Oscilloscope protection circuit - Page 1
Forum section: electronics / rf-microwave

author: Creedonics
author_rank: Newbie
timestamp: August 08, 2025, 09:47:07 am
content: Hello everyone,I'm currently an intern in a company and i was given a task, which is to create some sort of protection circuit for an oscilloscope that connects between the passive prob and the oscilloscope, the measured signal can go up to 1GHz, the idea is to keep the signal between 5V and -5V and block anything outside of this range, also there needs to be no change to the measured signal whatsoever ( no change of phase or amplitude ) and it has to be a passive circuit.I've thought to use a clipper circuit and it's probably gonna change the impedance so i might add an impedance matching network just after or before it, what do you guys think about this ?thank you in advance !

author: David Hess
author_rank: Super Contributor
timestamp: August 08, 2025, 10:36:29 am
content: If this is a 50 ohm system, then do what Tektronix did in their 1 GHz 7A29 vertical amplifier, as shown below.

author: Creedonics
author_rank: Newbie
timestamp: August 08, 2025, 11:03:50 am
content: are you talking about just the 4 diodes and 2 capacitors or the whole thing ?because i forgot to mentioned that it needs to be a passive circuit

author: jonpaul
author_rank: Super Contributor
timestamp: August 08, 2025, 11:17:45 am
content: Get and read the Tektronix Circuit Concepts books: 1980s, STILL excellent on these topics.https://w140.com/tekwiki/wiki/Concepts_SeriesOscilloscope Probe Circuitshttps://w140.com/tekwiki/images/6/62/062-1146-00.pdfProbe measurementshttps://w140.com/tekwiki/images/1/19/062-1120-00.pdfOscilloscope Vertical Amplifier Circuitshttps://w140.com/tekwiki/images/f/fd/062-1145-00.pdfYour proposed external protection is a poorly worded problem.Its unneeded and will result it loss of the carefully designed probe><Vert amp specs.At 1 GHz you must use a 50 Ohm input, TEK and other such scopes have built in power overdiss protection.At 1 Meg you ill not have a 1 GHz BW, and anything between probe and scope input will kill the specs.most 1 M scopes have a 400 V peak rating. With a 10X probe the probe rating dominates. Usually also 400VHave a great day,Jon

author: David Hess
author_rank: Super Contributor
timestamp: August 08, 2025, 11:22:49 am
content: The 350 MHz Tektronix 485 used a passive version, shown below, but it still requires higher supply voltages.I am not aware of any workable passive options, except the 485 example, which will support a 1 GHz bandwidth. The best third option is an active clamp using diodes or bipolar transistors.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 08, 2025, 05:46:14 pm
content: Quote from: Creedonics on August 08, 2025, 09:47:07 amIf its a super crap scope, maybe you can fiddle with the input and not detect a change. No change, seems impossible. I would start by defining the problem. What are the conditions you need to protect against? Are the probes a section of coax, or a 100x? Is the transient you are protecting against fast like ESD? How much voltage, energy..... Constrain the problem first.

author: hfleming
author_rank: Regular Contributor
timestamp: August 08, 2025, 10:17:03 pm
content: Ypu can make a PIN-diode clipper/clamper, but you really have to look at the requirements. Clamping a 1uS pulse is a whole different thing from trying to clamp a 100uS pulse…. And when you have to start clamping pulse trains, things can become really tricky…

author: jonpaul
author_rank: Super Contributor
timestamp: August 09, 2025, 01:45:59 am
content: SOLVED:Low capacitance bilateral TAZ.Rhom 12V SMD 5A 7 pF Bi-directional ESD ±8kV IEC61000-4-2https://www.rohm.com/products/diodes/transient-voltage-suppressor-diode/standard/vs12vba1hs-product#productDetailENJOY!j

author: Hamelec
author_rank: Regular Contributor
timestamp: August 13, 2025, 08:45:36 pm
content: 1GHz? 7pF? -- ENJOY

author: LaserSteve
author_rank: Super Contributor
timestamp: August 15, 2025, 01:18:01 pm
content: I found 1 pf and .26 pF protective diodes for Cell phones, but the question is, what do you do with a 1 Ghz or better op-amp to make an active low capacitance clipper... I'm genuinely curious.Steve

author: radiolistener
author_rank: Super Contributor
timestamp: August 15, 2025, 05:00:42 pm
content: I'm afraid passively keeping the input strictly within the ±5 V range at frequencies up to 1 GHz without any change to phase or amplitude is physically impossible. Any limiter (diodes, TVS, etc.) introduces nonlinearity and capacitance, even 0.5-1 pF at 1 GHz presents a shunt impedance of only a few hundred ohms, which degrades matching.As I remember, in one Soviet oscilloscope type, a special designed vacuum tube was placed at the input as a "hard" protection element. This provided exceptional robustness and good accuracy up to ~50 MHz. As far as I recall, those oscilloscopes could tolerate input voltages of kilovolts and proved to be remarkably robust in this regard. For 1 GHz, however, I think such an approach would be virtually impractical: parasitic parameters and construction constraints would prevent maintaining the required bandwidth and proper impedance matching.

author: LaserSteve
author_rank: Super Contributor
timestamp: August 15, 2025, 05:15:57 pm
content: 429 Ohms to ground at 1 Ghz plus parasitics...I'm curious how Tek and Lacroy do protection these days.

author: David Hess
author_rank: Super Contributor
timestamp: August 15, 2025, 06:08:07 pm
content: Quote from: LaserSteve on August 15, 2025, 05:15:57 pmThey probably implement protection of 1 GHz inputs in the same way Tektronix did it in 1979, which I showed. I do not know that there is a better way.I know the 1979 implementation was good enough at 1 GHz that Tektronix had to machine their own BNC connectors because even with the protection circuit, they could detect that no purchased BNC connectors were actually 50 ohms.

author: G0HZU
author_rank: Super Contributor
timestamp: August 15, 2025, 08:06:12 pm
content: To me it looks like the Tek protection circuits are going to be ineffective if you want to protect the very front end of the scope from big signals up at UHF. I don't think they are designed for this type of protection so I would suggest you don't consider them as viable for your application.The cheapest option would be a Zo probe followed by a passive diode clipper at the scope input. This would only have a medium input impedance of maybe 1.5k ohm so it may be no good.A basic (but well designed) clipper made with four 1N4148 diodes would be able to clip and protect up to maybe 300 MHz and it would have low VSWR and low loss (< 0.15dB) up to about 1GHz in a 50 ohm environment. But it wouldn't be able to clip and protect for signals up above about 300 MHz. A decent PIN diode clipper could be used here but it will clip at a lower level and it probably won't protect as well as the 1N4148 diodes at lower frequencies. It's possible to add an RF fuse inline with the limiter but this will add to the insertion loss.I would normally just use the Zo probe without the clipper for most things like this as it will act as a wideband voltage divider and this will protect the scope against fairly large signals. However, if you are probing really huge RF signals at up to 1 GHz then I'd recommend you think about why you want to do this as it is going to be hazardous for you as well as the scope.

author: radiolistener
author_rank: Super Contributor
timestamp: August 15, 2025, 08:23:09 pm
content: using limiting diodes will not protect the oscilloscope from kilovolt-level signals.And if a high-voltage source with sufficient energy is applied, it will ignite a spark or plasma from your protection diodes, which will destroy the oscilloscope’s input circuitry along with any protection diodes. Passive diodes can only handle small and low power overvoltages, they cannot prevent catastrophic failure from high-energy transients.

author: G0HZU
author_rank: Super Contributor
timestamp: August 15, 2025, 08:53:56 pm
content: Quote from: radiolistener on August 15, 2025, 08:23:09 pmIs he really going to be probing signals that big at 1GHz? 1kV at 1 GHz? Where did that come from?The diode limiter would be after a passive Zo probe (this probe has a big series resistance ahead of the clipper diodes that acts as a current limiter) but even a 100V pk sinewave would damage a typical Zo probe before damaging the diodes or the scope. It might be possible to make a Zo probe that could cope with 100V peak at its input at 1GHz.

author: G0HZU
author_rank: Super Contributor
timestamp: August 15, 2025, 09:07:20 pm
content: Note that a Zo probe needs to look into a very low VSWR in order to minimise the mismatch uncertainty between the large value series (current limiting) resistor and the input of the clipper. So the clipper would have to be designed to have a low VSWR all the way up to 1GHz.This is possible using four 1N4148 diodes arranged as two back to back pairs if you only needed protection up to about 300 MHz. To protect up to 1GHz would require better diodes but you would be trading ruggedness for bandwidth as the 1N4148 diodes are quite rugged when used as an RF clipper. Maybe use several PIN diodes in series to make up each element of the clipper? It might be a challenge to make this work to 1 GHz with low VSWR.

author: David Hess
author_rank: Super Contributor
timestamp: August 16, 2025, 01:02:19 am
content: Quote from: G0HZU on August 15, 2025, 08:06:12 pmI did not show it, but what Tektronix fully did was to use the diode bridge for instantaneous protection, while measuring the RMS power of the signal and disconnecting with a relay in front of the diode bridge. (1) They even made a custom IC for the RMS measurement.The stages in order were:Input BNCDelay Line or Optional Variable Delay LineInput Coupling Selection with PrechargeProtection Relay and PickoffInput AttenuatorsCurrent Driven Diode BridgeAmplifier with 50 Ohm TerminationThe specification state this was good enough for 10 volts RMS or 1 watt-second pulses not exceeding 50 volts peak.Their earlier design used a fuse, so I suspect they found that this was not enough for instantaneous protection, and later found that diode shunt protection was either not enough, or was limiting performance. Unfortunately they never published a detailed discussion of the design considerations for these amplifiers.I regard the overload protection of a high fidelity 1 GHz input as a best effort exercise.(1) The relay limited the maximum voltage because the contacts could not always survive a high voltage disconnect without damage. I do not understand how the diode bridge would survive such a high voltage, but apparently it does and is rated to 100 volts (!) while having a bandwidth of 7 GHz (!) which would have been necessary for good signal fidelity in a time domain instrument.

author: G0HZU
author_rank: Super Contributor
timestamp: August 16, 2025, 07:02:38 am
content: What I mean is that the Tek protection circuit looks to be designed to go after the input attenuators and not right at the scope front end. It only has a small forward bias so in normal use I suspect it isn’t meant to pass ‘normal’ signals above a small fraction of 1V. Otherwise I’d expect to see it add distortion to the signal. So it probably relies on the user to set the input attenuator correctly to stop it seeing signals above maybe 200mV.In other words, it won’t be much use if you fitted it right at the front end (before any attenuators) unless you only ever want to pass tiny signals through it in normal use or if you didn’t care about signal distortion.I’m basing this on a casual glance at the circuit and where it is located in the scope.

author: iMo
author_rank: Super Contributor
timestamp: August 16, 2025, 08:37:36 am
content: Could you point us to the diodes' spec/type in those TEK bridges? I made a sim with 1N5711 (the same as the popular HP-5082 series of schottky diodes used at the time), it "clips" somehow but the output signal is a crap.. Were those diodes PIN diodes?

author: David Hess
author_rank: Super Contributor
timestamp: August 16, 2025, 11:46:10 am
content: Quote from: iMo on August 16, 2025, 08:37:36 amTo support such a high frequency it was a hybrid with transmission line construction, so I was only able to find the specifications for the hybrid. The schematic shows as much information as is available. Given the voltage rating, I suspect the diodes are not too far off from common low capacitance silicon diodes.https://w140.com/tekwiki/wiki/155-0180-00Quote from: G0HZU on August 16, 2025, 07:02:38 amYou are probably right. It would require much higher bias to pass a larger signal.I have had good results using it to protect low impedance inputs and outputs, but did not require low distortion.

author: G0HZU
author_rank: Super Contributor
timestamp: August 16, 2025, 12:01:29 pm
content: It's probably going to be OK up to about 500 mVpk and above that there will be some distortion that would be visible on an FFT. Anything more than about 800 mVpk would show marked distortion IMO.The other minor issue would be the insertion loss as each diode would probably have an ESR of a few ohms at 8mA bias per diode plus there is the shunt loading of the 612R resistors. So it might have a typical loss of a dB or so and this would mean something like a -11% error in reading the input voltage. The input VSWR would also be degraded slightly but not enough to be overly concerned about.

author: radiolistener
author_rank: Super Contributor
timestamp: August 16, 2025, 12:14:40 pm
content: Quote from: G0HZU on August 15, 2025, 08:53:56 pmThe 1 kV example was not meant to imply a 1 GHz sinewave at that voltage - overvoltage can occur at any frequency. It could just as well be 50 Hz mains accidentally fed into the input, or DC, or RF from a high-power UHF transmitter, which would instantly exceed the safe limits of most probes and front-ends. Even if a diode could theoretically withstand high voltages, at 1 GHz this would translate into extremely high power levels during conduction due to the low impedance in the opened diode. Under such conditions, the diode would rapidly overheat and fail, often catastrophically.For example, the output of a 1 kW amateur radio transmitter aimed at EME (Earth-Moon-Earth) communications would produce far more than 100 V - at least 317 V peak. And who knows where else a user might connect an oscilloscope, accidentally or due to a lack of knowledge.I have, on more than one occasion, accidentally burned out fairly high-power attenuators made solely of resistors due to connection errors with a medium-power amplifier, so it goes without saying what would happen if diodes were at the input instead. Passive oscilloscope probes - which primarily contain resistor dividers and small compensation capacitors - often fail almost instantly when connected directly to the output of an amateur radio transmitter, often with visible smoke coming out of them.Incidentally, this is a fairly common situation: even technically competent operators often connect a standard 1:10 oscilloscope probe to the output of a 100-500 W transmitter, relying on the probe’s 600 V CAT rating, without realizing that the voltage limit drops sharply with frequency - down to 10-20 V at RF frequencies.

author: G0HZU
author_rank: Super Contributor
timestamp: August 16, 2025, 12:22:11 pm
content: Quote from: G0HZU on August 16, 2025, 12:01:29 pmThinking about this a bit more, it may be the case that the ESR of each diode is factored into the choice of the 612R shunt resistors so the top and bottom halves of the circuit then mimics a low loss T attenuator. So the VSWR might hardly be degraded at all. But I think there will be some insertion loss because of this.

author: iMo
author_rank: Super Contributor
timestamp: August 16, 2025, 12:54:06 pm
content: FYI - 200mV and 200V p-p with 500MHz square signal.The diodes are the fast HP 75V schottky from that time, imho..PS: added 1N4148 version

author: David Hess
author_rank: Super Contributor
timestamp: August 16, 2025, 05:04:26 pm
content: Quote from: G0HZU on August 16, 2025, 12:22:11 pmThe datasheet says 1dB loss. The plug-ins achieved 1:1.4 SWR worst case.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 18, 2025, 03:12:27 pm
content: Quote from: jonpaul on August 09, 2025, 01:45:59 amOP never provided further details on requirements. For all I know they want to connect coax to the wall socket.I think this is what I used on the low cost NanoVNA H4s I modified.https://assets.nexperia.com/documents/short-data-sheet/PESD5V0R1BCSF_SDS.pdf• Bidirectional ESD protection of one line• Extremely low diode capacitance, Cd = 0.085 pF at 10 GHz• Extremely low insertion loss: -0.25 dB at 10 GHz• Extremely low return loss: -19.4 dB at 10 GHz• Extremely low clamping voltage to protect sensitive transceivers• Extremely low leakage current < 1 nA at 5 V• Extremely low inductance protection path to ground

author: David Hess
author_rank: Super Contributor
timestamp: August 18, 2025, 05:18:28 pm
content: That should be good for ESD, but I suspect other types of even minor overload would wreck it. I do not see any power or current specifications, and the junctions must be tiny to get that low capacitance.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 18, 2025, 05:41:17 pm
content: Quote from: David Hess on August 18, 2025, 05:18:28 pmOP would need to define what the minor overload conditions are. Coax to the wall socket is out, but that may very well be what their manager was wanting... Managers are not always the brightest. At some point, proper training would seem like the answer.****I wasn't even sure if they were wanting to leave it in place with all their different probes. Like so many such posts, its a pointless discussion without the OP. That or the rest of us would need nail down some the requirements and post them, just for the sake of having some talking points.Here I show using a GDT/attenuator/TVS combination to protect a cheap VNA from some pretty large transients. It wouldn't survive the wall socket test. The video seemed to cause a bit of a stir with a few of the extra class hams.***the following link provides a discussion on input protection:https://nanorfe.com/forum/ESD-safer-circuit.html

author: LaserSteve
author_rank: Super Contributor
timestamp: August 20, 2025, 02:57:49 pm
content: I found an input protection /limiter MMIC.. DC-6 Ghz..Not so sure OP will like the low end rolloff...https://www.millermmic.com/product/limiter/MLT001But I remember a published article on scope front end design showing a separate LF path some place.Steve

author: joeqsmith
author_rank: Super Contributor
timestamp: August 20, 2025, 03:38:02 pm
content: Quote from: LaserSteve on August 20, 2025, 02:57:49 pmNo idea what OP needs but I would want DC. The datasheet talks about AC coupled inputs/outputs.

author: LaserSteve
author_rank: Super Contributor
timestamp: August 20, 2025, 04:26:52 pm
content: MLT001 is rated DC-6 Ghz.MLT002 is rated DC-12 Ghz,MLT003 is rated DC-18 Ghz.And so forth.I'm guessing the coupling cap is called for on ALL the data sheets, and the inclusion on the DC models is an error.Engineers miss the details sometimes when writing data sheets, and copy-paste is a common error.Steve

author: G0HZU
author_rank: Super Contributor
timestamp: August 20, 2025, 04:40:30 pm
content: For one thing, that limiter is a bare die device so the NRE costs would be huge unless you can buy an eval board with it already absorbed into a suitable enclosure. It’s also going to be quite fragile and would only be linear over a very small voltage range.There’s a link to QFN package but is this available?

author: LaserSteve
author_rank: Super Contributor
timestamp: August 20, 2025, 08:17:33 pm
content: That was the part closest to the previously mentioned hybrid that I could find.Steve

author: David Hess
author_rank: Super Contributor
timestamp: August 20, 2025, 08:31:23 pm
content: Quote from: LaserSteve on August 20, 2025, 02:57:49 pmThere are plenty of fully documented examples of dual path high impedance inputs, with an early design from the Tektronix 2232 series shown below. High impedance inputs are much easier to protect because their high input impedance allows for large values of series impedance to limit current. Continuous overload protection of 300 to 400 volts peak DC is common, and higher is possible, with derating as frequency increases.In a dual path design, the low frequency high impedance resistive divider before the slow DC path protects the operational amplifier, but the divider also multiplies low frequency noise.