Measuring Oscope Probe Bandwidth with an RF Sig Gen - Page 1
Forum section: electronics / testgear

author: killingtime
author_rank: Regular Contributor
timestamp: November 06, 2023, 11:56:37 pm
content: Hi,Bought some Oscope probes and wanted to measure their -3dB bandwidth. I know some people use a square wave generator and measure the edge rise time, but I don't have one, so I wondered if I could use an RF sign gen instead (sine wave) with a 50 Ohm o/p impedance and an oscope. Wind the frequency up until I see the voltage drop by x0.707.I've tried this using a probe to BNC adapter and I'm getting weird results. The plots don't look flat at all. Attached. Two probes. A genuine R&S 300MHz one and the new one.The RF sig gen (50 Ohms) has been tested on a spectrum analyzer. It's flat (into 50 ohms) across the freq range (+/- 0.5 dBm). No issues there.. The scope has a 300MHz i/p bandwidth.. The scope probe used as a 'control' has a 300 MHz bandwidth. 12pF tip capacitance. R&S RT-ZP03. Bought new. No reason to suspect it's faulty.. All probes set to x10.. I'm testing up to 200 MHz, so I'm well away from the -3db point on the mfr spec sheet of the control probe and oscope.. I'm measuring the peak to peak voltage displayed on the scope.. I'm aware scope probe tip capacitance will present a reactance that goes down as freq goes up, and that this will reduce the voltage across the probe given there's a 50 Ohm source resistance in the sig gen.I've run a sim in LTSpiceIV of the equiv circuit. Attached. The probe tip voltage is 20% down at 200MHz on account of resistive divider circuit formed with the sig gen o/p impedance. I think this is why the plots aren't that flat. Even before we get into bandwidth limitations (probe design, oscope front end etc) a lot of amplitude is lost to the sig gen o/p impedance.Would it be as simple as just subtracting the theoretical divider loss from measured Vpk-pk results to get the real probe -3dB bandwidth (Vpk down by 0.707)?Something else I've noticed is that the probe tip capacitance isn't constant. Not too sure on why though. I've swept a nanoVNA into the probe front end and I get 12pF back on the smith chart but it goes up with frequency to about 25pF at 175 MHzThanks.

author: TimFox
author_rank: Super Contributor
timestamp: November 07, 2023, 12:15:17 am
content: The usual rule-of-thumb that RiseTime = 0.35/Bandwidth is not true in general for oscilloscope amplifiers, but is commonly used.Measuring the bandwidth directly is accurate, so long as you know that the output of the generator is truly flat.Even then, there will be an unwanted lpf at the input, with a time-constant given by 25 ohms (50 ohm source into 50 ohm termination) x Input Capacitance (typically, 15 pF), or 424 MHz, that will have a small effect.

author: tautech
author_rank: Super Contributor
timestamp: November 07, 2023, 01:28:40 am
content: A probe isneverpart of a scope BW test system.50 ohm source terminated into 50 Ohms via coax, sinewave @ 1V and BW -3dB = 0.707 on the display.

author: noisyee
author_rank: Contributor
timestamp: November 07, 2023, 01:45:26 am
content: Hi,You would need a test fixture to proper test probe BW. It's an easy one with only a 50 ohm termination resistor and proper transmission line (or even a 50 ohm resistor bridging at a connector pin). Better measure its S11 before use.Don't be surprised if the result is weird. High BW passive probes have quite complex compensation network to compensate cable reflection and FR. The compensation network will only fit particular scopes.I had tested a claimed 500 MHz BW probe only have <200 MHz BW on my scope. Some hack needed to adjust the HF compensation (it has adjustable HF compensation but sealed), but the FR never become flat enough.That's why scope manufactures recommend particular probe for particular scope. If you don't use it, it's your own responsibility to verify the system performance.As for probe capacitance, manufactures specific it at LF. If you are interested in HF property, use a VNA to measure its S11, you can easily converted it into impedance curve. The curve should be closely but not perfectly match to an ideal capacitor. The impedance variation maybe the cause to non-constant capacitance measured on the smith chart.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 07, 2023, 02:14:46 am
content: One thing you can count on, all scope probes load your circuit.https://www.eevblog.com/forum/testgear/12-ghz-active-probe-project/msg4973839/#msg4973839

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2023, 02:57:20 am
content: Regarding capacitance:One possible conclusion, everything's always a lie. Perhaps it's measured at one particular frequency, and they don't care about it anywhere else.Another possibility: there is a more general truth underlying it, which you're either assumed to know, or which almost everyone is ignorant of (the latter is more likely, but a bit of both really, I think).The truth revealed: any impedance network can be approximated as an RLC lumped equivalent circuit. At low frequencies (below where transmission line properties of the internal components matter (1D fields), or let alone full 3D fields around them), especially when using lumped elements, this approximation can be done quite effectively, i.e. the accuracy of a curve fit improves rapidly as the number of elements used goes up.So it's an accuracy thing, and there is an "order" parameter, meaning the order of the polynomial of the transfer function: the number of reactive elements used to model the network.To summarize: if there is a lie, it's that an equivalent circuit only works over some frequency range; they might simply be leaving off further, extended details of that equivalent circuit -- whether because they don't care, or they don't know.When a scope is advertised as "1M || 20pF", or a probe as "10M || 14pF" or whatever, they're giving the DC equivalent (a resistance), and the MF equivalent (shunt capacitance).They are not telling you anything about the HF equivalent, what happens to that xx pF as its reactance comes down ever closer to Zo.(And for various reasons, you can assume there's a Zo somewhere in the 50-200 ohm ballpark, depending on cable construction, trace geometry, wires through free space, whatever. Or a bit outside that if you really work at it, i.e., more severe geometry; hence 600 ohm ladder line exists, or you can put twisted pairs in parallel forever, or make an ever-wider parallel-plate transmission line, to make arbitrarily low impedances.)There necessarily must be a loss element somewhere in the system, as you're reading a signal off the scope. That loss can be arbitrarily small, but most likely the front end levels off near 50 ohms, give or take a little peaking perhaps. Note a figure like 20pF has a cutoff with 50 ohms at 160MHz, so it wouldn't be very helpful for a 300MHz scope to have this much pure shunt capacitance and still be terminated into 50 ohms (say with a tee and terminator). It's perhaps possible that that capacitance gets switched out when the internal 50 ohm is switched on, but more likely it's always there, and just gets peaked out near cutoff; or it is itself the loss. In any case, the equivalent circuit will look more like 1M || (20pF + 50 ohm) up there, maybe with additional elements depending on peaking, front-end amplifier response, etc.Likewise the probe, the 14pF or whatever isn't a pure capacitance, it just approximates that over the say 1kHz-20MHz range, but above there, it has significant ESR. You can imagine a probe constructed from ordinary 50 ohm transmission line, which thereforemusthave an equivalent circuit of a Low-Z Probe (say, 450 ohms dividing into the 50 ohm cable). And so above ~20MHz, the equivalent circuit looks like R+C rather than R||C. (Real probes aren't ordinary coax, but a special high impedance lossy kind, cut to just such a length that the loss along it gives the 10x attenuation for free; this improves response, and reduces compensation capacitance. The equivalent might look more like 1k + 14pF at HF.)The probe tip impedance is further modified by its length, and that of the ground clip, which have inductance. The round trip of which might be 0.1-0.5uH depending on dimensions, so the probe further has a series RLC characteristic. This is generally made to peak at the probe's rated cutoff, introducing a zero to partially cancel out either the probe's own HF cutoff pole(s), or the scope's (in the hope that the scope's response is well behaved, which, it often isn't among DSOs..).The ground clip inductance, also introduces a common-mode error between probe ground / cable, and circuit ground; an error not shared by the probe tip (which being high impedance, carries hardly any current in response to CM), thus CM voltage (that is, voltage between circuit and scope grounds), at high frequencies, manifests as the inverse signal superimposed on the probed signal. Hence why you also measure such signals when probing ground -- what you would naively assume to yield zero volts always, but doesn't when voltage is dropped across that ground wire.Armed with this knowledge, you should be able to identify which poles are due to cable length, ground clip length, etc., and perhaps even compensate them further by adjusting those lengths -- or changing tack and using a coaxial adapter, rather than the probe tip/clip, to get the probe's internal response more simply.Tim

author: killingtime
author_rank: Regular Contributor
timestamp: November 08, 2023, 12:09:38 am
content: Evening Gents,I created another chart, this time accounting for the voltage drop resulting from the 50 ohm resistive source impedance of the RF sig gen. This voltage was then added onto the measured scope results. I got the voltage drop by running sims in LTSpiceIV at different frequencies. The chart attached assumes the probe capacitance remains constant over the freq range (12pF). It's much flatter and is what I would expect.Quote from: noisyee on November 07, 2023, 01:45:26 amWhy the 50 ohm termination resistor? Is that to reduce reflections back into the rf generator?You would normally only have to terminate into 50 ohms if you've got a long transmission line (coax say), to stop the line acting as a voltage transformer when terminated with a miss-match (SWR>1), but in my case the probe it stuck right on the end of the sig gen N socket, so there is almost no transmission line - less than 1/20th of a wavelength at the freq I'm using, so we can ignore it.Quote from: noisyee on November 07, 2023, 01:45:26 amI tried that and got some weird results at LF and VHF using a nanoVNA. Capacitance crept up. Might need to build a dedicated test rig. Too many projects on right now.Quote from: tautech on November 07, 2023, 01:28:40 amI did build that circuit and took measurements. Chart attached. Compare it to the probe plots. Quite different. The thing is, if I'm going to use a probe with a particular oscope, I need to know how the system will function with both combined. Probes do introduce gain and loss at various freq and add lots of rolloff at their -3db point. You have to add this to the scope response. Testing the scope without a probe only tells me the BW of the scope, not the whole system (probe included) - which is what I'll be using day to day.The other thing to note is that for scopes without a dedicated 50 Ohm termination (like mine), adding one yourself doesn't remove the 20pF on the scope front end, it's still there in parallel with the 50 Ohms feed thru termination added. That starts to dominate when the freq is high enough and creates a termination mismatch, SWR climbs and then that coax acts as a transformer changing the voltage seen at the scope. All you can do is keep the coax as short as possible, ideally less than 1/20th of the shortest wavelength you'll be using. At 250 MHz that's 6cm.

author: alm
author_rank: Super Contributor
timestamp: November 08, 2023, 12:16:32 am
content: Quote from: killingtime on November 08, 2023, 12:09:38 amFor best signal integrity, you want the stub length to be as short as possible. Back in the day Tektronix made a GR874-to-probe-tip adapter with 50 Ohm termination built in, so the termination could be closer to the probe tip then with feed-through terminator + adapter. In addition, this reduces the source impedance to 25 Ohms, which helps since capacitive loading is a big problem at the top end of the bandwidth.

author: David Hess
author_rank: Super Contributor
timestamp: November 08, 2023, 12:22:54 am
content: Quote from: TimFox on November 07, 2023, 12:15:17 amI have found it to be the rule rather than the exception, at least up to 300 MHz, but all of my oscilloscopes are old.The oldest oscilloscopes that I know of where it is not the rule are some of the Tektronix 11k series plug-ins, and the 250 MHz option version of the 7704A.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 08, 2023, 12:30:16 am
content: Quote from: David Hess on November 08, 2023, 12:22:54 amReally old...https://www.eevblog.com/forum/testgear/scope-speed-for-digital/msg3728101/#msg3728101

author: noisyee
author_rank: Contributor
timestamp: November 08, 2023, 01:30:09 am
content: Quote from: killingtime on November 08, 2023, 12:09:38 amIt's true when the amp of the sig gen is right after the connector. But some rf gen have a quite long pipe between amp and connector, which in some case can't be ignored.Verifying it or adding termination resistor at your preference.Quote from: killingtime on November 08, 2023, 12:09:38 amIn my test result, the slope of the impedance curve slowly decrease, which indicate an increasing of equivalent capacitance. So I think it's quite normal, like T3sl4co1l said:Quote from: T3sl4co1l on November 07, 2023, 02:57:20 amIn some case where probe loading should be treated seriously, manufacturer will provide measured impedance curve and equivalent circuit, or even snp files for simulation.

author: David Hess
author_rank: Super Contributor
timestamp: November 08, 2023, 09:31:56 am
content: Quote from: killingtime on November 08, 2023, 12:09:38 amMy Tektronix PG506 is designed that way also. It uses coaxial parallel source termination built into the BNC socket which is directly attached to the fast diode which disconnects the source, so it does not require a load termination to produce a clean output. However the load termination results in 25 ohms instead of 50 ohms driving the probe tip, so it doubles the bandwidth and duplicates how probes are specified.

author: Berni
author_rank: Super Contributor
timestamp: November 08, 2023, 09:51:32 am
content: Quote from: killingtime on November 08, 2023, 12:09:38 amThe internal guts of the signal generator are not 0mm away from the output connector. So not having a terminated output can also cause reflections inside the coax/traces that connect components inside the signal generator itself. A lot of reflection could also possibly upset a automatic signal level control loop inside the signal generator.A lot of things start to matter when you go into low single digit dB measurements.But in general you have independently discovered why passive probes become fairly useless at >100MHz. When you put a 100MHz sine wave into the 12pF of input capacitance the total probe impedance is 130 Ohm to GND. As you might imagine placing a 130 Ohm resistor to ground on your signal of interest is definitely going to affect the signals amplitude. So the probe is not wrong, it is measuring what is there, the problem is that the signal is loaded down once the probe is connected, so it is no longer the same shape/size.This is the reason why active probes are a very useful at high frequency. Yes they are expensive, bulky, fragile..etc but they also have only about 0.5pF to 1pF of loading. This brings our 100MHz sinewave example loading down a lot, to just 3000 Ohm.

author: David Hess
author_rank: Super Contributor
timestamp: November 08, 2023, 09:58:49 am
content: Quote from: joeqsmith on November 08, 2023, 12:30:16 amMind are old, but the even older exceptions I mentioned above did not have Gaussian or first order responses despite being analog oscilloscopes, (1) and the DSOs that I have do have Gaussian responses so do obey the 0.35 rule, as do many modern DSOs which eschew the processing needed after digitization to produce a maximally flat response.And then there are modern DSOs like the Rigol DS1000Z series which do not obey the 0.35 rule because their full power bandwidth is less than their small signal bandwidth, so their bandwidth varies with signal level.(1) These old exceptions were tuned for maximally flat response like the DSOs discussed in the article, and suffered more aberrations in their pulse response, just like described in the article.

author: David Hess
author_rank: Super Contributor
timestamp: November 08, 2023, 10:05:44 am
content: Quote from: Berni on November 08, 2023, 09:51:32 amThey are not, but as my example shows, it is practical to have an unterminated line length of only millimeters, which when combined with the limited bandwidth of the signal source results in no measurable reflections.General purpose signal generators will not be constructed this way, so either a load termination is required or bandwidths must be kept lower than in my 500 MHz example.

author: Berni
author_rank: Super Contributor
timestamp: November 08, 2023, 10:22:23 am
content: Well the mention that it is a N connector usually points to big heavy RF synthesizers, the general purpose ones usually have BNCs and don't usually go that high in frequency.For example the two Gigatronics RF synthesizers boatanchors (8 Ghz and 20 GHz) that i ended up with have a pretty long path inside. Behind the front panel N connector is a snaking rigid coax that takes twists and turns trough the instrument to end up at the switched attenuateor, then from there snakes around some more to end up at the directional coupler used for ALC, then snakes on some more to band switches and amplifiers...etc Most of these rigid coax lines taking some extra slack and bends in order to make them flexible enough to install without bending them out of shape. So you could quickly end up with half a meter of coax connecting the RF front end together, so lots of room for reflections to happen.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 11, 2023, 04:21:06 pm
content: Quote from: joeqsmith on November 08, 2023, 01:48:04 pmMy discussions with LeCroy and Tektronixhttps://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg593071/#msg593071