainly not designed for fast rise times and coupling your GPIO signal with who knows what and a measely 16ma drive output? It needs to have that rise time while driving a 50 ohm load, I don't know the impedance of the path between the raspberry pi CPU and out through that header, but I doubt its 50 ohms.Getting fast rise times is a pure analog thing, completely unrelated to what you use to trigger them, whether it be a computer like the raspi, a uC, or a plain RC delay. As in, you'd need a real driver even if you triggered from a raspi.

author: JBeale
author_rank: Frequent Contributor
timestamp: February 05, 2014, 02:08:58 am
content: Nothing wrong with your solution. The R-Pi happened to be convenient to me, and I know that a lot of folks have one already lying around, so it cost no money and very little time to connect up for a fast edge for a one-off measurement. For the kind of TDR I was doing, you trigger your scope off the pulse, it doesn't matter what latency the OS has. Of course the R-Pi was not intended for this purpose... but it works. The internal resistance of that particular GPIO pin driver seems to be about 44 ohms according to my measurement of the 10 nsec pulse. It drove my laser directly with just a 25 ohm ballast resistor, and it can also drive a 50 ohm cable with enough signal level for the scope to detect easily- without any additional driver.

author: ice595
author_rank: Regular Contributor
timestamp: February 05, 2014, 02:43:35 am
content: but does the R-Pi have a fast enough ADC to detect the delay between the transmitted edge and reflected edge?would be neat to make an inexpensive self-contained TDR.what great idea that would be for my final year college technical project

author: jwm
author_rank: Regular Contributor
timestamp: February 05, 2014, 03:10:11 am
content: Quote from: ice595 on February 05, 2014, 02:43:35 amI would think a fast comparator with one of its inputs driven by PWM to set its trigger level would be a better way to go. off the top of my head, Set your trigger level with the pwm, send the pulse and have the output of the comparator charge a cap, after it triggers use the slower ADC to read the voltage off the cap. sort of like an integrating ADC.... sort of.

author: JBeale
author_rank: Frequent Contributor
timestamp: February 05, 2014, 03:37:09 am
content: Quote from: ice595 on February 05, 2014, 02:43:35 amI was relying on the scope to do the measurement, the R-Pi has no ADC of any kind. It might be possible to do something clever with a fairly cheap addon, like those DVT dongles that be programmed as general-purpose 1500 MHz+ receivers. Note that the same R-Pi GPIO pin can also generate CW or FM around 100 MHz and has been used to generate FM broadcast-band signals.http://www.icrobotics.co.uk/wiki/index.php/Turning_the_Raspberry_Pi_Into_an_FM_Transmitter

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 05, 2014, 05:28:06 am
content: I built this many years ago; it used a PH2369 (close enough).Response on Tek 475, 10ns/div.I also built a fancier one (100V generator, pulse line option, and trigger input included).Note the avalanche element is a 2N3904 -- I have tried many transistors, most do not avalanche so dramatically or at all. I have been able to avalanche all 2N3904s that I have tested, which apparently is unusual for a planar transistor type. RF transistors seem to do it more commonly, which sort of explains 2N2369 (it's primarily a switching transistor rather than RF, but it's fast in any case). RF transistors also have lower breakdown voltages, which suggests it might be reasonable to build one of these with a 9-12V supply and charge pump rather than a flyback converter.Here's the response (350MHz TDS460), top being direct coax, bottom 10x probe on same node (obviously, the coax cable is longer than the probe cable, so it's time shifted).Tim

author: BravoV
author_rank: Super Contributor
timestamp: February 05, 2014, 05:46:30 am
content: T3sl4co1l, suggesting you to read theJim Williams Pulse Gen thread, if I'm not mistaken, you need to have tank capacitor to have a flat line after the pulse top to get accurate measurement.

author: BravoV
author_rank: Super Contributor
timestamp: February 05, 2014, 06:16:53 am
content: Alan, please, if you had the chance, really love to see the use of JW pulser at the 33GHz beast there to demonstrate the TDR, a centi meter resolution or use to profile the cable velocity factor maybe ?..<drooling>..Guys, just fyi, this w2aew's screen shot of the beast when he tested at his JW pulser, a braggin 150 pico second.

author: JBeale
author_rank: Frequent Contributor
timestamp: February 05, 2014, 07:22:54 am
content: I don't have any exotic scope, and with a different probe and cable setup I see only 4.6 nsec risetime. But just for reference I wanted to show that the Raspberry Pi can generate a pulse good enough for measuring cable length, without other circuits. In this case looks like I've got a 120 nsec round trip. You can see the pulse polarity switch when I change from end-open to end-shorted. I'm connecting the R-Pi GPIO directly to the SMA connector, so the negative-going return pulse gets clamped by the ESD-protection diode making it look more square.I also tried using a series resistor, a little 0603 part between the GPIO pin and the coax feed but it kept breaking off. With no resistor the return pulse amplitude is clamped more by the output driver trying to hold the pin at ground, but you can still see it well enough.

author: Redcat
author_rank: Regular Contributor
timestamp: November 29, 2014, 06:17:45 pm
content: Hi Alan, I have seen your video some days before and decided to build up such a nice fast edge oscillator circuit based on a 74AC14N today on this cold saturday morning.As always a great video and something interesting to play with. Had only a 100nF cap, so it's RT is more like 5-7ns but thats great for the moment.I would have a question: I have seen the so called "blue ring meter" (but i don't have one) on youtube, where you can measure an inductor/coil and see it's quality or condition based on the resulting ringing after a fast pulse was applied...Hmmm...it came to my mind that your TDR might give a similar result - so ringing or more ringing based on the coils condition (?) - I would like to test some e-motor windings.Can it be used for this application (would be great) and how would it be connected?Thanks and nice greetings, Tom

author: marcusj
author_rank: Newbie
timestamp: April 28, 2015, 09:35:56 am
content: For what it's worth, I also did a PCB build of Alan's circuit. I posted the full details onmy blog, but here's a couple of snaps of the finished result. I had thePCB'smade by Dirty Boards PCB's in China.

author: dom0
author_rank: Super Contributor
timestamp: April 28, 2015, 11:25:04 am
content: Quote from: ice595 on February 04, 2014, 03:18:47 amThose things are crap anyway. For fast signals you need a feed-through terminator.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 28, 2015, 11:26:45 am
content: Do any of the Rigol DSOs have TDR built into them?

author: tggzzz
author_rank: Super Contributor
timestamp: April 28, 2015, 12:33:01 pm
content: Quote from: Rufus on April 22, 2013, 02:01:30 amYes, it can:drive the circuit with a PRBS, and you will get the standard "lobed" spectrumdraw a straight line along the top of the lobes, slope 20dB/decadethe knee frequency, Fknee where the top of the lobe is 6.8dB below that linethe amplitude falls at >20dB/decade above the knee frequencythe 10%-90% transition time Tt = 0.5/ FkneeSource: Howard Johnson's "High speed digital design, a handbook of black magic", page 3

author: joeqsmith
author_rank: Super Contributor
timestamp: April 29, 2015, 03:39:29 am
content: Quote from: tggzzz on April 28, 2015, 12:33:01 pmExcellent book.

author: tggzzz
author_rank: Super Contributor
timestamp: April 29, 2015, 08:32:02 am
content: Quote from: joeqsmith on April 29, 2015, 03:39:29 amJust so. I saw mine on a shop's bookshelf in 1993, bought it, and currently it spends as much time off the shelf as on it.Over the years I've had similar experience with these other books:https://entertaininghacks.wordpress.com/library-2/bookshelf/

author: GeezerWhiz
author_rank: Newbie
timestamp: July 27, 2015, 05:46:59 pm
content: A very simple way tocomparereally fast rise/fall times is to dump your pulse into a short length of coax with a shorted end. Of course, at the "input" end, you see the sum of your input pulse and a delayed reflection of it. As rise/fall time gets faster, your reflected pulse is being subtracted from the lower voltage of a faster falling tail end of the "input" and eventually, it gets driven negative. A little fast diode detects this event as fall times decrease. (This was the basis of a fall time discriminator on a gamma ray detector, with about 20cm of coax putting the compare time out about a nanosecond.) So, coax with a well known pulse velocity can generate an approximation of rise/fall times.

author: hendorog
author_rank: Super Contributor
timestamp: July 27, 2015, 09:14:57 pm
content: QuoteThe DS4000 does have a fast rise time (500ps I believe) output which can be used for TDR. It works well with just a BNC 'T' and setting the scope input to high impedance.However it was better when I made a simple resistive splitter and set the scope to 50ohms. That removed the ringing.I don't think the DS2k has this output, at least the manual didn't show it. I don't know about the other models either.

author: S185149
author_rank: Contributor
timestamp: October 14, 2017, 02:31:44 am
content: I'm playing along with Alan's fine videos and put this circuit together and put it on a scope. I put in a pot for the 6.8k resistor so I can experiment with it. All I have right now is the 74HC14. I'm running it on a 5V source and channel one is connect to the back side of the BNC connector. I connect channel 2 to the BNC via a 3' length of coax. Print11, attached, is the screen shot of the scope. As you can see, the voltage off the coax is 10 times that off the back of the connector. In Print12, I added a 50 ohm termination at the scope. Both channel are reduced but channel 2 is still 10 time channel 1. Is the higher voltage from the coax the result of reflections adding up?Thanks,S18

author: Redcat
author_rank: Regular Contributor
timestamp: October 14, 2017, 10:25:54 am
content: @S185149I would guess, your scope is set to 10x probe on channel 1 and 1x on channel 2 (which would be right on 2).Just a wild guess.You can get even faster rise times with the 1GHz version of the 74HC14, the Potato Semi 74G14AS.I did rebuild this curcuit (actually the pcb looks similar to the one Alan got once from a viewer) a long time ago with this chip and it works nicely.

author: S185149
author_rank: Contributor
timestamp: October 14, 2017, 03:14:02 pm
content: Major brain fart on my part. I woke up this morning and, the manual probe setting bit me.

author: Redcat
author_rank: Regular Contributor
timestamp: October 14, 2017, 05:06:30 pm
content: You are welcome.It just looked so obvious...haha that can happen. Have fun with the TDR. Cheers

author: S185149
author_rank: Contributor
timestamp: October 14, 2017, 07:55:23 pm
content: Quote from: Redcat on October 14, 2017, 05:06:30 pmWould you believe, that's the second time I've fallen for that this week!