how to choose an active oscilloscope probe for 0-2v antenna measurements? - Page 1
Forum section: electronics / projects

author: k8943
author_rank: Regular Contributor
timestamp: January 22, 2019, 07:01:38 am
content: Following NXP guidelines for selecting the right passive components between antenna and Rx pins of PN7462 NFC controller, they recommend using an Active probe with a low capacitance (<2-3pF) for taking voltage measurements at the pins during communication.I looked on a few sites and had the impression that there are two kinds of active probes:1) those that are designed for lower voltages (say up to 20v or 50v) and cost around 1000USD upwards;2) those that seemed to be designed with measuring higher voltages in mind - and whose pricing starts lower down;The least expensive active probe on Digikey looks to be:https://www.digikey.be/product-detail/en/cal-test-electronics/CT4066/CT4066-ND/9342238Since this one seems to offer the required capacitance am wondering if it could do the job. (Which amounts to checking peak voltage levels at Rx inputs to the controller chip and making adjustments so they're somewhere around 1-1.5v.)Probably the best would be one of the "low voltage" probes but so far they all look a bit expensive (given this is all I really think I need it for)?One of the things that concerns me though is that it comes with two long wires which look like they're going to make a huge pickup loop for noise?Any recommendations?

author: macboy
author_rank: Super Contributor
timestamp: January 22, 2019, 03:20:56 pm
content: If the signal is single-ended and ground referenced, then you don't need a differential probe and you can probably use any ordinary FET probe. These typically have an input impedance of 100 kOhm to 1 MOhm in parallel with 1 to 2 pF capacitance, and have a typical bandwidth around 1 GHz. Of course a few very old or very cheap ones have less bandwidth, and the most expensive modern ones can go much higher. You should be able to get a used working one for $100-$250 USD. Note that all FET probes need a power supply, and the Tek ones usually require either a Tek scope with TekProbe connections or an external power supply such as Tek 1103. I use one of those with my Lecroy scope and Tek P6243 probe. Likwise, Lecroy active probes need a matching scope connection, and I haven't ever seen an external supply for those. Used Lecroy probes usually cost more than comparable Tek ones, but used Lecroy scopes themselves are usually a bargain.If you can't measure the signal w.r.t. ground then you will need a differential active probe, which will cost much more. You still need to think about the power supply.

author: cdev
author_rank: Super Contributor
timestamp: January 22, 2019, 04:08:28 pm
content: You didn't say what frequencies you want to measure.You can build your own FET RF probe for under $5 in parts or find kits on auction sites for under $15.A switch to a broadband high input impedance MMIC right at the input (assuming stability - likely only possible to assure with your own probes) might improve the flatness substantially at a tiny increase in cost. Also you could make small (loop and e-field, or a balun to give you a balanced input- but not a DC coupled one of course. Baluns are designed to cover a certain range - a balun that works great for HF likely wont work so well for VHF or UHF or LF, so you might want the ability to interchangeably use several of them) Making additional probes could extend the functionality a great deal. (That is how commercial antenna measurement equipment do things, some of that gear is basically just what I described, quite simple, but nonetheless, typically insanely expensive).

author: edavid
author_rank: Super Contributor
timestamp: January 22, 2019, 04:10:46 pm
content: Since the voltage you are measuring is reasonably high, and you don't need DC coupling, you can use a capacitive divider instead of a FET probe. Just put a 2pF capacitor between the measurement point and your lowest capacitance passive scope probe.You are correct in thinking that short leads are required.

author: Ice-Tea
author_rank: Super Contributor
timestamp: January 22, 2019, 04:22:06 pm
content: Do you need differential measurements? If not Testec may have something for you...

author: k8943
author_rank: Regular Contributor
timestamp: January 22, 2019, 06:10:52 pm
content: Great, this is really helpful.Ashamed to say not sure what the frequency of the Rx response is going to be from an NFC card. The requirement for an active probe was given on p45 of following app notehttps://www.nxp.com/docs/en/application-note/AN11706.pdfwhere they are talking about optimising the passives in the schematic on page 22 - but all the scope shots are too small to read the time scale!Seems the Testec TT-AF-1200 - can be had for about 700 money units.The Tek P6243 + 1103 power supply for about 300.Building something is enticing but will have to search to find something with appropriate input capacitance. So far just readhttps://www.instructables.com/id/DIY-1GHz-Active-Probe-for-Under-20/Fascinated by the idea of using a simple capacitive voltage divider. Not sure how this would work. From the description sounds as though the probe itself forms half of the divider. So in that case shouldn't one choose a capacitor of the same order of magnitude as the capacitance of the probe to have the best chance of cancelling out time delays?

author: edavid
author_rank: Super Contributor
timestamp: January 22, 2019, 06:23:32 pm
content: Quote from: k8943 on January 22, 2019, 06:10:52 pmNo. A pure capacitive divider has no phase shift, but why would you care if it did? You are just trying to measure voltage at the carrier frequency.NFC operates at 13.56MHz.

author: Ice-Tea
author_rank: Super Contributor
timestamp: January 22, 2019, 07:36:04 pm
content: Quote from: k8943 on January 22, 2019, 06:10:52 pmNot newhttps://nl.farnell.com/tektronix/p6243/probe-active-1ghz-3000-series/dp/207433

author: Ice-Tea
author_rank: Super Contributor
timestamp: January 22, 2019, 08:10:46 pm
content: To further on that notion: if you are anywhere near the Limburg area, you could pick up my Testec probe for a few days.

author: k8943
author_rank: Regular Contributor
timestamp: January 23, 2019, 09:28:42 am
content: @blueskull @ice-tea thank you for your kind offers. What great spirit in the EEV forum!Yesterday was preoccupied about designing a PCB but awoke realising have a dev board so could attempt some measurements straight away.First tried measuring Rx signal with passive probe (20pF 10M). Got readings just over 1 volt p-p, which is perfectly reasonable. As @edavid suggested soldered in a 2pF to attempt the capacitive divider trick. The readings were now VERY small. Scratched head and realised that maybe the voltage drop in a divider would be largest over the SMALLEST capacitor. So then dropped in a 2.2nF and now got voltage readings 10% higher p-p than before (i.e. 1.1v). Incidentally the waveform that I'm measuring seems to be 130kHz.Looks like a victory for the capacitive divider approach. Of course at some point it would be nice to check this against a real active probe, but first will try to understand this NFC stuff a bit. Right now the waveforms from the antenna and from the Rx mean less than a bowl of noodles.

author: David Hess
author_rank: Super Contributor
timestamp: January 23, 2019, 04:30:03 pm
content: Quote from: edavid on January 22, 2019, 06:23:32 pmThis would be a good candidate for a home built active probe like that shown below; replace Q1 and Q2 with your favorite RF JFETs. In the past when faced with this problem, I just included a JFET input based probe as part of the circuit which went unpopulated in production.

author: k8943
author_rank: Regular Contributor
timestamp: January 24, 2019, 11:39:21 am
content: Quote from: David Hess on January 23, 2019, 04:30:03 pmIf you were making large numbers of boards, might guess you had access to commercial active probes, so are you suggesting a built-in circuit gives a better result?Initially thought "yippee, maybe I can find a dual SMT package RF JFET.... and maybe the gain stage is not really necessary....". Then started looking at datasheets and became more concerned. Then followed the reasoning behind a related circuit in HH and now have the impression this is a project in itself!

author: k8943
author_rank: Regular Contributor
timestamp: January 26, 2019, 01:17:24 pm
content: @davidh A day or two later and find the appeal of your in circuit approach is only growing. Found modern SOT-23 equivalents (‎MMBF5486, ‎MMBT3904, ‎MMBT3906 - which assuage some of the earlier concern about unbalancing a finely equilibrated circuit with 47 and 51 ohm resistors) and a nice 3mm*3mm trimpot. Plenty of room left for two holes to wire in the bench PSU!Since my objective will specifically be to get accurate peak voltage readings (at 13.5Mhz) any recommendations on how to calibrate? Is it reasonable to use DC? Oh, and what about the optional 30M resistance at probe tip?

author: k8943
author_rank: Regular Contributor
timestamp: January 26, 2019, 01:35:43 pm
content: I should be able to get my hands on one.Is the idea to measure the sig gen output at DC and then crank it up to 13.5Mhz and assume the peaks are the same as voltage measured earlier? Then to calibrate the scope reading accordingly? Presumably one would use a sine wave since that's comparable to the kind of signal to be measured?

author: David Hess
author_rank: Super Contributor
timestamp: January 27, 2019, 03:34:09 am
content: Quote from: k8943 on January 24, 2019, 11:39:21 amUsing a probe would have required some type of probe tip adapter built into the circuit anyway and the built in circuit works better or at least good enough unless the bandwidth requirements are too high which is not the case for your application.QuoteIt is actually better to use separate JFETs in most cases to prevent coupling between them.Quote from: k8943 on January 26, 2019, 01:17:24 pmFor calibration short the input and trim for zero output.The input resistor is needed if the source is AC coupled to absorb the JFET's gate leakage.

author: David Hess
author_rank: Super Contributor
timestamp: January 29, 2019, 11:42:31 pm
content: I found a couple more examples of this kind of circuit worth studying in the LT1010 datasheet. Instead of using two JFETs with one correcting the offset of the other, a low input bias current operational amplifier is used to correct the offset of the JFET. As shown below, a chopper stabilized operational amplifier is used but any low input bias current part would work like a bipolar LT1008/LT1012/LT1097 or a JFET/CMOS part like a TL071. The trade off might be worth it to avoid the need for a matched JFET or trimmer.The disadvantage of this form of offset correction and why it is not used in oscilloscopes at least in this form is that if the input is overloaded, the operational amplifier's slow control loop causes very slow recovery time but this is probably not a consideration in your application.

author: k8943
author_rank: Regular Contributor
timestamp: February 01, 2019, 03:55:50 pm
content: Really interesting, thanks.The LT1010 / LT1050 combo explodes the BOM cost in a way noticeable in a throwaway in-circuit probe. Suggests building it standalone. Looked at datasheets but not sure how to figure out if would retain precision running at a voltage spread greater than the +5v/-5v cited.JFETs arrived but PCB stuck in China until after the end of their New Year's celebrations in a couple of weeks. Looking forward to testing - still struggle to believe the PNP/NPN pairs 1v:1v.

author: David Hess
author_rank: Super Contributor
timestamp: February 02, 2019, 02:08:53 am
content: Quote from: k8943 on February 01, 2019, 03:55:50 pmI meant that as an example to study and not a literal implementation. Replace the LT1010 with the cheap class-A 2N3904/2N3906 buffer in Bob Pease's design which I showed first and use your favorite JFET or superbeta operational amplifier like the LT1012/LT6010 in place of the LT1050.

author: k8943
author_rank: Regular Contributor
timestamp: March 03, 2019, 06:47:31 pm
content: Finally got to testing the probe built into NFC board. Whilst the attachment of a passive probe at the measurement point seems to reduce the reading on the active circuit by about 5%, the readings from both approaches vary markedly. (Image attached - CH1 Pease, CH2 standard probe connected upstream - note long timebase.)Very worthwhile! Thanks for this and look forward to taking it further as iterate the board/antenna and fine tune the Rx circuit.