Fluke 60MHz scope, amplitude of a signal gets lower as frequency rises. - Page 1
Forum section: electronics / beginners

author: Xupicor
author_rank: Contributor
timestamp: December 24, 2021, 09:56:08 pm
content: Hello all, long time viewer, new to the forums.I've always been interested in electronics, but recently I've decided to actually take the deep dive and see what's what. To that effect, over time, I've managed to put together an amateurish bench on a budget.I just got a Fluke PM3370B 60MHz combiscope delivered that I scored on ebay and it passes self-test okay, so I connected a JDS660 DDS Signal Generator/Counter (60MHz maximum) to it through unbranded Chinesium 1:1/1:10 probes that look exactly the same as UT-P04 UNI-T 100MHz probes I just looked up on ebay. The probes are set in 1:10 mode and the scope channel scaling settings are adjusted to reflect that.I've observed that up until about 1MHz, the signal amplitude on the scope matches the one set on the frequency generator (4Vpp here).However, once I start ramping the frequency up by 1MHz steps until the maximum 60MHz possible on the generator, I can observe the amplitude slowly getting smaller and smaller, finally reaching ~2.84Vpp at 60MHz signal.An obviously newbie question: is this behavior normal?As I don't have another frequency generator, nor another scope to verify either device, I come here to ask the question:Is this normal? 60Mhz is the max bandwidth of the scope, and also the maximum frequency I can generate from the freq. gen. - thus, is this just either one device or the other (or both) showing declining performance as they approach their maximum advertised numbers? Or is it because of the cheap probes? Would nicer probes even do anything much, considering the limited bandwidth of the scope?Is there an easy way I could verify it one way or the other without specialized equipment I obviously don't have?It doesn't bother me that much, even though the signal amplitude is lower (~2.84Vpp instead of ~4Vpp) the signal is still very much visible and clean (I can use "magnify" function to zoom in more on it), but I'd like to know if it's the generator sending a signal of a lower amplitude than advertised, or the scope seeing it as lower, because the frequency approached its maximum bandwidth.___I am aware of the sticky with a long oscilloscope tutorial that might shed more light on the effect I'm seeing here. I shall view the videos posted in the thread there. In the meantime, please, feel free to hop in and post a short explanation or point me in the direction of one. Thank you in advance.Oh, and Merry Christmas, everyone.

author: xrunner
author_rank: Super Contributor
timestamp: December 24, 2021, 10:24:16 pm
content: Yea that's normal. 60 MHz is the bandwidth of the scope, where the amplitude of the measurement will be 3 dB down. Your measurement is just about what is to be expected at 60 MHz.20 log (2.84 / 4.00) = -2.97 dB

author: bdunham7
author_rank: Super Contributor
timestamp: December 25, 2021, 12:02:44 am
content: Quote from: Xupicor on December 24, 2021, 09:56:08 pmI depends on what it does in between 1MHz and 60MHz. If it drops off a very little from 1MHz to say 30 or 40MHz and then rolls off to the 70% level at 60MHz, that's what you would expect. If it drops to 3V at 2MHz and then slowly tapers off after that, it would be odd.b/t/w, that's afantasticbudget electronics bench! You spend your money wisely.

author: Xupicor
author_rank: Contributor
timestamp: December 25, 2021, 02:13:15 am
content: Ah, of course. This is more or less what's between 1MHz and 60MHz:So I guess this is not _exactly_ ideal. Unfortunately I don't have any other probes to try, yet.

author: xrunner
author_rank: Super Contributor
timestamp: December 25, 2021, 02:33:10 am
content: Good observations from the start Xupicor. Hope you enjoy the hobby and I think you will be able to go as far as you want to.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 25, 2021, 11:51:56 am
content: Typically analog scopes are rated at 80% at their upper limit. About 80# of P-P. So this looks normal. You can usually get a decent looking wave with a somewhat higher freq though. Depends on the particular scope itself. I even have a digital 100MHz scope that has a very poor signal at 120 MHz and then has a better signal at 140 MHz. Go Think!Digital scopes do not do this but seem to just fall off at about their designated limit.I do not know if you have 50 ohm inputs on your scope. If you do have that setting, try it with a 50 ohm output generator with just a BNC patch cord (not the probes) to see the difference in the trace. You will be seeing the output of your signal generator into a 50 ohm load. You then can calculate power using Ohm's law.

author: Xupicor
author_rank: Contributor
timestamp: December 25, 2021, 03:41:39 pm
content: This scope has analog and digital mode, the peak-to-peak measurements above I've done automatically in digital mode (for convenience, many data points to enter, after all) but the amplitude is basically the same in analog mode if I just switch over.I don't believe I can switch over to 50 ohm inputs. On the tin it's 1MOhm 25pF, and the manual doesn't betray any way to change that. Neither can I see anything useful in the menus of the scope itself.Just out of sheer curiosity, I was thinking about maybe using a sweep functionality of my generator to somehow get a Vpp curve over frequency like I did above by hand, but on the scope itself. I'm not sure how I'd approach that, though. I guess... I could rectify the sinusoid wave, feed that back to the scope and use a very long sweep time in digital mode to get that? Heck, I'm just gonna try this out.

author: CaptDon
author_rank: Super Contributor
timestamp: December 25, 2021, 03:51:37 pm
content: Everything seems normal for that scope. First question, did you compensate the trimmer in the probe using 'Hopefully' a scope generated 1khz squarewave output test point?? Some scopes have a calibrator squarewave output at 2X line frequency. This will effect the rolloff of the probe which will then appear as rolloff of the scope.

author: bdunham7
author_rank: Super Contributor
timestamp: December 25, 2021, 04:07:54 pm
content: Quote from: Xupicor on December 25, 2021, 03:41:39 pmI doubt you have 50R inputs, but you don't need them to connect your AWG directly with a straight BNC to BNC cable. It will still work fine and eliminate any probe issues.As far as generating a frequency response curve with a sweep, you do it more or less like this:1. Set your generator CH1 for a 0-60MHz and a sweep time of 100ms. Then set CH2 for a short pulse with a repetition rate of 10Hz.2. Connect CH1 of the scope and CH2 of the generator to EXT TRIG (if you have) or CH2.3. Set the scope timebase at 10ms/div and set it to trigger on either EXT TRIG or CH2, whichever you have used in step 2.The sweep will 'paint' the scope with a filled-in pattern that will give you your FR graph. You won't be able to see the individual waveforms, just their combined effect. You can adjust everything from here to fine tune it.

author: Xupicor
author_rank: Contributor
timestamp: December 25, 2021, 05:03:07 pm
content: Yes, CaptDon, I've calibrated the probes according to the procedure in the manual, using the scope built-in "Probe Adjust" ~2kHz 600mV square wave output.There seemed to be 15mV..25mV of positive bias present in both channels, even in ground coupling mode, but after autocalibration it looks more like well under 1mV, so I guess that's plenty fine since that pretty much looks like noise now.I'll try your procedure, bdunham7, and will report back!edit: The best I could do is this:It's moving slightly, and that's with the trigger pulse at 9.90Hz (other values make it move to the right or left even more).Sweep timer only goes down to 0.1s, so I can't make it go faster. At slower speeds the trace sweep is... well, slow, a bit too slow to be of use.

author: Xupicor
author_rank: Contributor
timestamp: December 30, 2021, 08:07:24 am
content: Bumping to say thanks to everyone that answered, your time and effort is appreciated, thank you.Also, a bit of a new development:Okay, the problem with my function generator is that while the channels do appear to be synchronized in that when I have 1kHz on Ch1 and 2kHz on Ch2, I can stop and start one channel with the trigger being on the other, and the "phase shift" (they are different frequencies, but harmonic, so I guess it still makes sense to say that?) between them appears to be always exactly the same.But I don't have any control over phase shift, or rather, time delay (?) of Ch2 when I start a sweep on Ch1. So, sweep on Ch1 and a pulse on Ch2 seem to be completely unsynchronized and I can get the start of a sweep at different points in time relative to the pulse just by stopping and starting it manually. Sadly, I don't have dedicated "synchronized" outputs to give me a synchronized square wave that I could trigger on to know exactly when the sweep starts. I've seen other generators on youtube that are able to do so.I guess I could still do it manually by adjusting a frequency of the pulse to get the wave stable in horizontal domain and triggering on the falling edge which I could move with duty cycle to then adjust the waveform so that it starts on the left side of the screen, but because the UI of the generator doesn't let me tinker with the other channel while frequency sweep mode is engaged, I have to switch between sweep and duty cycle adjustments, which is kind of a bummer (and is really quite tedious).Still, just triggering on the swept channel and using dotted display in peak detect mode (or I could use envelope mode to get similar result), I could get this:which isn't half bad (this is 100kHz to 60MHz in one second sweep), though I don't know where the peak at the beginning comes from.So, there's that. I guess I can use that method to get a nice graph of various pass-through filters without spending money on a dedicated analyzer, so that's quite a bonus. Thanks, bdunham7!

author: george.b
author_rank: Frequent Contributor
timestamp: December 31, 2021, 01:26:23 am
content: And it looks pretty close to the graph you plotted before

author: Xupicor
author_rank: Contributor
timestamp: December 31, 2021, 01:43:20 pm
content: Yes, it's pretty similar -- again, save for the strange spike. In fact, it might be better than my manual jobbie, because even though I tried my best not to move the probes while pressing buttons, the setup I had was pretty touchy and amplitude could change quite a bit if the probes moved even slightly. I think it's the probe hooks that aren't all that great, because when I took them off it was much more stable.