Measuring medium-high frequencies (20-50MHz) -- probing questions - Page 1
Forum section: electronics / beginners

author: guymo
author_rank: Regular Contributor
timestamp: September 07, 2021, 11:27:57 am
content: I am a total neophyte with measuring higher frequency signals, and have been learning a lot in the last few days but would appreciate some advice on how to characterize the bandwidth of an amplifier that I've recently put together (with help from people here.)The amp is supposed to have gain of 100 from 2Hz up to some megahertz. It has an AC coupled input stage: 1uF cap in series, 1Meg resistor to ground, into the gate of a JFET. I'd like to know what its frequency response really is. The output comes from an op amp with a 47 Ohm (because I didn't have a 50 Ohm) resistor in series.I have a cheap function generator (Uni-T 962) and a Rigol DS1054Z scope (hacked for options + bandwidth) My issue is that different ways of connecting the amplifier output to the scope yield very different results at higher frequencies and I am not sure which one to trust the most.Today I ran some measurements with the following test setup. The function generator putting out 10mV pp sine wave, in "High Z" mode, connected to amplifier with 50cm of coax. I measured the peak to peak output of the amplifier for various signal frequencies and connections to the scope:with a direct coax connection (unterminated): 1MHz signal read 1.03Vpp; 20MHz read 1.01Vpp; 50MHz read 296mV pp.with a coax connection into a 50 Ohm terminator: 1MHz read 524mVpp, 20Mhz read 412mVpp; 50MHz read 188mVppwith a 10x probe: 1MHz read 1.01Vpp; 20MHz read 888mVpp; 50MHz read 496mVpp.[edit to add: I'm using the short ground spring rather than the long lead for this last set of readings](The apparent -3dB frequencies for the three connections were 32MHz, 26MHz and 41MHz respectively.)At 1MHz the readings are all consistent but as the frequency rises they diverge a fair bit. I'm reading all about reflections, transmission lines etc as well as having my own thoughts about impedance but I'd really like some hints as to what may cause the differences, which measurement setup is most likely to be trustworthy, and if it's possible to set things up better.Thanks in advance!

author: joeqsmith
author_rank: Super Contributor
timestamp: September 07, 2021, 12:01:51 pm
content: Someone will take a signal generator, set the output voltage, look at it with a scope, see double what it should be and then assume the generator is bad.Getting the impedances matched up is very important.Assuming the amplifier uses 50 ohms, you could maybe use a 10X probe to look at the signal assuming the amplifier is terminated to 50 ohms. How the probe is attached will play into it as well. As you are finding, as you go up in frequency the more details become important.In your case, you could start by adding a schematic for your amplifier and showing some pictures of how you are instrumenting it.Here is a small signal transistorhttps://youtu.be/GJNMnq8eD0E?list=PLZSS2ajxhiQD2gftdurGQoyGpUM_HobNI&t=2613Here is a much larger amplifierhttps://youtu.be/XaYBpPCo1qk?list=PLZSS2ajxhiQD2gftdurGQoyGpUM_HobNI&t=4484

author: guymo
author_rank: Regular Contributor
timestamp: September 07, 2021, 03:25:12 pm
content: QuoteThe core schematic from LTSpice is attached, for what it's worth. The actual device has an additional DC servo to null output offset.For the instrumentation, what kind of pictures would be useful? I've tried the following:1) a coax cable direct from the output BNC connector to the scope2) a coax cable from the output BNC connector to the scope, with a 50 Ohm termination at the scope input3) a x10 scope probe (Rigol PVP2150) on the pins of the BNC output connector (shown in a staged photo below; not sure how the solder mask got nicked, that's weird.)It's a bit tricky to take a good image in situ and I don't know what you'd be looking for really, but if there are important aspects then I can try.My gut is telling me that the x10 probe is the most reliable approach because it is designed with compensation and damping of reflections in mind, but I am unsure why the results from approach 2 are so far away from that.

author: ocw
author_rank: Regular Contributor
timestamp: September 07, 2021, 04:39:09 pm
content: I am not seeing a problem. Your measurements approximate the rated frequency response of a LT1819 with 20 dB gain.

author: guymo
author_rank: Regular Contributor
timestamp: September 07, 2021, 05:01:18 pm
content: Quote from: MasterTech on September 07, 2021, 03:59:02 pmThank you for this.While you were writing it, I guess, I noticed the should-have-been-obvious fact that the amplitude I was seeing dropped off with the loading of the circuit: 10x probe is highest, then direct connection, then 50 Ohm terminated connection. So maybe I am not seeing ghosts (transmission line effects) but just loading... in which case...Quote from: ocw on September 07, 2021, 04:39:09 pmThat's interesting. The LT1819 is indeed operating at about 20dB gain, the other 20dB coming from the JFET stage. How do we account for the differences in the readings e.g. between the 10x probe and the 50 Ohm terminated coax? Is it simply load on the op amp as proposed above? E.g. at 50MHz the 10x probe reading is 496mV pp while the 50 Ohm terminated coax is 188mV, which is quite a bit less than half the other reading. It's that discrepancy that I am wondering about.

author: ocw
author_rank: Regular Contributor
timestamp: September 07, 2021, 05:13:02 pm
content: The LT1819 is made for feeding a 100 ohm minimum load with -3 dB frequency response at 30 MHz with 20 dB gain. That (as well as for other factors) is why there is about a 50 ohm series resistor on its output. Load makes a difference at the frequency response limits.

author: guymo
author_rank: Regular Contributor
timestamp: September 08, 2021, 09:37:58 am
content: Quote from: ocw on September 07, 2021, 05:13:02 pmI've been thinking some more about this and went back to the datasheet. There is a graph of closed-loop output impedance which shows that at this sort of frequency the output impedance is around 20 Ohm. Loaded by the series resistor (47Ohm), then the 50Ohm terminator in parallel with the scope input capacitance (15pF), direct calculation gives an expected amplitude of just over 200mV, if we assume the unloaded output is close to what the 10x probe shows; I'm seeing 188mV and I think this is close enough to believe.The measured amplitude with the unterminated cable cannot be explained this way. But I guess the unterminated case will suffer from reflection effects.Is this a reasonable set of conclusions?1) the unterminated coax is a bad connection to use at these frequencies because of transmission line effects2) the 50-Ohm terminated coax correctly measures the output under load and the op amp output impedance has an impact3) the 10x probe is likely measuring the unloaded output of the amplifier fairly well

author: RoGeorge
author_rank: Super Contributor
timestamp: September 08, 2021, 10:39:30 am
content: Quote from: guymo on September 07, 2021, 03:25:12 pmNext time attach the .asc file from LTspice, so others can edit and eventually run the circuit's simulation.- The generator setting on high impedance or not does not really changes the impedance or the signal. It only changes the voltage written on the generator's display (the generator assumes you added the resistor terminator near your DUT when it's not set on high impedance). This is because the generator has inside a series 50 ohms resistor, like shown in the box named "Signal Generator", and this resistor together with the terminator you add externally, will make a voltage divider. The generator display tells the voltage you expect on the terminator's pins (if you have any terminator), that's what the generator's "impedance" setting is for.- Next thing to know for RF signals transport is that on any wire the signal travels with a finite speed. You can think about this like a wavefront going from one end to the other of your cable (talking here about the cable between the signal generator and the amplifier). Once the wave (signal) arrives at the other end, a part of it will enter into your amplifier, and some other part will reflect back. And so on, sloshing back and forth inside the cable, and overlapping with the generated signal.These reflections can alter the amplitude and the waveform of the test signal, so you want them minimized. To reduce the reflections, you need to match the impedance between a source and it's load. That means complex conjugate of the impedance (if impedances are complex, AKA they have an inductive or capacitive behavior, then keep the real part and change the sign for the imaginary part, or if the impedance is real, AKA pure resistive, then just match the real part and use the same resistance for source and load.Maybe this video can make all a little clear:#143: Transmission Line Terminations for Digital and RF signals - Intro/TutorialW2AEW is a reliable channel, search there for more videos about this subject.- To probe your circuit, use the 10x probe that came with the oscilloscope (osc probe should be calibrated with the square signal offered by the oscilloscope, as the user manual explains). No other 50 ohms needed between osc probe and oscilloscope.

author: StillTrying
author_rank: Super Contributor
timestamp: September 08, 2021, 02:07:29 pm
content: You can simulate most probing and cable effects to see the horrible results, in this simple one they're not too bad.

author: guymo
author_rank: Regular Contributor
timestamp: September 08, 2021, 04:12:33 pm
content: Quote from: StillTrying on September 08, 2021, 02:07:29 pmI've been playing around with this sort of thing this afternoon. Thanks.What's a good value of Td for a typical short length of coax? I read somewhere 1n per 200mm. Does that sound ok?[Reference was here:http://web.physics.ucsb.edu/~lecturedemonstrations/Composer/Pages/76.18.html]

author: TimFox
author_rank: Super Contributor
timestamp: September 08, 2021, 05:20:00 pm
content: 1 ns per 200 mm is a velocity factor of 0.66, which is correct for solid polyethylene (e.g., RG-58/U coax).Other popular dielectrics are faster, including semi-solid (foam) polyethylene, Teflon, semi-solid Teflon (Gore-flex), and air (factor 1). If important, check the manufacturer’s spec or industry standard.The speed of lightin vacuois 3 x 108m/s = 300 mm/ns, close to 1 ft/ns.

author: guymo
author_rank: Regular Contributor
timestamp: September 09, 2021, 08:55:41 am
content: Quote from: RoGeorge on September 08, 2021, 10:39:30 amThanks for your post. It gave me the impetus to add the measurement setup to my Spice simulations and that now gives results that are quite close to what I measure, at least in the two cases of the 10x probe and the properly terminated 50 Ohm coax. For improperly terminated coax I can't get the simulation to match what I'm seeing but I appreciate it is not the way to approach this anyway.I suppose the reason for preferring the probe over the coax connection is that it does not load the circuit -- the whole idea of the scope probe design.This raises a practical question for me. The amplifier I built was intended as a measurement preamplifier so as a result of this discussion I will mainly be using it with a 10x probe. What sort of connector would you put on the amplifier to make that as easy as possible? (I've currently got it with a BNC output, because I thought I'd be using coax cable.)

author: RoGeorge
author_rank: Super Contributor
timestamp: September 09, 2021, 10:40:18 am
content: BNC is a very good choice for the given frequency range.About getting good measurement results, probably the most important was to add the 50 ohms terminator.For the fun of it, you can simulate the transmission line alone (do a frequency swipe over a large range), with R_series and R_terminator of different values (no preamp), and probe in LTspice the voltage on the transmission line, in order to see how strange a simple cable (or else said a transmission line) behaves because of reflections.Any wire acts like a comb filter when the impedances doesn't match.Note that not only a coax will behave like that, but any cable and any piece of conductor will do the same. Even the empty space of a wave-guide will act like a comb filter if the impedance matching is neglected. This filtering behavior is sometimes used in RF/microwave by creating filters out of open or shorted pieces of coax, and in microwave by simply adding stubs to the shape of the PCB traces.What's even more interesting is that the notion of impedance extends in many other domains, not only electronics. Same principles of exploiting impedance (miss)matching can be applied anywhere, from electronics, to optics, to acoustics, to mechanics, even to sociology. Whether or not you want "something" to be absorbed or reflected/rejected, whether or not you want something to act like a filter, or maybe like a resonator, you can achieve any of that by playing with the impedance matching.Impedance can be defined as a much broader concept:the ratio between cause and effect, where the cause and the effect can be anything.Nice vintage video to exemplify the last two paragraphs:AT&T Archives: Similiarities of Wave Behavior

author: guymo
author_rank: Regular Contributor
timestamp: September 09, 2021, 12:35:56 pm
content: Quote from: RoGeorge on September 09, 2021, 10:40:18 amEven if what I want to do is use a 10x probe to my scope? It seems odd to add a connector that won't be used properly. Or should I be looking for a BNC adapter for the scope probe too?Just a few seconds in, I can tell the AT&T video is going to be great fun. Thanks!

author: RoGeorge
author_rank: Super Contributor
timestamp: September 09, 2021, 03:22:04 pm
content: Quote from: guymo on September 09, 2021, 12:35:56 pmI thought the preamp was meant to be standalone. If it's for a probe's tip, then anything goes, including adapters, or even a simple DIY like the one here:#111: How to make a high performance oscilloscope probe socket

author: guymo
author_rank: Regular Contributor
timestamp: September 09, 2021, 04:43:34 pm
content: Quote from: RoGeorge on September 09, 2021, 03:22:04 pmWell, it was, but the conclusion of this thread seems to be that the best fidelity at the scope will be from a 10x probe, right?The setup will always be signal --> preamp --> scope. If there's some better way to connect things please let me know!QuoteExcellent, thanks again.