Fast impulse from fast-rise-time square wave (microwave) - Page 1
Forum section: electronics / rf-microwave

author: ezalys
author_rank: Frequent Contributor
timestamp: October 31, 2018, 01:23:12 am
content: Suppose I have a very high bandwidth (harmonics up to/beyond 12.5 GHz) square wave (from say a ONET8501P). What's the most reliable way to derive a train of narrow impulses from this square wave? It seems to me that differentiating and rectifying might be a good way to go... but I probably shouldn't trust a single capacitor to be a good differentiator up to high frequencies. Any recommendations from the microwave gurus out there?

author: rhb
author_rank: Super Contributor
timestamp: October 31, 2018, 02:04:53 am
content: I'm certainly not a microwave guru, but if you need a signal source for testing I suggest you check out Leo Bodnar's fast rise time 10 MHz square wave modules.http://www.leobodnar.com/shop/index.php?main_page=index&cPath=124&zenid=58b771673c16e5a14713fc19019d82e8

author: ezalys
author_rank: Frequent Contributor
timestamp: October 31, 2018, 02:12:23 am
content: I ultimately want to build a sampler from a circuit like that (modified for differential out)... hence why I need to derive sharp pulses from it.

author: rhb
author_rank: Super Contributor
timestamp: October 31, 2018, 02:45:19 am
content: I assume you have a *very* fast scope. My BNC version was measured by Leo at 36 pS. Nominally that's a 14 GHz spike. My fastest scope is a LeCroy that goes to 1.5 GHz. This portends an interesting thread if you tell us what worked. I'm hoping to hack my LeCroy to 3 GHz.

author: ezalys
author_rank: Frequent Contributor
timestamp: October 31, 2018, 03:20:47 am
content: I have access to a modern 30 GHz sampling scope. I also have the sampling module from a frequency counter, which can be hacked into a high quality sampler with a bandwidth of 12 GHz. I also am not attached to retaining the full 12 GHz bandwidth of the square wave... if I can obtain a pulse train where the teeth fall 3 dB or so at 4 GHz I'd be very happy. 6 and I'd be ecstatic. Whatever I make will be on Isola FR408 so I'm not trying to be a hero.

author: awallin
author_rank: Frequent Contributor
timestamp: October 31, 2018, 05:43:33 am
content: seems like a comb-generator (impulse train) is slightly different from a sharp square wave generator (?)e.g.http://www.vk2zay.net/article/41orhttp://www.ke5fx.com/cg.htm

author: tomato
author_rank: Regular Contributor
timestamp: October 31, 2018, 07:02:47 am
content: Step recovery diode.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 31, 2018, 08:21:28 am
content: A tunnel diode might help, W2AEW have an intro with more resources linked in the video description:Another technique I would look into is the chirped pulse technique used in optics. In theory, it should work for microwaves, too.Do you have any specification numbers for the pulses you want to achieve?

author: tggzzz
author_rank: Super Contributor
timestamp: October 31, 2018, 08:42:43 am
content: Quote from: RoGeorge on October 31, 2018, 08:21:28 amIt certainly works for radars. In the 70s there were radars with 1MW peak and 10kW mean outputs.

author: tggzzz
author_rank: Super Contributor
timestamp: October 31, 2018, 08:55:36 am
content: You didn't mention voltages, so...Take the output, split into two, invert one, have slightly different delays on each output, recombine them. If you have a differential output, the splitting is unnecessary.Differential delay could be via wire length or a digital delay ic such ashttps://www.digikey.co.uk/products/en/integrated-circuits-ics/clock-timing-delay-lines/688?k=programmable%20delayThe details are left as an exercise for the student.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: October 31, 2018, 09:56:16 am
content: I have used shorted stub method (made from connectors lying around) just for playing purposes. Leo's pulser on 500MHz scope:

author: jungle vegetable
author_rank: Contributor
timestamp: October 31, 2018, 12:24:59 pm
content: Both small-value SMD capacitors and inductors (just a few mm of wire across the microstrip or connector) work quite well for this application (and, specifically, with ONET8501P). Inductors made from wire are easier since some tuning is required to obtain maximum pulse amplitude. Pulses, obviously, will be of alternating polarity, so a frequency doubler (or a passive mixer in general) might be required afterwards

author: jungle vegetable
author_rank: Contributor
timestamp: October 31, 2018, 12:40:59 pm
content: Here are some pulses made by shorting the SMA connector with a 1mm hex key that was lying around (100MHz square wave is from the ONET8501P, one division is 625ps, dots/samples are 5ps apart)

author: Marco
author_rank: Super Contributor
timestamp: October 31, 2018, 12:42:39 pm
content: If you are making a PCB any way, just put a shorted stub on the transmission line.

author: ezalys
author_rank: Frequent Contributor
timestamp: October 31, 2018, 02:27:19 pm
content: Why does a stub work? How long should the stub be?

author: Marco
author_rank: Super Contributor
timestamp: October 31, 2018, 03:01:33 pm
content: Quote from: ezalys on October 31, 2018, 02:27:19 pmGood question, I thought I understood why ... I only have a very approximate understanding though on reflectionThe principle is easy, the current pulse splits at the stub, then when the negative reflection comes from the grounded end it cuts the signal to the other branch to zero. So instead of a square wave you get pulses at the edges.That said, how exactly it works with impedances I have no idea. When I put it in a simulator it tells me the stub impedance should be 25 Ohm to make it work perfectly, with other impedances you get multiple reflections and a screwed up trailing edge. Is the simulator screwing up? I have no idea.PS. twice the length of the stub divided by (lightspeed*Velocity-factor) gives the length of the pulse, ignoring rise time.PPS. if you wonder why the 36 mA current source, the ONET8501P can have a 900 mV signal, which with CML, it's internal 50 Ohm resistors and assuming it's driving a 50 Ohm terminated transmission line means it's switching 36 mA.

author: mark03
author_rank: Frequent Contributor
timestamp: October 31, 2018, 03:12:43 pm
content: Quote from: tggzzz on October 31, 2018, 08:42:43 amAFAIK with radar you never have the pulse-compressed (non-chirped) signal at high power---there's no reason you'd want to. The chirp is collapsed as part of the receive/imaging path.

author: jungle vegetable
author_rank: Contributor
timestamp: October 31, 2018, 03:31:29 pm
content: Btw, LTspice is quite excellent at simulating transmission lines (it even has lossy ones, but they aren't really needed if you do short runs of FR4 or coax). Shorted stub effectively acts as a filter to block the DC component of the square wave (could also be approximated as a parallel LC notch filter)

author: Marco
author_rank: Super Contributor
timestamp: October 31, 2018, 03:41:17 pm
content: Quote from: jungle vegetable on October 31, 2018, 03:31:29 pmThat's the theory, but the moment you start contemplating the time domain impact of impedance mismatches it gets a bit headache inducing. From the point of view of the feed in transmission line, the stub is in parallel with the output transmission line after all ... so there's an impedance mismatch. That said, I think the simulator (simetrix in this case) has it right and the stub should be 25 Ohm impedance. Or you could put two 50 Ohm impedance stubs either side of the transmission line so you can use the same trace widths everywhere.

author: jungle vegetable
author_rank: Contributor
timestamp: October 31, 2018, 04:56:41 pm
content: Quote from: Marco on October 31, 2018, 03:41:17 pmSure, that is only an approximation and to get accurate result you need more than one LC circuit (it think SPICE simulators do something along these lines anyway). Interestingly enough, stub impedance controls the falling edge decay -- impedance above 25 ohm generates (stepped) exponential decay while below 25 ohm results in decaying oscillations

author: tggzzz
author_rank: Super Contributor
timestamp: October 31, 2018, 05:16:01 pm
content: Quote from: mark03 on October 31, 2018, 03:12:43 pmYou only have low power non-chirped signals in RoGeorge's graphic.But I agree with your statement. I believe the radar's chirped Tx signal (2 in the graphic) was synthesised rather than stretched from a pulse (1).

author: aandrew
author_rank: Frequent Contributor
timestamp: October 31, 2018, 05:43:34 pm
content: Marco, what software are you using for your simulation shown in the schematic and graphing output?

author: mark03
author_rank: Frequent Contributor
timestamp: October 31, 2018, 05:46:01 pm
content: Quote from: tggzzz on October 31, 2018, 05:16:01 pmAre you sure about this? I'm not familiar with the laser technique, but I heard the inventor interviewed on public radio and I thought they said the whole thing was designed for the purpose of achieving higher peak-power laser pulses. I think they were saying it couldn't be done before this invention because of some peak-power limitation intrinsic to the lasing apparatus.Back to the original goal, it would be cool if there were some sort of dispersive transmission line which you could drive with a chirp and get a real bang out the other end. I imagine obtaining the ultra-wideband chirp (DC-ish to 10s of GHz) in the first place is the hard part. Radar chirps are nowhere near that broadband AFAIK.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 31, 2018, 06:07:52 pm
content: Quote from: mark03 on October 31, 2018, 05:46:01 pmWith lasers people definitely recombine the amplified chirp into a short pulse with high peak power. The reason for this is to strongly drive non-linear interactions that depend on peak signal strength. For instance, if you want to do optical comb generation with a non-linear fiber, you want high peak power to drive high harmonic generation. Also, there are experiments people do on atoms where they need a high peak electric field compared to the electric field of the nucleus.Radar systems are (ideally) linear, so it would make more sense to transmit the chirp then apply the phase shift to bring it back to a pulse on receive. This avoids non-linear interactions while preserving the good time resolution of a fast pulse. If there are microwave applications where the high peak power pulse is desirable, you certainly could shift the chirp back into a pulse.

author: jungle vegetable
author_rank: Contributor
timestamp: October 31, 2018, 06:23:50 pm
content: Quote from: mark03 on October 31, 2018, 05:46:01 pmThere are NLTLs made from transmission lines and varactors:http://www.its.caltech.edu/~hajimiri/pdf/non-linear.transmission.pdfSome specific NLTLs manufactured on GaAs MMIC processes achieve sub-picosecond risetimes

author: Marco
author_rank: Super Contributor
timestamp: October 31, 2018, 06:37:46 pm
content: Quote from: aandrew on October 31, 2018, 05:43:34 pmThe demo version of Simetrix, it's limited in nodes but the GUI is the best I've found. It's also very resilient against convergence problems relative to other Spice simulators I tried.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 31, 2018, 08:13:55 pm
content: SimSmith might be a free alternative. I think is based on SPICE, same as LTspice, Pspice, etc.http://www.eas.uccs.edu/~mwickert/ece5250/notes/ADS_session2.pdfIn the first 5 minutes there is an intro about what can, and what can not do, by W0QE:

author: Marco
author_rank: Super Contributor
timestamp: October 31, 2018, 08:28:33 pm
content: Linear and frequency domain simulation are the exact opposite of what he needs.He needs an open source equivalent to Agilent ADS(Doesn't exist.)

author: ezalys
author_rank: Frequent Contributor
timestamp: October 31, 2018, 08:42:43 pm
content: I have access to ADS and microwave office. What I'd prefer to do is learn to design this with pen and paper and maybe do some final optimization in mwo.

author: rfeecs
author_rank: Frequent Contributor
timestamp: October 31, 2018, 10:12:36 pm
content: This is going off in a lot of crazy tangents. In reality, you don't worry about the line impedance so much. You don't have nice impedances on the different ports. You may be driving diodes, etc.People figured out samplers using diodes long ago, as in the sampling scopes of the 1960s. You can look at the manuals for Tektronix samplers like the S4 and S6 to see how they did it.The S6 used the short circuit transmission lines. They referred to them as "clip lines".The S4 used a "traveling wave" sampling gate, patented by Tektronix:https://patentimages.storage.googleapis.com/52/97/2c/b6fbddbb95dcb8/US4647795.pdfThe gate is generated using just one edge of the strobe line and is set by the length of a transmission line. Here's a very brief explanation:http://w140.com/frye_s4_gate.pdf

author: ezalys
author_rank: Frequent Contributor
timestamp: October 31, 2018, 11:09:04 pm
content: Yeah I'm kinda disappointed at how off topic people are being. You can't source cheap tunnel diodes or high frequency SRDs anymore in low volume and NLTLs and all that are very expensive and tricky and what the hell does chirping have to do with anything? I just want a fast pulse from a fast transition and to understand how such a circuit functions.But okay. Yeah. The S6 and S4 are nice designs that I've looked into before. The S4 seemed a bit complex but the S6 seemed pretty understandable. I like the clip-line idea a lot. I imagine I could prototype such a thing on a PCB using little pieces of coax. 100 ps seems really short... The scheme reminds me of finite differencing. You can choose a shorter dt (shorter clip-line) but then you sum with a less time-shifted version of the same signal, meaning if the rise time is a bit slow relative to l/c, you really kill the pulse amplitude. Is this intuition correct?

author: rfeecs
author_rank: Frequent Contributor
timestamp: October 31, 2018, 11:22:09 pm
content: Quote from: ezalys on October 31, 2018, 11:09:04 pmSounds about right. But ideally, if your rise time is much shorter than the clip line delay, you will get the full amplitude and the pulse length will mostly be determined by (2x) the line delay time.

author: Marco
author_rank: Super Contributor
timestamp: October 31, 2018, 11:25:13 pm
content: Quote from: rfeecs on October 31, 2018, 10:12:36 pmWhose reflections on the transmission line feeding them will ring with whatever you use to differentiate the pulse. You can't keep everything compact enough at these frequencies to avoid it, everything is a transmission line.QuoteWith a discrete design with modern components the design space is at the same time not what it was in the 60s, nor what it was for Tektronix once they started wire bonding hybrids and using MMICs. With the hybdrid in the SD-xx they could just scale their old designs, but even with 0201s that's unlikely to be optimal.

author: ezalys
author_rank: Frequent Contributor
timestamp: October 31, 2018, 11:26:34 pm
content: Okay that seems pretty reasonable. The onet8501p has a 35 ps max rise time so a 100-200 ps stub seems pretty reasonable.Also, I'm trying to scale in cost, not in frequency.

author: Marco
author_rank: Super Contributor
timestamp: October 31, 2018, 11:47:57 pm
content: Supposedly the designer of the early Tek sampling heads did a 20 GHz sampler on FR4 with discreteswell over 10 years ago. So in my opinion that's what you should shoot for

author: awallin
author_rank: Frequent Contributor
timestamp: November 01, 2018, 07:11:58 pm
content: seems radio-astronomers doing VLBI do what they call "phase-cal" with a pulse train that has harmonics up into the multi-GHz range.they make a square-wave with sharp edges, use a capacitor to differentiate the square wave, and then an RF-switch to pass only pulses of the desired polarity, see e.g.https://www.haystack.mit.edu/geo/vlbi_td/BB/023.pdf

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: November 02, 2018, 12:19:11 am
content: Looks like you get unpleasant component values if you want to differentiate in a 50 Ohm system.This is a 50 Ohm microstrip on 0.8mm FR4, the usual Bungard stuff that you use for self-etching in .de.First picture is just a scalpel cut through the stripline. That gives a nice peak, but watch the vertical scale.0.5 pF 0603 has the rising edge quite OK, but the decay is not differential-like, it simply passes thestep for the foreseeable future.Blue trace: reflection back to the TDR source,yellow = after 20 cm of coax + cut microstrip into normal 20 GHz sampling channel.I cannot easily attach a coax line in the middle of the board because there are no viasto the GND side. At least not today. Bedtime now.The board was an experiment to find the right eps-r of 0.8mm Bungard-FR4. 56 mil seemsto be the correct microstrip width for 50 Ohms. The circle was intended to be a resonatorthat could be seen on the VNA. That was before I had the TDR.regards, Gerhard

author: rhb
author_rank: Super Contributor
timestamp: November 02, 2018, 01:02:10 am
content: I'd like to note that a coax stub and mercury wetted relay were a common (only?) method for generating short impulses before semiconductors got fast enough to replace them.

author: Marco
author_rank: Super Contributor
timestamp: November 03, 2018, 10:35:21 pm
content: I don't think a volt or so is going to cut it to really kick a Schottky sampling gate into gear. If you are satisfied with single digit GHz you could copythis designusing the MSPD2018 Sampling Phase Detector, which is essentially a packaged sampling gate.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: November 04, 2018, 05:32:23 pm
content: Quote from: mark03 on October 31, 2018, 03:12:43 pmBut that's the point of the chirp, that you can run the transmitter longer at its abs. max. ratings and then compress the output in time to get a peak power larger than abs. max.Also, the transmitted signal would not be so visible to a simple diode detector.cheers,Gerhard

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: November 04, 2018, 09:26:33 pm
content: Quote from: Marco on October 31, 2018, 03:01:33 pmYour simulation was good, including the 25 Ohms.I used 28mm pieces of unknown 50Ohm cable, probably rg405.One 50Z-Line produced the messed-up trailing edge, two in parallel gave a nice pulse.Blue = reflections back into the TDRYellow = transmission into load channel.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: November 04, 2018, 09:34:01 pm
content: Then I shorted the lines to 9mm. That is no longer pretty. Too muchparasitics in comparison to line length. If you want to go faster, youwill probably need etched microstrip.cheers, Gerhard

author: ezalys
author_rank: Frequent Contributor
timestamp: November 04, 2018, 09:44:30 pm
content: Gerhard_dk4xp, I'm curious, what scope is this that you're using?

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: November 04, 2018, 09:49:23 pm
content: HP54750A with 18 GHz differential TDR plug in and a dual 20 GHz ordinary sampling plug in.(54754A and 54751A) I also have a 50 GHz plug in, but I avoid using it unless really needed.It has 2.4mm connectors and is easy to kill.regards, Gerhard

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: November 18, 2018, 09:53:08 am
content: I can see a lot of people are trying to use 50Ω microstrip or coax as a stub. However, Tek sampling heads use really wide and low impedance stubs - they call them clipping lines. Here is a snapshot of 40GHz sampling head front end. I have loosely commented relevant components. SRD generates the step pulse that gets differentiated and becomes a sampling pulse. Sampling signals are differential (unfortunate collision.)Differentiating differential pulse signal here you go.Leo

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: November 18, 2018, 10:18:30 am
content: Food for thought - the clipping lines are low impedance probably because SRD is a low impedance pulse source and 50Ω clipping line will not be able to fully suppress the signal.Notice that differentiating stubs are located close to the pulse source instead of dragging the pulse halfway across the head first.Microstrips delivering the gating pulse to the sampler are quite long - perhaps to buy the sampler some time to finish its sampling business until reflections start coming back. If the gate has closed by that time then they don't really matter as long as they don't reopen the sampler again.Leo

author: Marco
author_rank: Super Contributor
timestamp: November 18, 2018, 10:48:10 am
content: Quote from: Leo Bodnar on November 18, 2018, 09:53:08 amYeah, but their substrates are a lot thinner ... you can go lower than 50 Ohms, but not too much lower with a standard PCB. Flex PCBs are quite reasonably priced though, so that would work.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: November 18, 2018, 12:40:27 pm
content: Alumina's Eris almost 10 so they need to be thin. This seems to be using quartz which is closer to FR4. The main reason is probably low loss?I know you have figured that the stub in the middle of 50R line should be 25R impedance but I will throw in my simulation I did for somebody else anyway.It uses voltage sources with 50R impedance but it is equivalent to current sources with 50R termination and for this simulation makes no difference.With 25R clipping line you lose some of the pulse aplitude, of course.CheersLeo

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: November 18, 2018, 01:39:29 pm
content: I have 2 defective samplers from 83483A / 54751A plugins( I made one workingplugin from two semi-defective ones) . Part number is Agilent 5086-7699, available asa replacement part. Someone is selling on ebay for $120 (working with garanty),somebody else for $160 (defective) and someone for 530 Pounds ("untested", haha).This is the type of the samplers in the 20 GHz plugin that I used to make themeasurements above.Repairing the plugins is easy, you just need a Torx bit and a SMA/PC3.5 wrench.You can just exchange the SMA bricks (really PC3.5), nothing else.I decided to open one of them. Pictures are what my Sony xpedia Z5 cell phone can do,but even under the *50 microscope I cannot see all the details. The substrate seems to bequartz, it is translucent. The squares on the paper are 5mm.The cavities were filled with absorber foam to avoid waveguide resonance effects.I think that is outside of the diy area.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: November 18, 2018, 07:16:04 pm
content: similar sampler for a microwave frequency counter:http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1986-02.pdfcheers, Gerhard

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: November 24, 2018, 01:19:09 pm
content: I have tried stub pulse former yesterday and it did work for me too.Leo

author: ezalys
author_rank: Frequent Contributor
timestamp: November 25, 2018, 12:28:36 am
content: I have to imagine that the 25 ohms is necessary so the clipping line is matched to the parallel combination of the source and load, which are both 50 ohms. Suppose a wavefront propagating towards the junction coming FROM the short at the end. One wouldn't want reflections off the node where the clipping line meets the transmission line.