measuring high voltage with spectrum analyser - Page 1
Forum section: electronics / projects

author: fonograph
author_rank: Frequent Contributor
timestamp: May 12, 2017, 11:55:01 am
content: I need to measure low current,1miliAmp at most,high voltage 4KV peak,high freqency 100mhz signal.What probe should I use? Both Keysight and Lecroy have 4kv 400mhz 100:1 passive probes and 6kv 75mhz differential probes,I also found some high voltage CIC differential probes that can go to 100mhz.If I understand it correctly,differential probes are active,I fear they will introduce noise and distortion not to mention their much lower bandwidth.What should I do,what probe should I get?

author: ahbushnell
author_rank: Frequent Contributor
timestamp: May 12, 2017, 12:12:16 pm
content: http://www.highvoltageprobes.com/Try Northstar

author: w2aew
author_rank: Super Contributor
timestamp: May 12, 2017, 01:23:33 pm
content: 4KV peak at less than 1mA implies a very high source impedance, thus the input capacitance of any probe will significantly load down the signal, especially at 100MHz. Can you "probe" the signal with a near-field RF probe to get a relative reading?

author: fonograph
author_rank: Frequent Contributor
timestamp: May 12, 2017, 03:17:14 pm
content: no,I must connect it directly.What does load down mean? Is it that my source will output lower power as result of being probed? I think that will not be problem as long as it doesnt introduce more noise or distortion,I want to measure very high voltage amplifier

author: Molenaar
author_rank: Regular Contributor
timestamp: May 12, 2017, 03:24:04 pm
content: What is your load?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 12, 2017, 03:46:48 pm
content: Are we talking 100 mHz or 100 MHz, very different situations?

author: fonograph
author_rank: Frequent Contributor
timestamp: May 12, 2017, 04:15:40 pm
content: Its megahertz and the load is electrostatic speaker,the power only goes to charging the stators,it works kind of like capacitor,the air capacitor that is just metal plates with air gap between that is very similiar thing to what I want to do,another analogy is mosfet gate,the current doesnt flow any further that gate but you need some decent power to charge and discharge it fast,like millions of times a second.

author: Molenaar
author_rank: Regular Contributor
timestamp: May 12, 2017, 05:03:20 pm
content: For ESLs you need DC on the stators, as far as I know? Why would you need a 100 MHz signal there?

author: fonograph
author_rank: Frequent Contributor
timestamp: May 12, 2017, 05:26:22 pm
content: There are two types of electrostatic speaker,one that have DC on stator and AC on membrane and other is DC on membrane AC on stator.

author: Molenaar
author_rank: Regular Contributor
timestamp: May 12, 2017, 05:57:20 pm
content: Sure, but why at a frequency of a 100 MHz? Don't you mean 100 Hz or 100 kHz?

author: fonograph
author_rank: Frequent Contributor
timestamp: May 12, 2017, 06:28:27 pm
content: Its 100 megahertz,I dont want to tell you why.I understand that the reason you are asking is to help me and I appreciate it.

author: Hensingler
author_rank: Regular Contributor
timestamp: May 12, 2017, 06:50:59 pm
content: Quote from: fonograph on May 12, 2017, 03:17:14 pmWhat it means is the 6pF capacitance of the LeCroy probe you mentioned would draw 10A and 30kW of apparent power from your 4kV 100MHz signal.The idea of 4kV, 100MHz and low current is utterly ridiculous. Half a femto farad of stray capacitance would draw 1mA from that signal never mind radiation from whatever is carrying it.

author: Molenaar
author_rank: Regular Contributor
timestamp: May 12, 2017, 09:56:12 pm
content: Quote from: fonograph on May 12, 2017, 06:28:27 pmAside from the remarks above, I think the best way to measure such a fragile source would be to measure the 100 MHz in the far field. In air, however, this would be around quite far away.Are you trying to build a high frequency high voltage class D amplifier?

author: calmtron
author_rank: Regular Contributor
timestamp: May 13, 2017, 07:04:59 am
content: Never mind the probes, how are you going to set up the rest of the measurement enviroment considering half a fF of stray capacitance will totally swamp your signal? Have you done the maths on this?

author: fonograph
author_rank: Frequent Contributor
timestamp: May 13, 2017, 08:04:41 am
content: Yes,class D high freqency amplifier for ultrasonic electrostatic speaker,its not meant to play music at all,its not for humans.

author: PartialDischarge
author_rank: Super Contributor
timestamp: May 13, 2017, 08:32:14 am
content: Out of curiosity, what switching device in a class D amp can handle that many kVs at that frequencies you are trying to measure?

author: Molenaar
author_rank: Regular Contributor
timestamp: May 13, 2017, 08:53:52 am
content: I guess if you drive them really hard, a tube like the Eimac 8252w should work?I don't see why you would do this though, since these tubes are quite beefy and could be used in linear mode just fine.

author: mk_
author_rank: Regular Contributor
timestamp: May 13, 2017, 09:16:40 am
content: Quote from: fonograph on May 12, 2017, 11:55:01 amAs you are located in Austria I'm shure you know W. Ottinger. If you don`t know him - send me a PM.Anyway, I don't see a big problem as you can do a floating measurement for that mA and use that for some kind of analog transmission via optical media. Just find a way to modulate a LED/Laser whatever and receive that data and bring it back to analog. Gbit via optical media is well established.Anyway, your project is not something with a low cost-tag.

author: fonograph
author_rank: Frequent Contributor
timestamp: May 13, 2017, 11:18:25 am
content: Out of curiosity,you all act like its impossible and unusual thing to do,yet Keysigt,Lecroy,CIC and others make large number of various high voltage probes for oscilloscopes,not only that,there exist commercialy avaliable high voltage RF amplifiers,they make them blindly without measurement? My question is,what are these numerous HV RF probes supposed to measure if you say they cant measure my HV RF amp?You write that HV and high frequency is problem,but why do they bother to make them flat to hundrets of megahertz,why would company spend R&D money to create useless HV RF probe? And these high voltage low current RF amps,since its impossible to measure ,then I guess all the specs of distortion and noise must be lies since they cant test them.

author: mk_
author_rank: Regular Contributor
timestamp: May 13, 2017, 11:52:02 am
content: Quote from: fonograph on May 13, 2017, 11:18:25 amWell, do it yourself if it seems an easy task to measure 1mA @ 4kV roaring @ 100MHz. Tek Schematics for P6015 is aviable.

author: fonograph
author_rank: Frequent Contributor
timestamp: May 13, 2017, 02:39:32 pm
content: I never said its easy task,I didnt even disagree with anything that was written in this thread.Why did you come up with idea to DIY the probe? Why do you recommend obsolte tektronics probe that uses banned ozone depleting gas?Is there perhaps something special about that probe that makes it best choice in this situation? One thing I noticed in that graph is that the maximum voltage rating falls of rapidly at higher freqencies,I checked on Keysight and their probes fall off too,that is big problem I wasnt aware off.The 100mhz number isnt the switching speed,I just written 100mhz becose I wanted to see some of the harmonics too,not just fundamental,it switch much slower,around 12mhz.It was mentioned that the capacitance of the probe is problem,is then better to use active differential probe? Or how about making my own probe with super high voltage rating and make it very short,one banana long to be exact instead of 3m to reduce the cable capacitance.

author: azer
author_rank: Regular Contributor
timestamp: May 13, 2017, 02:53:49 pm
content: I believe you should be able to use a high voltage differential probe with the spectrum analyzer as they are buffered internally and don't rely on the 1Meg input of the scope for voltage division. Some of them probably still need 1Meg input impedance so you would need to add a buffer in between to drive the 50ohm input of the spectrum analyzer. You also would need some circuitry to power the probe through the proprietary tek/agilent/lecroy connector.Tek P5205 sometimes show up for cheap on ebay with cut cables, you just need to add new bnc and a power circuitry.https://www.eevblog.com/forum/testgear/tek-p5205-hv-differeantial-probe-teardown-btw-what-are-the-red-and-brown-wires/

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 13, 2017, 03:01:22 pm
content: You're just a victim of physics. I was going to post the probe derating curves, but I see you already looked at them. There's nothing wrong with the old Tek probe, but they do have a newer model that's permanently filled with non-ozone-depleting material. It has an input capacitance of 3 pF, too much for what you're doing. It's also too slow. If your fundamental is 100 MHz, where's the first harmonic? Yes, you're going to need a much faster probe. An active solution, unless quite complex/expensive won't handle your voltage level. You say you have to directly connect the probe, but this seems like something where a non-contact method would be far more suitable. It's not like you don't have a huge amount of field available! In the RF world I believe this sort of thing is most commonly done by sampling the signal via coupling, not resistive dividers like most probes. Please don't use millihertz (mHz) when you mean megahertz (MHz). It hertz the eyes of those used to the difference.

author: Kalvin
author_rank: Super Contributor
timestamp: May 13, 2017, 03:15:19 pm
content: Could you measure the current using a wide-bandwidth signal transformer? Just select a transformer with proper insulation. Or something like this:https://books.google.fi/books?id=V1jbeiR8nZEC&pg=PA83#v=onepage&q&f=falseAfter all, a capacitor is a current driven device.

author: Kalvin
author_rank: Super Contributor
timestamp: May 13, 2017, 03:44:00 pm
content: If the low side of the speaker is connected to the ground potential, just put a small resistor in series with the speaker's return signal and measure the voltage across the resistor with the network analyzer, or use a wide-bandwidth signal transformer for isolation if needed.

author: fonograph
author_rank: Frequent Contributor
timestamp: May 13, 2017, 03:59:08 pm
content: Conrad Hoffman,in my last post I explained that fundamental is only 12MHz and that my requirment for 100MHz was out of desire to see harmonics.Yes I would now write MHz and mHz properlyThe more I think about using non contact method,the more attractive that options becomes.At 12MHz and 4 thousand volt,like you said the field around should pretty strong,I am noob so I wonder,if I do the non contact method,how is it going change the measurement? For example,how different will freqency response be than if I measured directly? Is it going to be shifted?Becose I know low freqency AC radiates less photons than higher freqency AC,so the 12MHz fundamental wouldn be displayed lower and high harmonics at 100MHz higher relative to their true amplitude? Is this done capacitivly or using radiowaves/photons or magneticaly by induction? How much harmonic distortion and noise would this non contact method cause,more, or less than HV probe?

author: PartialDischarge
author_rank: Super Contributor
timestamp: May 13, 2017, 04:12:38 pm
content: Regarding the mentioned derating of these resistive HV probes I've opened a new thread, as I've always been curious about that graph...https://www.eevblog.com/forum/projects/tek-high-voltage-probes-derating-with-frequency/msg1207712/#msg1207712

author: w2aew
author_rank: Super Contributor
timestamp: May 13, 2017, 05:24:30 pm
content: Quote from: fonograph on May 13, 2017, 03:59:08 pm4KV is nothing to fool around with if you are a noob, mistakes can be fatal.Non-contact sensing is certainly safer. It won't change the voltage at your device, and frequency response will be a function of the near field probe / antenna. Since you're dealing with high voltage, low current, the near field energy will be dominant an E-field, thus you'd want to use an e-field probe.Here's a video I did on near field probes.

author: fonograph
author_rank: Frequent Contributor
timestamp: May 13, 2017, 07:20:08 pm
content: that was excellent video! I instantly subscribed,I hope your ankle is healed.These sniffer magnetic and e-field probes seem perfect,how is the noise and distortion performance when you use these instead of directly probing? Who makes the best sniffer probes?

author: Zero999
author_rank: Super Contributor
timestamp: May 13, 2017, 10:03:36 pm
content: Quote from: fonograph on May 13, 2017, 03:59:08 pmYes, go with the non-contact method. When you're in the near field (<<1/4wavelength), you can make the assumption that a metal plate, placed near what you're measuring is just a capacitor in series with it. If you know the area of the plate/wire and the distance, you can easily work out the capacitance.I'm a bit confused about whether you're measuring voltage or current? 4kV is easy to measure with non-contact methods but 1mA is much more difficult.QuoteNot quite. The amount of photons is the same, it's just that lower frequencies generate low energy photons, which are much more difficult to detect individually than high energy photons.

author: tautech
author_rank: Super Contributor
timestamp: May 13, 2017, 10:19:30 pm
content: Quote from: fonograph on May 13, 2017, 07:20:08 pmHere's a thread that should give you some guidance:https://www.eevblog.com/forum/testgear/new-affordable-emc-probe-set-at-a-bargain-price-from-ariel-rocholl's-lab/

author: Molenaar
author_rank: Regular Contributor
timestamp: May 15, 2017, 08:47:52 am
content: Quote from: Hero999 on May 13, 2017, 10:03:36 pmBut wouldn't a near field measurement load down the source too much for a reliable measurement, since in this case both the frequency and the source impedance are quite high?

author: Hensingler
author_rank: Regular Contributor
timestamp: May 15, 2017, 12:24:38 pm
content: Quote from: Molenaar on May 15, 2017, 08:47:52 amThe OP doesn't have a 4kV 12MHz source and never will. This whole thread is ridiculous.