DIY Injection Transformer for Power Supply Control Loop Response Measurements - Page 1
Forum section: electronics / projects

author: diyaudio
author_rank: Frequent Contributor
timestamp: October 09, 2016, 05:54:13 pm
content: Its clear an Injection Transformer is required to perform signal isolation when performing loop response measurements,however they cost big bucks, a Picotest J2101A costs $525.00https://www.picotest.com/products_J2101A.htmlI want to start building a DIY Injection Transformer for my MSOX3104T, does anyone know what transformer material I can use to support a 1:1 10Hz to +/-40MHz signals.

author: jahonen
author_rank: Super Contributor
timestamp: October 09, 2016, 06:00:11 pm
content: I have just used an ordinary mains transformer (230/5 volts or something like that, voltages are not that critical), I think it works sufficiently well up to at least few tens of kHz, but that is often enough for switch mode regulators. When both reference and output are measured, the transformer passband flatness is not so critical. I actually used a 192 kHz sound card and ARTA software for the measurements, good enough for most of my needs. This ordinary mains transformer is also used in this TI appnote:http://www.ti.com/lit/an/snva364a/snva364a.pdfRegards,Janne

author: 2N3055
author_rank: Super Contributor
timestamp: October 09, 2016, 06:29:40 pm
content: a bifilar winding on a toroidal ferrite core... Maybe a bit of experiment as to how many turns...

author: MatteoX
author_rank: Regular Contributor
timestamp: October 09, 2016, 08:09:17 pm
content: It is extremely difficult to acheive 10Hz - 45MHz transformer bandwidth (that is 6 1/2 decades or 22 octaves!).Picotest mentiones use of specially annealed cores. If you look at Picotest J2101A Data Sheet, you will noticeresonance at 11MHzhttps://www.picotest.com/downloads/INJECTORS/J2100A-J2101A_Spec_Sheet_FinalV2_Email.pdfMany years ago I tried to make an isolation transformer for the same purpose. My goal was 10Hz to 1Mhz bandwidthbut I couldn't reach it so I give up after several experiments.There were several discussions on the Internet that critiqued TI app note listed above and its use of mainstransformer.I would be really curious if someone knows how to acheive such bandwidths.Maxim App note 325https://www.maximintegrated.com/en/app-notes/index.mvp/id/3245hassome interesting details

author: 2N3055
author_rank: Super Contributor
timestamp: October 09, 2016, 08:27:50 pm
content: 50-100Hz to few Mhz shouldn't be problem on common ferrite cores...45 MHz not so easy, but I see no reason to that high on testing control loops on PSU...22 nsec rise time in PSU control loop not so common... Most PSU have bandwidth in 10s of kHztesting to 100-200 kHz to make sure all is stable is good practice...Testing to few MHz for fast PSU I get... 45 MHz i have no use for..

author: 2N3055
author_rank: Super Contributor
timestamp: October 09, 2016, 08:42:27 pm
content: Quote from: MatteoX on October 09, 2016, 08:09:17 pmWell probably it is Rogowski transformer or a compensated multi coil on a special core...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2016, 08:57:30 pm
content: Curious how much isolation capacitance it has.That's a good price for such a fine transformer. If you actually need that bandwidth, I suggest buying it!If you don't, then reconsider how much bandwidth you actually need, and go from there.Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 10, 2016, 10:39:03 am
content: It's the low end of the frequency range that is the hardest to achieve with a transformer. This is because operating down to a low frequency requires a very high permeability (mu) core and lots of turns, but a high mu core tends to have high losses vs. frequency, and lots of turns means lots of distributed capacitance (ie - both act to roll off the high frequency response). Note that you must design the transformer for the lowest expected frequency to avoid saturation.If you want to test low bandwidth switchers like, e.g., boost PFC converters, then you really do need bandwidth to extend down below 1Hz, because the crossover frequency for the voltage loop is usually in the range of 5-15Hz.Conversely, bandwidth exceeding a few hundred kHz will rarely be necessary unless you are designing state of the art GaN or Class-E/F switchers running at 1MHz+ - at a minimum you only need to test a little beyond the crossover frequency to ensure loop gain remains below unity without popping back up again or that the phase margin is adequate in and around the crossover region.All that said, this would be an impossible task for a single transformer except for one thing: the signal level is usually in the range of 10mVpp (and never more than 100mVpp; at least not in my experience). This means you can get away with relatively few turns without running afoul of Faraday and Lenz.Alternatively, you can use an isolation amplifier to inject the signal; an option that is arguably more popular these days.

author: Floyo
author_rank: Regular Contributor
timestamp: October 10, 2016, 11:03:51 am
content: I dabbled a bit with this after being unsatisfied with the mains transformer technique, or using a common mode choke as Jay_Diddy_B did in one of his threads. I then found some inductors I have had lying around for ages, I think they came out of some telecom gear. They have a stupid amount of primary turns, and a stupid high permeability. the primary inductance is around18 Henry(!). I plotted the response with my analog discovery, see attachments.The secondary is terminated into 50Ohms, and the primary is being fed in "low impedance" output mode of the AD signal gen. Resonance happens at around 5.5Mhz, and the low cutoff is around 5 Hz, though you could get a bit more out of it by virtue of the loop measurements being relative, and by changing the drive impedance. If there is interest I could give some more details, but I'm at work right now, so that will have to wait a bit.

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 10, 2016, 01:04:05 pm
content: Quote from: AcHmed99 on October 10, 2016, 11:27:36 amNot sure where you are going with this... the isolation transformer we are talking about here is for injecting a very small signal into the feedback network which results in a much larger variation in the switcher's output voltage (or current, if that is what the control loop regulates).

author: Kleinstein
author_rank: Super Contributor
timestamp: October 10, 2016, 04:44:47 pm
content: If you don't use the transformer very heavily, you could get away in splitting the measurement and use 2 separate transformers, one for the low frequency (e.g. 1 Hz -1 kHz) and one for the higher frequencies (e.g. 100 Hz - 10 MHz). This very much eases on the design. Some overlap can also help to check for limitations.For the low frequency range one could even use some kind of balances current injection or other electronic circuit instead of a transformer - so one could measure down to essentially DC.

author: 2N3055
author_rank: Super Contributor
timestamp: October 10, 2016, 04:51:04 pm
content: AcHmed99,I think you didn't quite understand how it's done..You don't inject current into PSU output.. You take PSU, you break feedback loop and insert injection transformer in series with feedback loop.. So it gets amplified by error amplifier, not suppressed......

author: diyaudio
author_rank: Frequent Contributor
timestamp: October 10, 2016, 05:09:19 pm
content: Quote from: Floyo on October 10, 2016, 11:03:51 amThis looks very interesting, I too saw some possible isolation transformers in modem equipment I may possibly use one, can you upload some more details on your transformer?As a side note: I don't think I need that high frequency response for the transformer is required...up to 1MHz signal will do to test compensation loops.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 10, 2016, 05:32:25 pm
content: Hi,In this thread:https://www.eevblog.com/forum/projects/dynamic-load-bode-plot-using-hp-35665a-dsa/msg309192/#msg309192I showed how I used a common mode choke for injecting the signals.In this message I showed how to estimate the useful bandwidth of the transformer based on magnetizing inductance and leakage inductance.https://www.eevblog.com/forum/projects/dynamic-load-bode-plot-using-hp-35665a-dsa/msg310991/#msg310991I prefer to use my HP3577A VNA analyzer for doing this measurement. It has tracking filters, which can be set as narrow as 10Hz, these reject all the frequencies except the one that you are injecting. The narrow bandwidth helps lower the noise floor.The injection transformer does not need to be flat. The transfer function of the transformer is not part of the measurement. You are measuring on either side of the transformer with respect to ground.The injection transformer is connected between the output and the top of the divider resistor. The control loop will try and hold the top of the divider constant so all the injected signal should appear on the output. At low frequencies the voltage on the divider should be very small. I normally inject a signal that is 0.5 to 1% of the output voltage. so for a 5V supply I will use 25mV rms. If the loop gain is 40dB only 25/100 =0.25mV is at the top of the divider. As the injection frequency is increased, the loop gain is reduced and the ratio of the signals changes. You do not want to make the injection amplitude too big. If you do you may not be measuring the small signal behaviour.Since this I have developed a new injection transformer that uses a common choke with 6.5mH of inductance.I believe that the Picotest injector, must use special high frequency techniques, like a Pearson current transformer.I would love to see a teardown of Picotest transformer or any of the other picotest units.There is another way of doing this, using op-amps. This would work very well at low frequencies, but I have forgotten the details.Regards,Jay_Diddy_B

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2016, 07:20:47 pm
content: Quote from: diyaudio on October 10, 2016, 05:09:19 pmBeautiful! We have all the numbers we need to work with!Suppose the transformer is 1:1 at 50 ohms. It should be made of 50 ohm transmission line, in maximum electrical length 1/8 wave at 1MHz. If velocity factor is 0.67, then 25 meters is the allowable length.Which is a pretty damned good bit of wire!It should be something like #34 AWG or finer magnet wire (cough*), twisted together. This won't be quite as low as 50 ohms, but it'll be close enough.Good cores:Hi-mu toroids, especially nanocrystalline (obtain from Mouser or Elna Magnetics?)Silicon steel, with fine laminations (under 10 thou), GOSS or betterPot cores (hi-mu ferrite, e.g. 3E27 or better)Making a core size choice isn't obvious. You do have a known quantity of wire to wrap around the thing. You can probably get on the order of 500 turns around a 5uH/t^2 core. Which is 1.25H, which gives a -3dB cutoff at 6.4Hz. Not too bad. Actually half that, because you'll have a 50 ohm signal generator, acting in parallel with the 50 ohm secondary termination resistor. So 3.2Hz.You should be able to obtain higher inductivity (>20uH/t^2) in the fancier steels (NiFe, permalloy) and amorphous and nanocrystalline materials. This would push the cutoff below 1Hz.The winding should be bifilar, wound in layers (with tape between each), to prevent too much crosstalk from adjacent layers. That will help avoid weird dips and peaks in the response.*Checking... you may actually want much heavier wire, since 25m of #34 is 21 ohms. And that's just for one winding! #28 is about the smallest you can get away with (< 3dB insertion loss), and #25 would be ideal...Needless to say, larger wire will invite much larger cores and windings. A UR style core (the kind they make flyback transformers with) might be attractive, because of the wide cylindrical winding area available.Needless to say, any excuse you can make to reduce the winding length will save on both resistance and HF response.Tim

author: 2N3055
author_rank: Super Contributor
timestamp: October 10, 2016, 07:29:03 pm
content: Quote from: AcHmed99 on October 10, 2016, 05:08:59 pmI thank Jay_Diddy_B for explaining it, my English is failing me sometimes (most of the time).. I wanted to say that injected signal gets superimposed on top of normal feedback voltage (vector added to be more precise).. As Jay_Diddy_B explained, result is modulation of output, using feedback amp and pass transistors (let's presume linear PSU for a moment) as amp with unity gain..For audio enthusiasts A class amp

author: Floyo
author_rank: Regular Contributor
timestamp: October 10, 2016, 07:48:49 pm
content: Quote from: diyaudio on October 10, 2016, 05:09:19 pmI think the specific core I used is a TDK RM-8 in H5C2 material, the data can be found in this pdfhttps://product.tdk.com/info/en/catalog/datasheets/ferrite_mz_tl_rm_en.pdfthe AL is pretty high. I don't know exactly how many turns it has, but going from the 18H primary inductance and the specced AL that comes to around 1000 turns. The secondary being about 26db down in the plot gives a pri/sec ratio of around 1/19.5 so that makes the secondary around 50 turns. Wire size is "really thin" and airgap is "0". I hope that helps a bit, I didn't arrive at this transformer by any maths, just experimentation, and luckily the 1000 turn primary was already done when I found the thing in my junk bin.

author: 2N3055
author_rank: Super Contributor
timestamp: October 10, 2016, 08:12:37 pm
content: The attenuation part..What you said is perfect if you take low impedance signal generator and just modulate output.. Regulator "fights the change" and will negate your disturbance, and will suppress it as a part of regulation.If you inject into the loop, you simply modulate feedback signal , and by measuring frequency and phase response to that signal (minus DC of PSU) you are basically measuring it for PSU loop..I apologize if there is misunderstanding...

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 10, 2016, 09:39:17 pm
content: Hi group,I have put together an LTspice simulation to illustrate how this works. By studying an op-amp circuit we easily determine if we have the correct answer.In the model I have used an op-amp to amplify a 10V reference to 20V a gain of two. I have included a network analyzer in the model, connected between nodes A and B. The LTspice will measure the loop gain, in the time domain, the same way as if I used my HP3577A in the lab. There is no need for an isolation transformer, because in SPICE the voltage source is floating.If we start at node B and go clockwise around the loop. R1 and R2 are a divider, divide by 2 or -6dB. It has an output impedance of R1 in parallel with R2, R1//R2, = 10k.The divider resistance combined with R3, 40k, has a gain of 4x or +12db. The total gain in resulting from the resistors is -6dB + 12dB = 6dB.There is a low pass filter caused by C2 and R3F= 1/2 x Pi x 1600pF x 40,000 = 2.5kHz (-3dB)There is a zero at the frequency caused by R3 and C1F= 1/2 x Pi x 0.16uF x 40,000 = 25 Hz (+3dB)The results from LTspice are:If we look at the relative size of the signals at A and B we see:The units are dBV, that is dB relative to 1V rms.And if we look at them in the time domain, at one frequency, 316 Hz we see:The vector sum of the signals A and B is always equal to the injected signal.If this works for this example, it will work for more complex circuits too, where it is difficult to know the right answer.I have attached the LTspice model.The model could be expanded by adding the transformer model between the source Vac1 and points A and B.Regards,Jay_Diddy_B

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 10, 2016, 10:42:16 pm
content: Quote from: 2N3055 on October 10, 2016, 04:51:04 pmYes! Thanks to you and Jay_Diddy_B for explaining this while I was off doing chargeable work!

author: MatteoX
author_rank: Regular Contributor
timestamp: October 11, 2016, 01:46:17 am
content: Few references for electronic signal injection:Galinski "Use Op-Amp Injection for Bode Analysis"http://www.edn.com/design/analog/4369185/Use-op-amp-injection-for-Bode-analysisKeeney "Simple Signal Injection Aids Controol Loop Analysis"http://powerelectronics.com/power-electronics-systems/simple-signal-injector-aids-control-loop-analysis

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: October 11, 2016, 03:52:37 am
content: What about using a linear optocoupler instead of a transformer?

author: LeonV
author_rank: Contributor
timestamp: October 11, 2016, 04:11:41 am
content: Any one know what the frequency response from an air core transformer would be?Or what if you wind a cooper loop into another loop? Like copper wire wound into a toroidal shape, then a winding around that as if it was a toroidal core.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 11, 2016, 06:23:37 am
content: Quote from: LeonV on October 11, 2016, 04:11:41 amAwful. I'd be impressed if you got bandwidth more than 10 times the center frequency.Tim

author: rx8pilot
author_rank: Super Contributor
timestamp: October 12, 2016, 02:43:58 am
content: I have been using a Jensen VB-1BB isolation transformer used for analog video. Not perfect, but not bad assuming that 10hz is not a necessity. It does ok down to rather low frequencies though. I paid $20 on eBay. BNC 75 Ohms in/outhttp://www.jensen-transformers.com/product/vb-1bb/I tried to use it with the Keysight power application on my MSOX6000 but was quickly stopped by it's inability to deal with the noise in SMPS. The newer software on the 3k's and 4k's seems to be a lot better at filtering and allowing a LUT for the injection voltage to get a better SNR on a range of frequencies. I was able to get measurements, but manually stepping through the frequencies, injection levels, averaging, and math filtering. A bit slow, but I think I was able to get believable measurements. After getting some pricing on dedicated FRA's, I am determined to figure out how to get what I need on the scope. I will design about 4-5 SMPS's per year so I need this capability for a grand total of maybe a week per year as I work out the supply design.Having a real VNA would be nice, but certainly not a must.

author: diyaudio
author_rank: Frequent Contributor
timestamp: October 13, 2016, 12:28:33 pm
content: Quote from: rx8pilot on October 12, 2016, 02:43:58 am-- Johnnie HancockYes, we recently updated the firmware in the 3000T and 4000A X-Series. We implemented a completely different measurement algorithm that improves the dynamic range of frequency response measurements significantly. We also implemented a much better user-interface, as well as overlaying the gain and phase traces on the same plot. We plan to update the firmware in the 6000 X- Series next year. I don't know exactly when, but when it comes out, it will be a free upgrade.?

author: rbola35618
author_rank: Frequent Contributor
timestamp: October 13, 2016, 03:51:54 pm
content: Hi EEVBLOGGER,I posted this video awhile ago here on the blog. I wound my injection transformer using a P42/29 core. I basically replicated a Venable injection transformer that we have at work. I get good results with the transformer. I now use the Cleverscope to do my FRA measurement and do not need an injection transformer. The Cleverscope has an isolated function generator output and all I have to do is connect across the injection resistor. Since there is no injection transformer used, I get a bandwidth all the way to DC. The Cleverscope work well for me.

author: diyaudio
author_rank: Frequent Contributor
timestamp: October 13, 2016, 05:10:32 pm
content: I found a unused complete Panasonic EMI Line filter core (from the junk box 10 years old), doesn't look bad at all.http://www.datasheetspdf.com/PDF/ELF18D290x/530267/3

author: rx8pilot
author_rank: Super Contributor
timestamp: October 13, 2016, 06:06:15 pm
content: Quote from: rbola35618 on October 13, 2016, 03:51:54 pmRobert -Are you allowed to describe how the Cleverscope signal generator is isolated? Guessing it is not a transformer and some sort of isolation amplifier? The wide band electronic isolators used for this purpose are double or triple the cost of the the transformers, but seems unlikely they are terribly difficult to design and manufacture (could be sadly mistaken of course).On the more traditional route, how critical is the transformer performance relative to the goal of FRA on an SMPS control loop? I do not have much knowledge of designing magnetics, but I can intuitively understand there is a challenge making a very wide bandwidth transformer like commercial models. If there is no need to test active PFC's, the low frequency is seemingly unimportant. I am not sure why you would need to go to 45Mhz on the high end either. In a practical sense, what bandwidth would you say is reasonable for the budget minded?BTW - thank you for your video lessons on YT and the offline pointers you have shared.

author: Bud
author_rank: Super Contributor
timestamp: October 13, 2016, 06:28:40 pm
content: Quote from: Jay_Diddy_B on October 10, 2016, 05:32:25 pmWhat is the point of having an "isolation" transformer if the measuring inputs not isolated?Or theay are?

author: rbola35618
author_rank: Frequent Contributor
timestamp: October 13, 2016, 06:46:07 pm
content: The designer of the Cleverscope is Bart Schroder. I approached Bart several years ago about implementing the FRA option. He redesigned the function generator so that the grounds on the function generator would be isolated from the scope grounds. He used an opto-isolator to do the isolation. You can float 300Vrms on the function generator. The isolation ideal was originally proposed to Bart by a colleague of mine Robert Raulk.Regarding the loop measurement, if you are doing PFC then you need to go down to 1 to 5Hz. In my case, I do not do PFC since my design are only DC to DC space environment. So when I do a loop plot, I do them from 100Hz to 100KHz. My crossover frequency are around 1KHz to 10KHz. So I do not really need that high of a frequency. The Venable injection transformer state a bandwidth from 10Hz to 10MHz. So 10MHz is well above the frequency that I am interested.In addition to the FRA, I use the Cleverscope as a LCR. Bart implemented a LCR option. I built a jig with a current sense resistor and I am able to characterize my magnetics, and capacitors. You need very high frequencies, (The Cleverscope goes up to 65MHz) to characterize the magnetics and capacitance. When characterizing the magnetics, I am interested in the parasitic such as the leakage inductance, ESR, the parasitic capacitance (self resonance). The parasitic usually can only be seen at very high frequency where they become dominate.I can see where Keysight can also implement the LCR option into their 3000T series. This would great if they could do that. I have the regular 3000 series and it can not do the loop analysis. I asked Keysight if they could add the loop analysis to the regular 3000 but they said the FPGA was not powerful enough. But that is OK since I have the Cleverscope.

author: rbola35618
author_rank: Frequent Contributor
timestamp: October 13, 2016, 06:52:43 pm
content: Hi Bud,The input do not need to be isolated. The ground of both channel 1 and 2 are connected to the power supply ground while the positive probes are connected to an injection resistor. Keep in mind that the resistor is 10 ohms to 100ohms and if you are monitoring a 5V output, then both end of the injection resistor will be close to 5V. You then need to connect a signal source across the resistor which as I mentioned is riding on 5V. So if you connect a non-isolated function generator across the resistor, you will end up shorting your 5V output to ground thru the function generator's ground. Therefore you have to use an isolation transformer. I hope this make sense. Robert Bolanos

author: Bud
author_rank: Super Contributor
timestamp: October 13, 2016, 07:10:27 pm
content: Thanks Robert. Basically you are saying transformer is a DC block. I interpreted In the context of this thread that the purpose of transformer is galvanic isolation from the power supply for safety reason.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 13, 2016, 07:52:48 pm
content: The transformer is there to add the signal to the 5 V output with all its higher frequency components. So it is not just to block a clean 5 V, it is there to inject a differential signal. The links to the electronic replacements could help to understand the purpose:They have an amplifier that adds the driving signal and outputs it to the other side.For a low frequency circuit this could work quite well. But with a faster circuit the amplifier needs to be pretty fast, not add a significant phase shift that might upset the DUT. This might even apply to frequencies higher than you are interested in. So for slow circuits this electronic way might be an alternative to the injection transformer. The amplifier way also has another limitation: it does only work at low voltages, so the transformer can provide this level including DC.

author: rbola35618
author_rank: Frequent Contributor
timestamp: October 13, 2016, 08:04:15 pm
content: The only problem that I see with the Keysight is , if I am not mistaken, the resolution of the ADC is only 8 bits. The dynamic range and the ability to get the signal out from the noise is not as good as a regular FRA or the Cleverscope.My Cleverscope has a 10bit ADC and I can upgrade it to a 14 bit ADC. So theoretically, the Cleverscope should be more sensitive. I have done some test where I can a good loop plot using only 1mV on input stimulus signal. I then tried doing that on out 30K Venable and it was not able to get a good plot.I think some of the higher end Keysight may have a higher resolution ADC. I would assume that those with higher ADC would work much better.RB

author: rx8pilot
author_rank: Super Contributor
timestamp: October 13, 2016, 08:09:16 pm
content: Quote from: rbola35618 on October 13, 2016, 08:04:15 pmTo be fair, the resolution of the ADC is only a useful spec if the analog circuits in front of it are better. With full manual control on the 8bit Keysight using averaging and filtering I was able to pull out some really small signals.

author: rbola35618
author_rank: Frequent Contributor
timestamp: October 13, 2016, 08:34:21 pm
content: Hi rx8pilot,What would be an interesting test would be to see how low of a stimulus signal will get you a good plot.If you get a chance and are interested in finding out, do some plots and decrease the signal and see where it starts to plot noise. I would be interested to find out how sensitive it is.RB

author: Kleinstein
author_rank: Super Contributor
timestamp: October 13, 2016, 09:09:11 pm
content: The 8 Bit converters in the scopes are really fast. So they can do a large amount of oversampling to reduce noise. An 8 Bit resolution looks low, but at GHz sampling the noise density is not that bad. It is well possible that the noise from the (switching) supply itself is higher than the ADC / Scope noise. Often 1:10 probes already give more noise than the scope itself. There is not much to gain with a low noise scope, if there is more noise from the DUT.The important part could be how good averaging and harmonic analysis is done and thus what is the effective bandwidth. A slower measurement with longer averaging on each point can reduce the noise - especially if it is slower for the points with poor S/N only. So it could be just a question of how much time is spend for the curve. It's only improving like the square root of the time, but there is still a last resort in using extra time - up to the point of over night.

author: rx8pilot
author_rank: Super Contributor
timestamp: October 13, 2016, 10:15:31 pm
content: Quote from: rbola35618 on October 13, 2016, 08:34:21 pmUsing the built in application, there is essentially no solution for SMPS. By the time I got a usable SNR, the loop will have long since been clipping. The 6000 scopes do not yet have the new power application that the 3000's and 4000's have. I could try to setup some manual tests to see what happens on one of my boost converters. I will be in the CNC shop for the next few days, so possibly on Sunday.It would be great to throw the results and method into this discussion. Not a whole lot of conversation to be found on loop response. TALKnerd(1000) = AUDIENCEsmall.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 14, 2016, 03:42:56 pm
content: I looks like isolated generator is an $400 option. So if you have that type of scope, this is an attractive option.If you want to build the transformer yourself, I would consider a more limited frequency range and than the option to chose from a few. It should be easier to make two of them for separate frequency ranges than one with very high bandwidth. Depending on the DUT you might have to choose the suitable one.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 14, 2016, 06:34:50 pm
content: Usually there is not need for isolated probes. It is just that with that special scope (cleverscope) you have the option to use the isolated generator option instead of the injection transformer and a normal (grounded) generator.For a normal scope setup this is less of an option. It might be possible in the manual mode, as a very low cost option, to use an isolated (maybe even battery operated) generator - but this would be without special scope functions and one might still need the trigger signal to do at least multi period averaging. For the trigger a fast opto-coupler could be an option - so all the way down to DC.

author: r0d3z1
author_rank: Regular Contributor
timestamp: October 24, 2017, 10:55:11 am
content: Quote from: rx8pilot on October 12, 2016, 02:43:58 amThis is a similar product: Model 0017CC (10 Hz to 5 MHz, 50 ohm to 50 ohm, BNC to BNC) by North Hills Inc.

author: rx8pilot
author_rank: Super Contributor
timestamp: October 24, 2017, 03:13:58 pm
content: Quote from: r0d3z1 on October 24, 2017, 10:55:11 amInteresting option. Strange that these are targeted to video applications but 50 Ohms.This model seems like a better fit than the JensenSent from my SM-G892A using Tapatalk

author: David Hess
author_rank: Super Contributor
timestamp: October 24, 2017, 03:27:09 pm
content: Picotest should redraw their diagram to be even more confusing.A 60 Hz AC current transformer can work as inexpensive injection transformer.

author: rx8pilot
author_rank: Super Contributor
timestamp: October 24, 2017, 03:28:56 pm
content: Picotest wants to get a lot of money for a transformer....Sent from my SM-G892A using Tapatalk

author: David Hess
author_rank: Super Contributor
timestamp: October 24, 2017, 05:18:54 pm
content: Quote from: rx8pilot on October 24, 2017, 03:28:56 pmIt is not a lot of money for someone who gets paid for the sort of work where the transformer would be useful. Greater than $5000 DSOs are not expensive in that respect either. Think of the cost in comparison to the salary of the engineer using it.For hobbyists, there are other less expensive but more time consuming methods to use like driving the injection point with a transconductance output or using a 60 Hz AC current transformer in reverse. I usually get by with just measuring the small signal and large signal load transient response.

author: rx8pilot
author_rank: Super Contributor
timestamp: October 24, 2017, 08:47:28 pm
content: Quote from: David Hess on October 24, 2017, 05:18:54 pmTrue enough.I am in the purgatory of power electronics design. I just don't do it enough to justify the bench full of specialized test gear, so I muddle through the best I can. I looked into renting the Bode100 VNA with all the accessories and it is about $750-$850 per month rental. About 8 rentals would pay for it. I would use it for 3-4 designs per year - so buying it breaks even at around 2 years for the dedicated FRA.Measuring transient response will definitely reveal most control problems - I do the same over a range of operating conditions. I have been able to get some use out of the FRA built into my Keysight scope although it takes some doing when testing noisy switching supplies. The automated FRA application cannot deal with the noise, but manually I can step through the frequencies and use math filters and averaging to get a measurement of gain/phase in the control loop. Slow - but eventually gets to a reasonable answer of phase margin.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 24, 2017, 08:59:12 pm
content: Hi,I described my injection transformer in another thread.https://www.eevblog.com/forum/projects/dynamic-load-bode-plot-using-hp-35665a-dsa/Here are some of the details:Schematic:Board Design:ConstructionI normally use an HP 3577A VNA for measuring control loop response. The HP3577A does a great job, but it is big and heavy.Jay_Diddy_B

author: rx8pilot
author_rank: Super Contributor
timestamp: October 24, 2017, 09:23:21 pm
content: Other than size - the 3577A is an interesting option since it goes to 5hz.Any maintenance issues with it?Your injector looks pretty cool.....off to read that thread now. Thanks for posting that one.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 24, 2017, 10:19:44 pm
content: Quote from: rx8pilot on October 24, 2017, 09:23:21 pmThe only issue that I have had with the 3577A is the CRT. The CRT assembly is the HP1345A display module. I changed mine with one I got from another instrument. If it happened again I would fit one the LCD retrofit kits.It is a great instrument.Regards,Jay_Diddy_B

author: rx8pilot
author_rank: Super Contributor
timestamp: October 25, 2017, 10:27:18 pm
content: Look what just arrived.....got it for a few peanuts and it does not boot up. If I am lucky, it is a simple thing. For the cost of 1 hour of driving - it was worth a look.HP 3577A

author: BFX
author_rank: Frequent Contributor
timestamp: November 03, 2019, 07:13:21 pm
content: Little bit of research done and here is the resultAny similarities are purely only by chance