High (>1Gohm) impedance scope probe? Why is this not a thing? - Page 1
Forum section: electronics / testgear

author: @
author_rank: Contributor
timestamp: July 14, 2025, 07:11:17 pm
content: I recently looked into ultra low power oscillators such as this design:http://www.discovercircuits.com/H-Corner/Ultra%20low%20current%20oscillator.htmThere are two variants, one with 100M resistors and one with 1Gohm, consuming 30nA and 3nA respectively. To better understand the circuit, I would like to use my 125MHz BW scope with 1Mohm inputs to probe the interesting nodes of the circuit.Trying to probe such a circuit with a common 10X passive probe is a no-go, obviously. 100X probes are readily and cheaply available but a 100Mohm input impedance is still too low. 1000X passive probes do exist but are quite bulky, because they're intended for high voltage signals. (On second thought: do these probes even have a higher DC impedance? The Tek probe circuits guide mentions a divider made up of 9M9 + 111k for a 100X probe - is that how all 100X probes are made, instead of a 99Mohm resistor?).Looking at active probes, these mostly seem to address different needs, such as differential probing or high speed signals (~GHz). The input impedance is still in the Mohm range, though.Ideally what I would want is a low-voltage (<12V), single ended, DC-coupled, moderate BW (50..100MHz) probe with high input impedance (>1Gohm). This doesn't seem to exist either commercially (for a hobbyist budget) or as a DIY/open hardware project.Am I overlooking something that would make this impossible? Is my goal somehow misguided?Or could this be made with a decent opamp (such as an OPA810) connected as a unity gain voltage follower to the circuit under test? I would then feed it through a coaxial cable and connect it with an in-line 50ohm load to the scope input. I'm hoping for someone to tell me why this won't work ;-)

author: TimFox
author_rank: Super Contributor
timestamp: July 14, 2025, 07:38:29 pm
content: What frequency do you need? At 1 MHz, a 1 G\$\Omega\$ capacitive reactance implies 0.16 fF capacitance.1 pF at 1 MHz and 50 MHz is a reactance of 159 k\$\Omega\$ or 3.2 k\$\Omega\$, respectively,A good active probe has a tip capacitance of roughly 0.5 pF in parallel with a reasonable DC resistance.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 14, 2025, 08:41:46 pm
content: From OP's link, the oscillator frequency measured a low 1Hz frequency. Pulse width is 1ms.

author: TimFox
author_rank: Super Contributor
timestamp: July 14, 2025, 08:51:48 pm
content: Hewanted to probe with a normal scope.If he only needs 1 Hz. why not use a 10 uF capacitor to feed the 1 meg scope input?

author: Zero999
author_rank: Super Contributor
timestamp: July 14, 2025, 08:54:54 pm
content: Add an FET op-amp buffer to the input, heck even the old TL072 will do. It will need a +6V and -3V power supply for a 0V to 3V input and output swing.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 14, 2025, 09:22:01 pm
content: Quote from: TimFox on July 14, 2025, 08:51:48 pmThe OP's post, and link they provided was pretty clear.You could try rolling one of Peace's old probes without the option loading, or buffer as suggested.https://www.eevblog.com/forum/beginners/rf-active-probe-input-impedance/

author: TimFox
author_rank: Super Contributor
timestamp: July 14, 2025, 10:09:01 pm
content: If DC accuracy is not needed, but response to frequencies << 1 Hz is required, a simple JFET source follower running from a split supply (two 9 V batteries, for example) driving the DC-coupled oscilloscope would be good for signals up to a few V, with very high DC resistance and reasonably low capacitance. One could leave the gate "floating" and return the common node ("ground") from the two batteries to the ground of the circuit under test.Only 1 JFET, 1 resistor, and a couple of bypass capacitors.

author: @
author_rank: Contributor
timestamp: July 15, 2025, 06:32:22 am
content: Thanks everyone for your comments.Quote from: TimFox on July 14, 2025, 07:38:29 pmLet me see if I understand you correctly: You simply plug the numbers into the formula for capacitance reactance Xc = 1/(2 * pi * f *C) and obtain a capacitance that the probe would have to have, which is way too low? That means my assumption that 100X and 1000X probes would use 99Mohm and 999Mohm resistors is wrong. Good to know!Quote from: TimFox on July 14, 2025, 08:51:48 pmYes, the oscillator has a frequency of 1Hz, but the pulse is 1ms and I would like to observe the transient behaviour, and also tune the circuit for higher frequencies. Also I would like it to be DC-coupled.Quote from: joeqsmith on July 14, 2025, 09:22:01 pmI had actually found the Pease probe in that forum post but dismissed it because I assumed the DC gain would be arbitrarily determined by the component tolerances. But maybe I need to study that some more - should be easy to breadboard for my low-frequency application.Looks like a FET buffer is the way to go. Apart from the dynamic characteristics, I assume an opamp would need to have enough oomph to drive the 50 ohm load that I would put at the scope input, right? Or I could keep using the scope's 10X probe and not worry about output current. Might even use a discrete JFET then.Now I'm just wondering why something like this isn't a readily bought accessory - maybe my needs are just too rare...

author: joeqsmith
author_rank: Super Contributor
timestamp: July 15, 2025, 01:40:37 pm
content: QuoteThere may be some specialized probes available but I am not aware of them. SPICE may be a lot cheaper if you just want to play with the circuit. If I need to work with really high impedances, it's at basically DC and I am using an electrometer. My old Keithly has an analog output that could drive a scope, but your never going to see that 1ms pulse with it.I had built one of Peace's probes back in the 80s when he posted it in EDN. That was my first active probe.***With the modern scopes being so low noise, I wonder if there is enough gain if you just add 1G to your 10X probe, would it be good enough?

author: Performa01
author_rank: Super Contributor
timestamp: July 16, 2025, 09:58:28 am
content: Quote from: joeqsmith on July 15, 2025, 01:40:37 pmDepending on the input capacitance of the 10X probe, the bandwidth of such a probing solution would be about 10 Hz, whereas the TO dreams of 50-100 MHz...

author: joeqsmith
author_rank: Super Contributor
timestamp: July 16, 2025, 12:41:22 pm
content: Quote from: Performa01 on July 16, 2025, 09:58:28 amI was thinking just to see anything with that 1ms pulse on the cheap. I wonder if OP has actually built the oscillator they linked and has tried to make any measurements yet.

author: David Hess
author_rank: Super Contributor
timestamp: July 16, 2025, 02:48:23 pm
content: There are such probes, however for oscilloscope applications the capacitance at the probe tip is more important than the input resistance. Making a probe which will perform better than anything you can buy is simple, as shown below.Quote from: @ on July 15, 2025, 06:32:22 amThe Pease probe uses source and emitter followers, so gain only depends on the alpha of the transistors and is always slightly less than one. This could be improved by replacing the emitter loads with constant current sources.QuoteUsually the oscilloscope input would be kept in high impedance mode for operation below 100 MHz, so the operational amplifier would have no trouble driving it, or drive a 10x probe from the buffer circuit.There are high impedance low input capacitance fast operational amplifiers now which could be used for the input stage. They use a bootstrapped input stage to achieve even lower input capacitance. Bootstrapping could be applied to Pease's probe to further reduce input capacitance.Analog Devices has the LTC6268 which achieves 350 MHz with a 3 femtoamp 450 femtofarad input, however its maximum supply voltage is 5 volts. Achieving higher voltage with your required performance is going to require a discrete input stage.

author: @
author_rank: Contributor
timestamp: July 16, 2025, 09:28:14 pm
content: Quote from: joeqsmith on July 16, 2025, 12:41:22 pmI have -- adding a 74LVC1G14 Schmitt trigger inverter to the output so I can observe the behaviour without disturbing it too much. Because the signal is mostly close to one of the rails, that barely increases the power consumption (at least as far as I can tell using a cheap DMM with 10nA resolution).The behaviour of the circuit was obviously very dependent on the supply voltage; but also the component values (such as the capacitor at the base of the NPN) had a non-intuitive influence... which is why I am trying to hook up my scope. You would think that decreasing that capacitance value would increase the frequency of oscillation in a reciprocal manner, but it's not that simple. As for the suggestion (by joeqsmith) of simulating it, I had some difficulty there, but if anyone's interested, someone else has tried this already:https://github.com/rabbithat/3nanoAmpOscillatorQuote from: David Hess on July 16, 2025, 02:48:23 pmThanks for the suggestion. 5V supply should be plenty.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 16, 2025, 09:54:24 pm
content: Took a few minutes and put it into Micro-Cap. Swept C1 from 2 - 6nF, 2nF steps. Easier than trying to sort out how to probe it.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 17, 2025, 12:58:27 am
content: I built the 100M oscillator using the parts called for with your original post except the 0.1 was swapped for a 0.15. 178 showing a standard 10x probe attached to the output. 177 showing the RAP active probe attached.Also shown is a 10:1 divider made from 10pc of 10M. 180 using RAP active probe, 181 standard 10x.Network analyzer supports 10Meg input. 10x probe was not compensated, long dangling ground lead, RAP probe missing dampening resistor, poor construction..... Still, good enough to give you some idea. RAP probe build with 2N5486.

author: David Hess
author_rank: Super Contributor
timestamp: July 17, 2025, 04:10:24 am
content: Quote from: @ on July 16, 2025, 09:28:14 pmWhat matters here is that with a 5 volt supply, the common mode input range of the LTC6268 is only a couple of volts. This can be increased with a compensated input divider, but this would limit the input resistance to lower values and likely add capacitance.Older designs bootstrapped a high voltage, +/-10 volts, 100+ MHz buffer, but this is a tricky piece of engineering to accomplish without VHF oscillation.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 17, 2025, 01:20:24 pm
content: I added a dampening resistor and ground strap to the RAP probe, then calibrated the network analyzer. It peaks to 3dB at 50MHz. Much better than before but needs some work. The input resistance is a bit over 30Gohm.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 17, 2025, 03:02:17 pm
content: Changing 0.1 to 0.15 (as built on breadboard) and zooming into pulse region.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 17, 2025, 04:36:14 pm
content: Quote from: @ on July 15, 2025, 06:32:22 amShown with 100p / 22p caps, keeping other components as listed. Period is now 16ms. The original article, seemed like they were trying to get as low a power draw as possible and still oscillate.SPICE may save you time/money. I'm surprised the breadboard Nazi's haven't chimed in yet saying how worthless they are..

author: David Hess
author_rank: Super Contributor
timestamp: July 17, 2025, 06:37:36 pm
content: Quote from: joeqsmith on July 17, 2025, 12:58:27 amQuote from: joeqsmith on July 17, 2025, 01:20:24 pmThe 2N5486 by itself has a calculated bandwidth of 93 MHz as a source follower, (1) but when driving the additional capacitance of the current sink and 2N3904, will be about half that, so very close to your measurement. Higher bandwidth will require a faster JFET, like a 2N4416 or 2N5397, which somewhat defeats the idea of having the lowest possible input capacitance.The 2N3904/2N3906 could be replaced with the faster MPSH10/MPSH81 which are more common in 100 MHz circuits.The input resistance has to be made as a difference measurement to remove the effects of leakage. So measure the input leakage while changing the bias voltage and then divide the change in leakage by the change in voltage.(1) Ft = Gm/(2*Pi*C)

author: @
author_rank: Contributor
timestamp: July 17, 2025, 06:49:37 pm
content: Thanks Joe, I've quickly built your variant with 100pF / 22pF. I'm seeing a period of 11ms if I measure after a Schmitt inverter. Without the buffer, loading the circuit with a 10X probe, the period is roughly halved.Current still seems quite low (tens of nanoamps), but the 74LVC1G14, if used as a buffer, now dominates with about 4uA at 3V. I can run the circuit at 1.7V at 0.3uA.So I guess I will have to build an active probe to measure it properly, that is without the buffer and without loading the circuit significantly. (Or just supply the buffer separately, but the point was to get analog signals at various points in the circuit).

author: joeqsmith
author_rank: Super Contributor
timestamp: July 17, 2025, 08:35:00 pm
content: What's your end game for it? Just playing or you actually have a use?

author: @
author_rank: Contributor
timestamp: July 17, 2025, 08:58:11 pm
content: Quote from: joeqsmith on July 17, 2025, 08:35:00 pmJust sort of a fascination with low-power electronics.I might do a variation on this project:https://hackaday.io/project/169882-ultra-low-power-led- which is misusing an LED as a flyback diode in a very dim light, which can run for years on a coin cell battery. It uses a quartz RTC module which consumes 45nA but is really overkill. Dave Johnson's oscillator (with your suggestion of picofarad capacitors) could hopefully be used. I'm wary of the 74LVC1G123 consuming more current if the signal isn't CMOS-friendly.More projects in the same vein:https://hackaday.io/list/194395-ultra-low-powerHere on the eevblog forum I've also come across the nanoWatt clock by Howardlong:https://www.eevblog.com/forum/projects/ultra-low-power-mcu-design-the-nanowatt-clock/Obviously for a clock you need a precise time base, but for some other application of PIC XLP + LCD an oscillator made of a few discrete parts could be useful. That one seems to require about 2kHz though.I guess it's like asking a golfer about why they try to hit a ball as few times as possible...

author: mawyatt
author_rank: Super Contributor
timestamp: July 18, 2025, 01:20:14 am
content: Quote from: @ on July 17, 2025, 08:58:11 pmMight like this DARPA project from awhile backhttps://www.darpa.mil/news/2015/n-zero-aware-electronicshttps://www.eetimes.com/darpa-research-advances-for-near-zero-power-sensors/https://fedtechmagazine.com/article/2017/06/military-iot-darpas-n-zero-initiative-aims-conserve-power-iot-sensorsBest

author: David Hess
author_rank: Super Contributor
timestamp: July 18, 2025, 03:52:01 am
content: You might be interested in this application note from Jim Williams where he measures the RMS current of a 32 kHz crystal in a micropower crystal oscillator.https://www.analog.com/en/resources/technical-articles/sub-ua-rms-current-measurement-for-quarts-crystals.htmlBob Pease's low capacitance probe could be improved with bootstrapping to further reduce its input capacitance and increase its input resistance. A small signal MOSFET could also be used for further improvement.At lower frequencies, the previously mentioned LTC6268 could be bootstrapped for higher voltage operation. I am not sure how this would compare to Bob Pease's probe except that it would achieve microvolt accuracy with its much higher alpha as a follower.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 18, 2025, 05:41:00 am
content: Beware of passive transmitters, too:https://en.wikipedia.org/wiki/The_Thing_(listening_device)

author: mawyatt
author_rank: Super Contributor
timestamp: July 18, 2025, 12:13:36 pm
content: Quote from: RoGeorge on July 18, 2025, 05:41:00 amThat's not really a "Passive Transmitter" but more of a "Pressure Sensitive Resonator/Reflector", also don't quite "buy" the Wiki detailed description of the operation!!Very clever device tho, reminiscent of the Laser Reflector concept utilizing a Window as a pressure sensitive diaphragm!!Best

author: joeqsmith
author_rank: Super Contributor
timestamp: July 18, 2025, 12:47:57 pm
content: Quote from: David Hess on July 17, 2025, 06:37:36 pmNotice the probe was peaking up, not rolling off.Quote from: David Hess on July 17, 2025, 06:37:36 pmR=V/I? In the context of loading, I don't think it matters much. The measured resistance could be much higher, but for sure it is not low enough to significantly effect OPs measurements at low frequencies, which was really the only point. Normally, I would run a test like this in a shielded box like the one shown. Note the sheets of Teflon inside the box. Measuring small currents isn't trivial. Everything was still setup, shown with the same measurement with the RAP probe on top of some foam. Resistance is now about double.That electrometer can source well over 20V. The output voltage of the probe will roughly track the input. It may not be very linear in this region but OP was working at 3V.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 18, 2025, 01:54:01 pm
content: Just a quick sanity test using a 1G 5% resistor. Measures 1.001Gohms directly. Measured a 18650 batteries open circuit voltage with Bymen at 3.99V (closer to OPs operating voltage). Placed the battery in series with the resistor and measured the current at 3.94nA.From previous measurement, 3.986V / 75.5G = 47pA. Placed battery in series with the RAP probe and measured the input current at 3.70pA. I suspect that the high test voltage from the electrometer is causing a problem when measuring the RAP probe direct. In the context of what OP was asking, it doesn't really matter.***added missing photo

author: David Hess
author_rank: Super Contributor
timestamp: July 18, 2025, 05:30:11 pm
content: Quote from: joeqsmith on July 18, 2025, 12:47:57 pmYea, so there is more going on with yours, which is hardly surprising. The formula is useful as an estimate of what is possible with a given transistor and explains why oscilloscope 1 megohm inputs were stuck at 200 MHz for a long time.QuoteIt could be important in the sense of a fair comparison with operational amplifiers which define common mode and differential resistance that way. Otherwise the input bias current interferes with the measurement.Incidentally, impact ionization causes orders of magnitude increases of JFET gate leakage as the drain voltage and drain current increases. This can be controlled by bootstrapping the drain to operate at a constant voltage below 10 volts.QuoteI am jealous of your 614. My hacked together transimpedance amplifier for current measurement, shown below, has a useful resolution down to 10 femptoamps if I am very still. I need a shielded enclosure for it to do any better and I calculated that could in theory give a 1000 times improvement.If I did that, then I would make all kinds of other improvements. I have an idea for adjusting the input leakage current by bootstrapping the supply to the operational amplifier.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 18, 2025, 11:26:48 pm
content: Quote from: @ on July 14, 2025, 07:11:17 pmQuote from: TimFox on July 14, 2025, 08:51:48 pmHard to get down to DC with a series capacitor.Shown is HP34401A attached directly to the Fluke 1VDC standard (no warmup time + dangling leads). Next the same 1Gohm resistor was placed in series with the standard and meter. Last the 1Gohm was placed in series with the RAP probe and HP34401A was connected to its output. As expected, RAP probe presents little loading at DC.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 18, 2025, 11:42:16 pm
content: Quote from: David Hess on July 18, 2025, 05:30:11 pmThe 614 was dead when I received it. Batteries were of course bad. It uses a high voltage op-amp that was also bad. I turned to China to get a replacement (parts were no longer available). Expecting to be scammed, when the parts arrived, their markings appeared legit. Made a jig to test them against the data sheets and all was fine. Big thanks to China.On the 20pA scale, it has a resolution of 10fA. Like pretty much all of my home test equipment, it is not in calibration.

author: coppercone2
author_rank: Super Contributor
timestamp: July 19, 2025, 12:02:45 am
content: I made this thread a little bit agohttps://www.eevblog.com/forum/projects/electrometer-front-end-for-oscilloscope/different idea, but similar solutionsI still don't know if I wanna start that project. It is low/medium use/interest to me.high impedance is a .. dry. The circuits are lazy and do no work.

author: Zero999
author_rank: Super Contributor
timestamp: July 19, 2025, 10:44:34 am
content: A plain old TL072 will do, as it's low frequency and decent enough amplitude not to worry about the offset voltage.I put it into LTSpcie, but it didn't simulate very well, which I suspect is due to the huge difference in magnitude between the RC time constants in the circuit.

author: iMo
author_rank: Super Contributor
timestamp: July 19, 2025, 11:06:53 am
content: You have to experiment with TL072 models, it seems. The libs I use (Bordodynov) includes various versions, for example:

author: Zero999
author_rank: Super Contributor
timestamp: July 19, 2025, 11:09:49 am
content: Quote from: iMo on July 19, 2025, 11:06:53 amThat's better, but I found it behaved the same, irrespective of whether the TL072 circuit was connected or not.Please post the .asc file.

author: iMo
author_rank: Super Contributor
timestamp: July 19, 2025, 11:13:51 am
content: It is your asc file..

author: Zero999
author_rank: Super Contributor
timestamp: July 19, 2025, 11:44:24 am
content: Quote from: iMo on July 19, 2025, 11:13:51 amThe different models don't explain why your results are different to mine. Which version are you using? I've recently updated LTSpice and it didn't make any difference.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 19, 2025, 12:04:50 pm
content: The original .osc oscillates apparently at random, even when the opamp input is disconnected from the oscillator. So before choosing a different opamp model, it's the Q1 Q2 oscillator that doesn't simulate well, in the first place.Instead of.tran 20s, use.tran 0 20s 0 1ms(or set in the GUI 'Maximum Timestep:' 1ms), or else the solver will skip large chunks of time, and miss any fast changes that might happen in that circuit. By setting max step 1ms, it will work just fine with the TL072 model already included in the original .asc file.(also, if the 1ms max timestep is specified, then the ic=0 initial condition for C1 is no longer necessary for the oscillator to start)

author: Zero999
author_rank: Super Contributor
timestamp: July 19, 2025, 12:14:51 pm
content: Quote from: RoGeorge on July 19, 2025, 12:04:50 pmI forgot to post my results, which I've added to the previous post.Yes, that works. I should have known to try that, as I'm aware of that issue.iMo hasn't done that though. His screenshot just shows .tran 20.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 19, 2025, 12:52:09 pm
content: Quote from: Zero999 on July 19, 2025, 12:14:51 pmThe timestep is not constant, it varies at each calculation step. The engine tries to guess the next timestep by observing the variations in the entire circuit. If the state of all the circuit's nodes look pretty stable, then the engine will "risk" a bigger timestep, based on the entire duration (20s) and the speed of the recent variation.My guess is, by replacing the model for TL072, that new model contained some faster internal variations somewhere in its internal nodes (when compared with your TL072 model), and thus, the engine noticed a fast variation in some of the nodes inside the new TL072, and "thought to itself": OMG! Things are moving fast in this schematic, I'll be prudent, and only make a small timestep to calculate my next point.For example, if you add a simple 1kHz AC voltage source with a 1kohm load attached to it (separated from the rest of the circuit), you can use.tran 20sand Q1 Q2 will oscillate without specifying any max timestep. This will work because the AC source will produce noticeable variations at the timescale of 1ms, the engine will notice the speed of at least one node varying fast, and will use a finer timestep. Since the same timestep is used to solve the entire circuit, the finer timesteps enforced by the AC source will have as a side effect the correct simulation of the oscillations in Q1 Q2.

author: mawyatt
author_rank: Super Contributor
timestamp: July 19, 2025, 02:47:36 pm
content: The "Look Ahead Timestep" SPICE algorithm is linked to all the time domain signal sources, so adding a time domain source will alter the Look Ahead dynamics.This Look Ahead algorithm is a very powerful feature of SPICE and utilizes a number of convergence parameters to "guess" at the next best timestep. Some of the more sophisticated algorithms will detect if an "event' was missed and step backwards in an attempt to recover. Long ago Cadence created their version that was quite remarkable in detecting subtle circuit nuance events that other SPICE flavors would completely ignore and step over. Of course Cadence wasn't plain vanilla SPICE, or free, or Open Source, so one needed to pay for this, but proved it's worth many times over in advanced IC design!Best

author: joeqsmith
author_rank: Super Contributor
timestamp: July 19, 2025, 03:30:08 pm
content: Quote from: Zero999 on July 19, 2025, 10:44:34 amAs David suggests, there may be better suited op-amps available today.Quote from: David Hess on July 16, 2025, 02:48:23 pm

author: iMo
author_rank: Super Contributor
timestamp: July 19, 2025, 03:33:07 pm
content: Quote from: Zero999 on July 19, 2025, 11:44:24 amHere is the LTspice 24.0.9And this is with 100secs transient.. It seems the numerical errors causing this..

author: RoGeorge
author_rank: Super Contributor
timestamp: July 19, 2025, 06:14:22 pm
content: Enforce a small-enough Maximum Timestep, 1ms or less in this case, and the simulation will become correct. The.opt numdgt 15won't help, because the errors are not numerical, the errors are caused by the (wrongly estimated) look ahead (time steps) algorithm. It jumps too far in time when the algorithm decides the next point to be calculated.These jumps are not equidistant. Their length is decided (as a best guess) after each calculated point, and the guess is based on the total time, here 100s. 100s is huge relative to the expected speed of the signals. It is about 1 million times larger than the meaningful details for a correct simulation, sub-millisecond range.Give a little hint to the look ahead algorithm, by enforcing the maximum timestep to 1ms or less.

author: Zero999
author_rank: Super Contributor
timestamp: July 20, 2025, 12:33:34 pm
content: Quote from: joeqsmith on July 19, 2025, 03:30:08 pmYes there are better op-amps available, but is it necessary, when the TL072 might do? I haven't built the circuit in real life, so I don't know for sure, but it's worth a try.Quote from: iMo on July 19, 2025, 03:33:07 pmTry deleting the TL071 and just simulating the oscillator?I think RoGeorge is right. Adding the TL071 model, caused the SPICE engine to decrease the time step, which had the effect of making the oscillator simulate properly.It's possible it didn't work with the model I used, because it's much simpler.Please post the library file you used the the TL071.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 20, 2025, 02:24:55 pm
content: Quote from: Zero999 on July 20, 2025, 12:33:34 pmOP would need to chime in. I think we all agree that the >1Gohm impedance the title calls for with a 50MHz BW would be impossible with what is available today, outside of simulation. Tim's (2nd post in the thread) describes the problem. OP required a DC coupling and describes using a digital gate to buffer the signal but wanting to look at the analog signal. We know it was a 1Hz pulse signal with a 1ms pulse width but OP wants to run it faster. The DC requirement seems like a must have. With circuit using 1G values, it also seems that the DCR needs to be >1Gohm. Beyond that, your guess is as good as anyone's.Showing RAP probe connected to LiteVNA. Measures roughly 1.5pF. May give the OP some idea how quickly the impedance drops with frequency.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 20, 2025, 02:31:57 pm
content: Would be interesting to simulatate the output impedance of each node in the oscillator (no opamp), to see if the Pease's high impedance probe can be used in practice or not.I mean, if a node in the physical oscillator has an impedance in the range of 1G\$\Omega\$ or so, and we try to measure that node with the high impedance JFET adapter, the 50MHz bandwidth becomes irrelevant, because the bandwidth response of the probe will be limited anyway by the input capacitance and the G\$\Omega\$ impedance of the node to measure. They make together an RC low pass filter, and the bandwidth will be severely limited. Instead of the measured 50MHz from a 50\$\Omega\$ generator, in practice the probe will have a bandwidth of only about 400Hz when the generator gas G\$\Omega\$ range impedance.400Hz, not MHz, not kHz. 400Hz, maybe less for higher than 1G\$\Omega\$ nodes!Code:$ # the -3dB BW for a 1Gohm output generator, when adding all the parasitic capacitances at the input of the JFET probe$ qalc '1/( 2*pi* 1Gohm * (0.29 + 0.06)pF )'1 / (2 × pi × (1 gigaohm) × ((0.29 + 0.06) picofarads)) ≈ 454.7284088 Hz$ # the -3dB BW for a 1Gohm output generator, when adding a 30Mohm in parallel with the JFET probe's input$ qalc '1/( 2*pi* (1Gohm || 30Mohm) * (0.29 + 0.06 + 0.08)pF )'1 / (2 × pi × ((1 gigaohm) ∥ (30 megaohms)) × ((0.29 + 0.06 + 0.08) picofarads)) ≈ 12.70772026 kHzLess intuitively, by adding to the probe those optional 3x10M\$\Omega\$ resistors at its input, the bandwidth willincrease(up to 10-15kHz or so when measuring G\$\Omega\$ range nodes). The smaller the added resistance at the input of the probe, the more bandwidth. However, the smaller the parallel resistance added, the smaller the signal to measure will become (for a source like this circuit with G\$\Omega\$ range resistors in it).Those 50MHz measured are great for the probe, but they are only possible for low impedance signals. In this oscillator, the usable bandwidth of the probe will be of only a few hundreds of Hz, maybe tens of kHz at best.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 20, 2025, 02:50:58 pm
content: Yep, was covered by Tim. Just a shorter version of it.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 20, 2025, 04:03:07 pm
content: Just to show there is no free ride per Tim's second post. The network analyzer was swept from 10Hz to 100kHz and normalized using a 10x probe. The 10x probe was then attached to RAP probe with a 1Gohm series resistor.3dB cutoff should be roughly 1/2pi(1Gohm*1.5pF) or 105Hz. Instead I measure 70Hz. This suggests the probe is closer to 2.3pF. The problem with making this measurement with the VNA is how I calibrated it (basically removing a thru connector when making the measurement which lowered the capacitance). But just for a ballpark measurement, you get the jist.Moving to a better op-amp may squeeze a bit more out of it but I can't see ever getting close to that >1Gohm impedance at 50MHz with todays technology. SPICE is really the best approach.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 20, 2025, 06:55:44 pm
content: Using the NanoVNA H4 with old mixers and added filter caps. Calibrated with homemade standards and RAP probe soldered to test fixture to avoid changing connectors.

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2025, 08:54:21 am
content: Quote from: joeqsmith on July 20, 2025, 04:03:07 pmPease was going for the simplest possible probe using readily available parts.Bootstrapping the JFET can remove more of its input capacitance, and is a common feature of higher performance probes and input circuits, but the examples I see look more like medium frequency compensation and might be to reduce settling tails. (1) The source follower by itself is bootstrapping its own input capacitance but is limited by its alpha, which is improved by using a current source as its load.Bootstrapping an operational amplifier does even better because its alpha is so much higher. A step up from this is to make the gain of the bootstrapping connection slightly adjustable so the input capacitance can notionally be trimmed to zero.A coaxial shield could be added to the probe input and then bootstrapped to cancel stray input capacitance, as shown below for a similar probe designed by Jim Williams.I wonder how close we could get with a better circuit. 1 picofarad at 50 MHz is 3.2 kilohms, so 1 femtofarad is 3.2 megohms. Somehow a high alpha at high frequency will need to be achieved.(1) I think the last detailed schematics I have of active probes stop in the 1970s. I studied them specifically because they can be fabricated with standard surface mount parts and 1 GHz is nothing to laugh at.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 21, 2025, 11:07:42 am
content: Quote from: David Hess on July 21, 2025, 08:54:21 amEven if you could achieve that, it's more than two orders of magnitude from what the OP was asking for. If you have any ideas on how to pull it off, I'm sure a lot of us are very interested in hearing them.

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2025, 03:22:46 pm
content: Quote from: joeqsmith on July 21, 2025, 11:07:42 amI know that an alpha high enough to divide the capacitance by 1000 times is possible out to 10 MHz; this is better than the common requirement for video amplifiers, but not by a lot. I think that could be improved by another 10 times because we have access to better parts now, but after that, the frequency would have to be reduced. So that gets us up to 32 megohms at a frequency somewhat lower than 50 MHz.That Linear Technology operational amplifier gives us about another 10 times improvement by starting with a lower capacitance, so that gets us up to 322 megohms, again at a frequency somewhat lower than 50 MHz.That is my Fermi answer. Beyond that I would have to do some clipboard engineering and work with real numbers, probably starting with the limitations of the Linear Technology part.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 21, 2025, 05:41:45 pm
content: Quote from: joeqsmith on July 20, 2025, 02:50:58 pmSorry, we were posting at the same time, mine was about the posts before yours.The idea was not covered, I mean the first phrase: Would be interesting to simulatate the output impedance of each node in the oscillator (no opamp), to see if the Pease's high impedance probe can be used in practice or not.Did that now, out of curiosity, and looked at the AC impedance of each node of the oscillator circuit. This was made with the purpose of deciding what parameter we should have for a probe, such that it wont alter too much the working of the oscillator while probing at different nodes.The .AC in SPICE considers only the DC operating point of a circuit, so it will miss any impedance variations of the transistors becoming on/off. I think this can be took into account (at least partially) by running a .tran first, writing down on a paper each node's voltage for the 2 distinct states of the oscillator, 1 and 0. Then forcing the above values into an .AC run (using .IC directives for each node), to find out the expected (dynamic) impedance of the nodes )corresponding to each of the 0 and 1 states of the oscillator output.The above simulation was made for 3 situation (each schematic for one of the following 3 situations):- the .AC impedance of nodes N1..N4 is calculated for the oscillator state when its output (node N4) is at zero )all nodes names end in "_0"- the N1..N4 nodes impedance is calculated without enforcing a high or low state, at the bias point found by LT spice- same as first, but this time the .AC is forced for the logic 1 (the high 3V out state) instead of 0What is my interpretation of those plots? The impedance of the nodes are way less than 1G, rather in the M\$\Omega\$ than in the G\$\Omega\$ohm range. Then why the OP is seeing a big difference when using a simple CMOS gate at the output?My guess is the oscillation period is altered because of the extra capacitance added by the gate, and NOT by the parasitic resistance of the gate. Which means that we can still use a CMOS gate if we manage to isolate the gate input capacitance from the oscillator.The easiest solution to isolate the gate capacitance: add a resistor between the output of the oscillator and the input of the CMOS gate, right near the output of the oscillator. How big the resistor? I didn't simulate the gate, and I don't know by heart what input capacitance to expect from a CMOS gate. I would start with a 100k\$\Omega\$, and increase as much as possible, up to M\$\Omega\$ or even G\$\Omega\$, until the gate doesn't switch any more. Maybe use a trigger Schmitt gate if you are looking for overall low power (to avoid any slow transient 0->1 or 1->0 states).

author: joeqsmith
author_rank: Super Contributor
timestamp: July 21, 2025, 06:46:54 pm
content: Quote from: RoGeorge on July 21, 2025, 05:41:45 pmYou have been busy!! I think I would phrase it "...to see if the Pease's high impedance probe can be used in practice or not, on that one specific circuit" OP posted about wanting to play with the circuit and change it. Trying to design for a moving target is no fun, but yes, I get your point.***Further increased values in sim... Wouldn't want to attempted it in real hardware. Not even sure if SPICE models are accurate enough to know that it even oscillates in real life..

author: @
author_rank: Contributor
timestamp: July 21, 2025, 07:32:42 pm
content: Wow I seem to have sparked quite a bit of discussion. Thanks to everyone involved. I'm still learning new things...The bandwidth "requirement" of 50..100MHz I put in my original post was just kind of made up on the spot, don't get hung up on that.I have simulated the circuit with ngspice integrated into KiCad, see screenshot. I like that you can have a regular schematic/layout file and then just run SPICE on part of it. Average current consumption also seems accurate. Downside is having to hunt for component SPICE models, as these don't come with KiCad. With a different model I found for the BJTs, it just wouldn't oscillate.I'm thinking that the 4.7Mohm resistor should probably be reduced in value, if one wants to feed the output to a CMOS circuit. Having the idle level closer to the upper rail would reduce the leakage current in the CMOS, surely? For the 1Gohm variant it wouldn't really matter.I've also drawn a PCB for the Pease probe with SMD components (MMBF5486). Going for a compact design, supplied by three CR2032s that will give an asymmetrical supply of -3V...+6V. (Not posting that yet because there are likely embarassing goofs, so I want to test it first. Waiting for ordered components and PCBs to arrive...)

author: @
author_rank: Contributor
timestamp: September 02, 2025, 06:27:47 pm
content: I have now built a "Pease" probe. Functionality-wise I consider it a success. The form factor needs improvement...To illustrate a use case:Over at hackaday.io, several people have built and documented ultra-low power LED drivers, where a LED is driven with a very low duty cycle (e.g. 3µs / 20ms). Current consumption can be well below 10µA, depending on desired brightness. I have now added a variation using the Johnson oscillator instead of PIC microcontrollers or RTC modules, which other people have been using:https://hackaday.io/project/203844-yet-another-ultra-low-power-ledInstead of having to program a microcontroller or RTC, one just needs to assemble a few jelly-bean discretes. I have drawn a ⌀20mm PCB but haven't had it made yet, so I'll attach a 3D render.Thanks to Bob Pease's probe design, I am able to probe Dave Johnson's oscillator circuit without changing the output frequency (or even stopping oscillation) as before with my passive oscilloscope probes.c1-charge.png shows the timing capacitor at the base of Q1 charging up until it triggers both transistors to conduct. Probing that with a 10X passive probe is not possible, as the capacitor is charged through a 100Mohm resistor.The other two plots zoom in on the output of the oscillator (channel 2, blue). This is fed into a 74LVC1G123 monostable for generating precise pulses, which are also shown (channel 1, yellow). My circuit compensates for battery voltage drop by increasing the pulse width, in order to achieve constant brightness as the battery discharges. The voltages shown don't vary because most of the circuit is on an 1.8V LDO. A voltage drop of 3V -> 2V would otherwise change the oscillation frequency by a lot.The probe circuit uses 3x CR2032 batteries for an asymmetrical supply of -3V / +6V. I'm mostly interested in using it in single-supply circuits <5V, so this is enough, with some headroom. The PCB shape is rather goofy though and difficult to hold. The heatshrink was kind of an afterthought and had to be applied after installing the batteries.I hope to make a better layout, maybe with some smaller batteries like LR44. The device should probably be made to fit inside a metal tube, both for shielding and ergonomics. Ideally it would accept one of these short ground spring wires that come with scope probes.You can find the KiCad files and schematic PDF here:https://github.com/sw/pease-probeAs I said, the layout is not very good, but the schematic may be useful for someone.Thanks again to everyone who has chimed in here.

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2025, 01:03:50 pm
content: Quote from: @ on September 02, 2025, 06:27:47 pmI have been thinking more seriously about an active probe design and think I have everything figured out except for the power supply. In the past, Tektronix got away with an external +/-15 volt supply brick, or having the oscilloscope provide the bipolar supply, but the former option does not seem practical today. I am not sure I can get low enough noise with a built in switching supply or an inverter running on external 5 volts or a battery.

author: JacobPilsen
author_rank: Regular Contributor
timestamp: September 03, 2025, 01:50:28 pm
content: I would prefer small Li-ion accumulator.

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2025, 03:09:14 pm
content: Quote from: JacobPilsen on September 03, 2025, 01:50:28 pmMaybe, but a higher performance design will still require a higher voltage bipolar supply so an inverter or switching regulator will be needed even for battery power.I also would not design something with a rechargeable battery unless the user can replace it.

author: MathWizard
author_rank: Super Contributor
timestamp: September 05, 2025, 10:51:04 am
content: In LTspice I made the low power osc. w/ BC547/556, and with 3 emitter-followers driving 50 ohms. And the osc works better than it did without it, although the rise/fall times of the 50R is in the hundreds of microsec, so yeah that could be a lot better. Without it tho it was 10's of ns.I made one of those DC-100MHz JFET probes, with THH parts and a 2N5485 from ebay, dead-bug style on a copper clad board. It seemed to do ok upto near 100MHz.I'm forgetting how high the input impedance of it is. And I already have 200MHz, probes, so I haven't tried to improve it yet. Now that I have some legit SMD JFETs, I want to make an all SMD version, and make an enclosure/case for the probe.

author: coppercone2
author_rank: Super Contributor
timestamp: September 07, 2025, 06:04:17 am
content: Is it possible that the capacitance of a fluke 289 meter can turn on a IGBT? I Just fired a power transistor probing a non functioning circuit. I don't think I shorted anything out with my probes. Basically its in some fault state where voltage is present but it does not operate. If you some how turn it on, it will burn stuff out.I am wondering if a active probe should be used for those circuits, even if its not required at first glance. This was @ 300V with the DC setting.

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2025, 12:35:07 pm
content: Quote from: coppercone2 on September 07, 2025, 06:04:17 amIt is possible. Say the meter has 200 picofarads of capacitance, and a small IGBT has 400 picofarads of input capacitance and is fully turned on with 6 volts. Then a voltage of 18 volts across the meter capacitance will charge the IGBT input capacitance enough to turn it on. A large IGBT might have an input capacitance of 10 times that so would not turn on unless the voltage across the meter was 180 volts which is unlikely.The driver circuit should be low impedance and absorb the charge from the meter, assuming it is connected.

author: coppercone2
author_rank: Super Contributor
timestamp: September 08, 2025, 01:42:47 am
content: thats a good clue, disconnected driverThis is a TO247 package.Still bad to have 'self destruct if probed' nodes on a circuit that is being repaired. To me it means the meter is not good for the job... ?I understand being careful with current, and HV nodes, but having... capacitance sensitive nodes to worry about is a bit much? New threats meters not equipped to deal with...