Calibrating a scope with out the right tools.  - Page 1
Forum section: electronics / general

author: Zenwizard
author_rank: Regular Contributor
timestamp: February 04, 2022, 07:45:48 pm
content: To start. I KNOW I should not however I am getting a LOT of questions about how to do this given my videos. So here is a questions that is being posed and looking for advice on signal sources to add to the lab for a viewer requested topic:A question for the viewers: I have been getting a lot of questions about scope calibrations and repair. Currently I have gone through all the equipment needed to calibrate a scope under 1 Ghz of bandwidth and have videos showing up to 500 Mhz frames. I have gotten a lot of questions from viewers about how to do a scope calibration without the correct gear. Seeing as the channel is for the viewers, I am curious as to what people have in their labs and where I should target some gear so that I can attempt to make a video about calibrating a scope without the recommended signal sources. My current function generators in the lab I would say are woefully underpowered for this task only hitting up to 2 Mhz and an unstable 20 Mhz. So I am looking for recommendations as to what to acquire for this challenge.The signals needed are fairly mundane however The high frequency compensation of the vertical will need to be some fast edges for the rise time. Other than that, a descent function generator should get us at least in the ball park.I have the stable clock problem solved with an upgrade to the 10 Mhz reference in the lab Video will be out on that in the near future.Looking to see what everyone things we should add as a function generator for the lab. The one requirement I would like to have is a 10Mhz input. Let me know what you think.Zen

author: bdunham7
author_rank: Super Contributor
timestamp: February 04, 2022, 08:01:43 pm
content: Are you talking about calibrating older analog CROs with physical trimmers? Or computer-controlled analog CROs like the Tek 24xx series? Or DSOs?The answer for the first and maybe the second is you can mostly get by with a good RF generator and a PSU, plus either an AWG or a pulse generator. You may need a fast-edge generator of some type as well. There's no need for 10MHz references or accurate timebases--the adjustments are 1-3% at best.For DSOs, you'd have to read the manual. They're all over the place.

author: Zenwizard
author_rank: Regular Contributor
timestamp: February 04, 2022, 08:22:32 pm
content: I personally have all the tools needed to calibrate anything in the Textronix 5xx 4xx 7xxx and 22xx scopes. I don't have any of the 22xx examples yet but this is for a lack of scopes to cal not from the equipment needed. The question I am getting asked often is how do I do it with out a PG506, SG503, SG504, TG501, and TD pulser including a type 284.The current state of the lab is I have 2 x Tek 2 Mhz FG's and a 20Mhz B&K that is nowhere near stable enough. My requirement for the 10 Mhz input is because I am planning on sourcing something to add to the lab to compare how close you can get with out the proper tools. I have an SRS FS725 Rubidium reference so anything I buy I would like to take advantage of that. So anything that I source I will be using for future work on the bench so having it backed up by the Rubidium would be nice.Yes and modern function generator or awg is what I am looking for. The end goal of this project is how close can you get with out blowing the budget. I know this can be done with a 5 and 6 figure unit but If I was going to go that route I would just get a fluke scope calibration and call it a day. This would however defeat the purpose of this project.Zen

author: bdunham7
author_rank: Super Contributor
timestamp: February 04, 2022, 09:25:28 pm
content: I don't have any of those 'correct' tools. Up to 100MHz, you can do what I think is a fine job with just a hacked Siglent SDS2142X, a Leo Bodnar pulser, a PSU, a DMM and a decent DSO. For higher bandwidth, like the 485 or the 7xxx series, I add an old HP 8656A RF Generator that I repaired and calibrated myself with equally horrifying ad-hoc DIY methods. I'm also able to get the 24xx types through their unforgiving automated calibration using this set of tools. The thing about calibrating analog oscilloscopes is that it is easy enough to check the result.The SDS2000X AWG has a 10MHz input/output, but it won't do you as much good as you might think. The outputs may be stable, but the frequency values may vary a bit (sub-ppm) in unexpected ways. None of that can possibly matter to an analog CRO since their minimum errors will be 10,000X as great.

author: niconiconi
author_rank: Frequent Contributor
timestamp: February 04, 2022, 10:22:51 pm
content: My understanding. The calibration and adjustment process of a modern sub-GHz DSO is pretty simple, all you need is calibrated DC and AC sources, the software handles the rest. For example, for a traditional Tektronix TDS/TBS scope, you need to:1. Send a bunch of accurate DC voltages into the scope, e.g. 20 V, 2 V, 1.6 V, 0.8 V, 0.4 V, 0.32 V, 0.2 V, 0.16 V, 0.08 V, 0.04 V, 0.03 V, 0.02 V, 0.015 V, 0 V, both polarities.2. Send a bunch of leveled AC sinewaves into the scope, e.g. 5 KHz, 1 MHz, 50 kHz, 20 MHz, 100 MHz (max bandwidth), at amplitude 1 Vpp, 2.5 Vpp.This is 90% of the work. Then you also need:1. 1 KHz, 0 mV - 800 mV edge signal from a pulse or square wave generator for calibrating the trigger signal.2. Time-mark generator for checking timebase accuracy.Then you're done.I think the time-mark generator is the easiest test equipment here. Many oscilloscopes only have a low-performance timebase, to verify its accuracy you only need a 10 ppm reference clock. in 2020 this is almost trivially solvable. For best results you can use a $50 crystal oven timebase for sub 1 ppm accuracy. In this age, any signal generator powered by a Direct Digital Synthesizer can meet the requirement with a good timebase. However, in calibration procedures, it may need to output not a standard square wave but a special pulsed waveform, a suitable trigger output may also be necessary, plug-and-play may not work. But it should be an easy issue to solve with suitable configuration (e.g. programming a waveform) or building an external circuitry. Alternatively, a complete DIY time-mark generator can be built for $30.The edge signal is only a minor detail, any signal generator should be okay.Then you need an accurate DC source, 0.5% accurate. Getting an accurate DC source is difficult and expensive, but I think you can always adjust a voltage manually with a multimeter. Even a relatively cheap meter can easily reach 0.5% at DC. For manual performance verification that's all you need. For automatic adjustment, the voltage should be stable for a few minutes to allow the scope to complete one adjustment step.Finally, a leveled sinewave generator, which is basically an RF signal generator, but it needs to have an amplitude accuracy around 3% to 7% from 0.5 Vpp to 5 Vpp, from near-DC to sub-GHz. This is the most difficult and challenging test instrument in the entire calibration procedure, 80% of the signal generators will NOT be able meet this requirement. 3% is 0.256 dB in power, but most RF signal generators are only 1 dB accurate. There are several workarounds:1. Used equipment. Tektronix SG50x series is the vintage classic. But the premise of the question is "without correct equipment", so...2. Leveling the amplitude by manual adjustment with an accurate RF power meter, as long as the signal generator's amplitude can remain stable for a few minutes. For example, R&S NRP-Z81 RF power meter has an amplitude uncertainty around 0.05 dB within +20 to 25 degrees C. But RF power meters are expensive, especially if you want a meter you can trust to be completely within the specifications. But at least they're standard lab instruments and have many uses, unlike a leveled sinewave generator or a scope calibrator.3. Some really high-end RF signal generators allow you to connect an external power meter or sensor and use that for automatic level control. Basically just like 2, but now it's automatic.4. If the RF signal generator supports amplitude modulation with DC. By building a suitable error amplifier, you can DIY the automatic level control.But note that 2, 3, 4 have pitfalls. You must ensure that the signal the oscilloscope is seeing is exactly what you're measuring with the meter. All adapters, cables, connectors, power dividers, and especially attenuators (for RF leveling since 5 Vpp is too much for most RF meters) will introduce amplitude errors due to VSWR and insertion loss. Any extra attenuation must be zeroed out manually.Oscilloscope calibration procedures already say you must use known cables type and length, known power dividers, adapters, etc., specified by the manufacturer, so that attenuation won't create error. Doing all of these manually with impoverished procedures should definitely be possible, but will have more pitfalls.Of course if you're not a voltnut and you just need to fix a broken scope, I don't think 4% leveling is really needed, a RF signal with 0.5 dB flatness should be good enough. But this is the Metrology forum.BTW, right now I'm playing with the idea of whether it's possible to reach the amplitude accuracy required with inexpensive circuitry. My idea: feed a CW signal into a variable attenuator, split the signal into two paths, one to the receiver and another to a detector. The detector can be an inexpensive log converter (Analog Devices' high performance RF log amps are 0.25 dB accurate). Finally, control the variable attenuator with the error signal from the detector, close the control loop, and you get leveled sinewave. With this design, it's possible to perform the same task with a standard signal generator. But I don't really expect a success.

author: Zenwizard
author_rank: Regular Contributor
timestamp: February 04, 2022, 11:13:02 pm
content: I am mainly getting questions about 465 475 and 7603. Which is fine due to them being 100 and 200 Mhz the DB flatness is REALLY only needed for 1 step actually. That being the bandwidth check. Other than that it is mainly "create a signal of 6 division of display." The only critial signals are timing of some sort. Known amplitude for the vertical gain calibration and substantial checks. This could be done with DC honestly, and the fast edge for a HF adjustments.Also the scopes in the 4xx and 7xxx need to hit a tolerance of 3% which with modern gear should be relatively easy.I disagree with the edge signal being a minor detail. On these old CRO scopes you can loose 30-50Mhz on a 100Mhz not having the vertical amp peaked correctly.

author: jonpaul
author_rank: Super Contributor
timestamp: February 05, 2022, 08:08:51 am
content: Bonjour just saw this note:Been working with TEK scopes for decades, mostly 7000 series and 2465/7/A/B.We have all the TEK recommended CAL instruments and know very well this problemA few notes please:1/ The BW is not as critical as transient response, so a fast clean pulser is required. The very dine Leo Bodnar 40 pS pulser is prefect and very economical. 49 L UK!http://www.leobodnar.com/shop/index.php?main_page=product_info&products_id=2952/ leveled generator ... 1980s HP 8640B, RF gen 512kHz..512 or 1024 MHz.https://www.hpmemoryproject.org/timeline/bob_devries/hp8640b_01.htmBeware that SG504 MUST have the leveling head, rare and expenseive to find and seldom comes with the plugin, but DIY versions have been made.3/ Follow the TEK CAL procedure in the serv manual exactly, many steps in the 2465x need some explanation, search tis form as well as the Tektronix at groups.io.4/ certain steps in 2465x need a PRECICE 50% duty cycle precise freq and certain exact amplitude levels or the CAL step WILL NOT finish.This is in 2465B but may affect other variations as well.Bon chance,Jon

author: David Hess
author_rank: Super Contributor
timestamp: February 05, 2022, 08:58:29 am
content: The peak-to-peak output of a function generator can be calibrated with an average responding AC voltmeter by reading 11% high. This combined with a switched attenuator can produce all of the calibration voltages needed for vertical attenuator calibration.Input capacitance compensation requires a normalizer however a x10 probe can be used as a normalizer if its attenuation is acceptable. Otherwise a normalizer is easy to make with a precision 1 megohm resistor and trimmer capacitor.And timebase calibration requires a precision variable frequency source however this is readily available in many forms. A higher frequency function generator can serve here.Transient response calibration is the most difficult since it requires a fast enough reference level pulse generator.If a leveled RF source is absolutely necessary, then it can be provided by using any RF source and an RF sampling voltmeter or sampling oscilloscope to calibrate the level.

author: jonpaul
author_rank: Super Contributor
timestamp: February 05, 2022, 09:37:45 am
content: David we used the very fine Keysight DVM 6.5 digit 34465A to check the RMS levles up to 300 kHz, as well as DC and frequency whne calibrating.We found the transient CAL more important than BW as a goal for alligningmt of VERT.Kind Regards,Jon

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 05, 2022, 01:17:21 pm
content: On CRT scopes the bandwidth is correlated with some percentage of the incoming signal.Usually this is 80% or so. Better scopes are better. The response between low freq and the highest is not perfectly "linear" alsoThis means that a 100 MHz scope can show 80% of amplitude of the incoming signal at 100MHz and still be "good"The freq of the horizontal is not exact either and depends on the freq standard used, which sometimes is not too good.The response of the HV CRT tube is not entirely linear and can vary a little across the "sweep"There is a lot of fluctuation or jitter in freq that makes exact freq readout not perfectly accurate in a lot of cases

author: jonpaul
author_rank: Super Contributor
timestamp: February 05, 2022, 01:31:57 pm
content: For fast transients the abberation, Tr Tf, flatness and overshoot are important.We adjust our scopes and plugins as per TEK service manuals, all have many transient trims, which interact.With a very clean 40 pS Tr source from Leo Bodanr, we can get very fine transient response, beyond the TEK specs on 7904, 7104, 7A29, &A24.The BW is not adjusted.Jon

author: mawyatt
author_rank: Super Contributor
timestamp: February 05, 2022, 02:33:22 pm
content: This gets into the discussion of what is the scope BW relative to the pulse rise time. The old 1st order model for the scope used the rise time {ln(0.9)-ln(0.1)}/(2*pi) or 0.35 relationship, however this implies only one adjustment since it's a 1st order model.If the scope has multiple pulse response adjustments like the faster scopes, this implies a more complex multi-order model that may deviate from the 0.35 relationship. Seem to recall that Tektronix used a different factor than 0.35 between pulse rise time and bandwidth for the faster scopes, something ~0.4 but don't remember the exact relationship.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: February 05, 2022, 02:50:51 pm
content: Quote from: jonpaul on February 05, 2022, 08:08:51 amUsed just about every analog scope Tektronix made, they were the absolute Masters of the Time Domain. HP were the equal in the Frequency Domain, both tried tocross overbut neither was successful in theotherdomain at that time.Best,

author: Zenwizard
author_rank: Regular Contributor
timestamp: February 05, 2022, 03:45:46 pm
content: Jon, I can agree with this but I also disagree with the terminology. The transient response IS the bandwidth calibration. This step is where a ton of bandwidth is gained or lost. A bad adjustment of the transient response will chop 30Mhz of a 100 Mhz scope. It is true when you peak the transient response you are maximizing the rise time a direct response to this is also maximizing the bandwidth.A question on the leo pulser as I have 2. This works and can be used? I was under the impression this was not a good use case as this was an avalanche pulses and the wave shape needs to rise then level off. I have the Tek TD pulser and a working Type 284 in the lab. Though the Type 284 needed a major overhaul. It has been repaired.

author: Zenwizard
author_rank: Regular Contributor
timestamp: February 05, 2022, 03:51:52 pm
content: Mawyatt, On the Tek scopes there is many adjustment points that effect rise time both R and C. The adjustment procedure calls for square wave with a flat top. As the Tech your goal is to get the rise time as fast as possible with the squarest corner with aberrations of less than 2% on the wave form. Miss adjustment can cause some horrible ringing. Of the adjustment is made for a to slow rise time the VERY next step in the adjustment process is Check bandwidth to make sure the scope meets rated spec. Transient response has a direct impact on bandwidthZen

author: bdunham7
author_rank: Super Contributor
timestamp: February 05, 2022, 03:55:23 pm
content: Quote from: Zenwizard on February 05, 2022, 03:45:46 pmThe Bodnar pulser is a 10MHz square wave.

author: jonpaul
author_rank: Super Contributor
timestamp: February 05, 2022, 04:50:39 pm
content: Rebonjour a tous:We use TEK PG 506A and TM501, but the Leo B is faster and better than the PG506.For BW we have SG503 but prefer our old HP8640B with 1024 MHz capability and no leveling head.My understanding is that the fast V amps like 7A29, 7A24 and scopes like 2465/7 need transient alignment, at various delays after a leading edge.I trim all the transient trimmers and afterwards measure BW. Of course there is a tradeoff and max BW will not give best time domain response!The many R and C trims interact so about 3-6 passes are needed for best results.I have not seen a bandwidth trim in any TEK cal procedure, only a performance verification.Finally one of the CAL steps in 246X/A/B requires a very precice duty cycle/level and frequency wave or the CAL step will not complete (out of range)This may not apply to all scopes but for sure the CTT options need this.I am sure our TEK veterans like Chuck H can further comment!Bon week-end!JonPS: Scope photo is 7104 with 7A29 plugins, driven from Leo B 40 pS pulser thru Min-Circuits HAT-20 20 db attenuator to reduce VSWR,

author: mendip_discovery
author_rank: Super Contributor
timestamp: February 05, 2022, 06:15:22 pm
content: How about picking up a Bradley 192.

author: Zenwizard
author_rank: Regular Contributor
timestamp: February 06, 2022, 03:09:15 pm
content: Hmm, Looks like I will have to dig out my Pulsers and take a look at them again. I have an 8657A in the lab that I have used for BW on the 7K frames even better because of some leveling that can be attained through the calibration fixture.

author: jonpaul
author_rank: Super Contributor
timestamp: February 07, 2022, 10:59:41 am
content: Any comments on the transients photo, interested in others fast transients calibration.Jon

author: dl1640
author_rank: Regular Contributor
timestamp: February 07, 2022, 12:18:13 pm
content: Wavetek/Fluke 9500 + 9510 Head

author: mawyatt
author_rank: Super Contributor
timestamp: February 07, 2022, 02:29:07 pm
content: Quote from: jonpaul on February 07, 2022, 10:59:41 amThose are excellent examples of an analog scope proper pulse response. Your calibration efforts are well represented in those 2 waveforms!!Well done sirBest,

author: Zenwizard
author_rank: Regular Contributor
timestamp: February 07, 2022, 03:15:09 pm
content: Those do look great on the transient response. I can take some pictures when I am back in the lab but my analog tops out at 500Mhz for 1Ghz I need to kick over to my DSA620AZen

author: jonpaul
author_rank: Super Contributor
timestamp: February 10, 2022, 10:02:56 am
content: Rebonjour Cher Zen wizard et Mawyatt, mille mercis pour le gentil réponse!We worked on optimization of digital audio transformers and transmission systems since 1987, research and writing papers for AES, SMPTE, NAB, especially on pulse response and abberation.That affects the jitter in Manchester coded Signals eg digital audio.We were lucky to acquire 7104, 7904, 7A29 at ham Fleas and silent key estates. So, digital scopes cannot represent the fine details as well. To optimize transients response a system approach is needed, pulse generator, connections, scope will each affect the pulse rise time and abberation.The T coil peaking and adjustment proceedure of the TEK scope mainframe and plugins vertical amps is iterative. Each adjustment affects a different time eg 1...2..5..10...20 ns due to the vertical amplifier design.Chuck Harris has mentioned that transients response adjustment is an art rather than a science.We used a combination of BNC connectors to replace cables. Finally, an old Tek scope camera hood could shade room lights and stably position an iPhone parallel to the CRT of 7104 or 7904, for fine scope photos.The Mainframe graticule illumination, vs readout and scope traces intensity need careful balance.This work extended over a long period.Again many thanks for your kind comments!Bon SoiréeJon

author: mawyatt
author_rank: Super Contributor
timestamp: February 10, 2022, 02:04:08 pm
content: Quote from: jonpaul on February 10, 2022, 10:02:56 amRecall the Ross solution to the T coils that Tektronix kept as a trade secret, supposedly only 4 folks at Tek knew the solutions which was locked up. That with the Ft doubler amplifier kept Tek way ahead of all others, and took HP nearly 20 years to duplicate the Tek real time scope bandwidth performance.This was the basis of my earlier comment about Tek being the Masters of the TD and HP Masters of FD way back then.Also, completely agree with ChuckBest,

author: 2N3055
author_rank: Super Contributor
timestamp: February 10, 2022, 03:47:54 pm
content: Quote from: jonpaul on February 10, 2022, 10:02:56 amSure they can. In last 30-40 years lots of stuff happened.Same signal source with attenuator. On a modern 12Bit scope.That being said, I'm still absolutely amazed what those wizard in Tek could do in purely analog domain. Absolute genius..

author: David Hess
author_rank: Super Contributor
timestamp: February 10, 2022, 11:45:49 pm
content: Quote from: 2N3055 on February 10, 2022, 03:47:54 pmMy ancient DSOs handle fast rise signals just fine. I think it only became a problem with modern real time only DSOs that eschew equivalent time sampling.

author: Zenwizard
author_rank: Regular Contributor
timestamp: February 11, 2022, 06:12:06 pm
content: This test will be interesting to run on my 7854 once I get the calibration tuned up on it. both of the 7854's I have a WAY off in the digital sections.

author: tautech
author_rank: Super Contributor
timestamp: February 12, 2022, 10:57:55 am
content: Quote from: 2N3055 on February 10, 2022, 03:47:54 pmNo attenuator, no special settings just plug and play with a 1 GHz 8 bit DSO and Leo's pulser.However measured risetime is 10x Leo's spec.