Not Enough Detail On Scope Capture - Page 1
Forum section: electronics / beginners

author: george7941
author_rank: Contributor
timestamp: May 12, 2025, 09:43:38 pm
content: This is the fuel pump current on my 2006 GMC Sierra truck, captured by my Rigol DS1054Z scope. The current clamp is a OTC 60A 3820-06. The pump motor has a 10 bar commutator.As you can see, two of the ten humps are flattened. What causes it?Expanding the waveform does not yield any more detail. Do I need to upgrade to a better scope? The two I am considering are the Rigol DHO802 and the Picoscope 4425A, the Pico being much more expensive. They are both 12 bit vertical resolution scopes. Would they yield better results?

author: BillyO
author_rank: Super Contributor
timestamp: May 13, 2025, 04:13:14 am
content: A 12-bit scope with better sensitivity and more memory depth would certainly help. What does it look like it you go to 50mV/div on the vertical and double the rate on the horizontal (1ms)?If you want to go to a new scope, consider a Siglent SDS800x-HD series scope. Considered by the majority (me too) to be better than the Rigol.Also, how do you have your setup? There seems to be some stray signal being picked up, like a low frequency sine wave. Can you show us a picture of your probing?Also, you are at 2ms/div horizontal. Why is the scope showing 10MS/s? It should be showing 1GS/s at that sweep rate. Maybe go into the acquisition menu to see if the sampling rate is set properly.

author: BennoG
author_rank: Frequent Contributor
timestamp: May 13, 2025, 05:18:52 am
content: My guess would be the frequency bandwidth on the current clamp.A quick search did not reveal any info on this. So it could be the max bandwidth of the probe is somewhere between 1kHz and 100kHz.This is for a sinus wave so if you want to see square waves your bandwidth of the probe need to be at least 10 times higher than the frequency of the square wave.Benno

author: george7941
author_rank: Contributor
timestamp: May 15, 2025, 09:44:32 pm
content: The current clamp I used is this onehttps://www.pdimeters.com/products/pdi-ca60-ac-dc-amp-probeIt mentions a frequency response of 20 KHz. A tenth of that is 2 KHz. The wave I am looking at is about 1 KHz (each wave is approx 1 ms). So would that make for adequate frequency response?I have the manual that came with the clamp. The pic shows the specs. I used the 1mA/10mV rangeHere is another puzzler. I have the scope, both signel and ground terminals, hooked up to the cold water copper pipe. I am getting a spurious wave. The copper pipe is one with the underground buried copper pipe bringing water into the house. I was expecting a flat line.I have the Acquisiton menu on the screen and it automatically sets the sampling rate at 25 Ms/sec, with no other choice.

author: BillyO
author_rank: Super Contributor
timestamp: May 15, 2025, 10:04:59 pm
content: 20kHz is enough for what you want to do.I'm not sure what I'm looking at in that 2nd picture, but I do see what amounts to a large near-field antenna.However, I really wanted to see how the probe was being attached to the scope and how the prob was situated on the vehicle. I'm assuming (maybe incorrectly) that the scope is being powered by a properly grounded power cord. So, to get that assumption taken care of, ensure that the scope is attached to a properly ground outlet via a properly grounded cord. Then sow us how you are attaching the current clamp to the scope, and how you are using the current clamp on the vehicle.

author: george7941
author_rank: Contributor
timestamp: May 15, 2025, 10:23:07 pm
content: The second pic has nothing to do with the truck fuel pump current.I hooked up the scope, in my basement, today, to the water pipe to see if it would pick up any stray signal and it is doing that, as shown in the third pic. I assume that the near field antenna you refer to is the white wire that is connecting the ground terminal to the copper pipe. This white wire is picking up the signal shown in the third pic? The scope was plugged into a properly grounded 120v outlet. The voltage probe seen is one that came with a Fluke 98 scope.For the truck fuel pump current, I was running the scope off a 150W inverter plugged into the cigarette lighter socket in the truck. So the scope was not grounded at all.

author: BillyO
author_rank: Super Contributor
timestamp: May 15, 2025, 10:33:15 pm
content: Quote from: george7941 on May 15, 2025, 10:23:07 pmYes. There could also be a tiny voltage drop between the copper pipe and the ground where the scope is plugged in.Quote from: george7941 on May 15, 2025, 10:23:07 pmThat could be a real problem. Ditch the inverter and use a good grounded extension cord. The output of hose inverters is usually just horrible and with the scope ungrounded it becomes susceptible any and all noise the inverter creates.

author: radiolistener
author_rank: Super Contributor
timestamp: May 16, 2025, 12:40:33 am
content: your screenshot is captured with 10 MS/s sample rate, this is too small, try to capture it at higher sample rate to see more details.

author: george7941
author_rank: Contributor
timestamp: May 16, 2025, 01:03:09 am
content: The scope automatically sets up the sample rate and record memory, with no user control possible.From a newbie's perspective, 10MS/sec sounds like a lot. So does 12M pts record memory depth.

author: BillyO
author_rank: Super Contributor
timestamp: May 16, 2025, 01:28:54 am
content: Quote from: george7941 on May 16, 2025, 01:03:09 amWell, that really sucks. I just tried on my cheapest Siglent and the sample rate for 2ms/div was 500Ms/s - fifty times higher. That particular Rigol is a bit of an old timer and may not be up to the scopes of today, but it was a star in it's day and surely it can produce better resolution than what we are seeing. I'm not intimately familiar with it so having a Rigol fanboy chime in might be useful. Just something to check, but do you have the memory depth properly set too? See if it's set to maximum. That will affect how the scope set the sampling rate.

author: radiolistener
author_rank: Super Contributor
timestamp: May 16, 2025, 02:30:08 am
content: Quote from: george7941 on May 16, 2025, 01:03:09 am10 MS/s is too low to see millisecond pulses in detail.And as I can see, he probably captured waveform at about 0.1 sec/div and then scaled it to 2 ms/div, so this is the reason why oscilloscope switched to so low sample rate and why it has so bad resolution. It also may contain waveform aliasing distortions, due to this so low sample rate.

author: BillyO
author_rank: Super Contributor
timestamp: May 16, 2025, 02:37:59 am
content: Quote from: radiolistener on May 16, 2025, 02:30:08 amGood observation. I'll bet that's it.

author: george7941
author_rank: Contributor
timestamp: May 16, 2025, 02:58:13 am
content: QuoteI think that is what I did. So a longer time base leads to lower sampling rate? Is that because of the record memory depth and more memory depth would have led to higher sampling rate?I am going to try again with the scope plugged into 120V AC and at 2 ms/div. It will be a week or two before I can get to it.Thanks for all the help so far.

author: george7941
author_rank: Contributor
timestamp: May 16, 2025, 03:03:39 am
content: Also I think AC coupling will produce more detail, so that is what I will do.

author: BillyO
author_rank: Super Contributor
timestamp: May 16, 2025, 03:08:45 am
content: Quote from: george7941 on May 16, 2025, 03:03:39 amWell, these current probes are balanced output devices, so it should not matter.I'm still a little leery about the noise in your first post. The proper connection to a grounded AC supply should help, but it would be informative to see how you connect the probe to the scope. No rush, but this is important. You would not believe the thigs I have seen....

author: radiolistener
author_rank: Super Contributor
timestamp: May 16, 2025, 03:27:50 am
content: Quote from: george7941 on May 16, 2025, 02:58:13 amyes, when memory is not enough to capture waveform at full sample rate, oscilloscope decrease sample rate, it leads to worse details and introduce some waveform distortions, but there is no other way to capture long sample with low memory size.So, you need to check sample rate when set slow time/div.Technically it should apply low pass filter when reducing sample rate, but since it require powerful and very fast DSP and oscilloscope don't have it to do it at full sample rate and don't have enough memory, it just skipping some samples, which leads to signal distortions.

author: george7941
author_rank: Contributor
timestamp: May 17, 2025, 09:24:10 pm
content: First two pics show hookup, with the scope still powered from an inverter. B+ is fed to the fuel pump through a jumper and the clamp is on the jumper.Pic 3 is the result.Pic 4 is when I expand the waveform. The result is less than satisfactory, with the stepped waveform. I take it that it is because of the limited 8 bit vertical resolution.Powering the scope from 120v will have to wait a bit since I live downtown in a big city and I park on the street.

author: george7941
author_rank: Contributor
timestamp: May 17, 2025, 09:28:36 pm
content: Thought I had attached this pic earlier

author: BillyO
author_rank: Super Contributor
timestamp: May 17, 2025, 09:35:19 pm
content: The probe to scope cable is suspect. It looks like it was walked on for a month then taped up and used to tow something around the back yard. You might have to open the probe to check for continuity on both the conductors. Especially if the shield is broken, you will pick up a tone of noise.What is with the huge length of brown and yellow wire? That can't be good.

author: george7941
author_rank: Contributor
timestamp: May 17, 2025, 09:56:21 pm
content: The original BNC connector on the end of the clamp cable became defective and I had to replace the connector. I kept the unshielded length of the repair as short as possible. The repair looks very messy because heat shrink could not be used. The cable insulation started melting when trying to heat the heat shrink.I have a unmolested Gossen Metrawatt clamp and it produces a bit more hash than the OTC clamp when used for fuel pump currents. I think it is because the Metrawatt does not have a low amp range. So I use the OTC clamp for low amp measurements and the Metrawatt for starter currents.The long paired cable is attached to a momentary switch I used to turn on the fuel pump for scoping. You can see the switch on top of the battery.

author: george7941
author_rank: Contributor
timestamp: June 11, 2025, 03:16:57 pm
content: Got some captures with the Rigol hooked up to 120V. Pic 1 shows setup. Pic 2 shows DC coupled 100A Metrawatt clamp. Pic 3 shows AC coupled 60A clamp with the repaired lead. The repair does not seem to have affected the quality.Is this about the best quality I can expect? Would a 12 bit scope. whether Siglent or Rigol offer significantly more detail?

author: tggzzz
author_rank: Super Contributor
timestamp: June 11, 2025, 05:18:11 pm
content: Quote from: BennoG on May 13, 2025, 05:18:52 amThe bandwidth required depends solely on therisetimeyou wish to observe; the period/frequency is irrelevant. BW=0.35/trisetimeAnthropomorphic explanation: the circuit and scope neither know nor care when the next transition might occur. Consider, for example a 1Hz signal with a 1µs width; a 10Hz instrument will show very littleFor a little theory, plus scope/spectrum analyser traces demonstrating what happens, seehttps://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/

author: radiolistener
author_rank: Super Contributor
timestamp: June 11, 2025, 05:34:06 pm
content: Quote from: george7941 on May 17, 2025, 09:24:10 pmBased on your screenshots showing the "stair-step" effect on the waveform, it's clear that the oscilloscope was initially set to a low sensitivity (high volts/div setting), and the waveform was later zoomed in. This approach prevents you from seeing fine details, as oscilloscopes typically use 8-bit ADCs. When recording a waveform with low sensitivity, you lose resolution from the start, and no amount of zooming will recover the lost detail. To capture maximum detail, the volts/div setting should be as low as possible - just enough to fit the waveform on screen at the time of acquisition. This way, you won’t need to zoom in later, and the captured waveform will retain its fine structure.But avoid waveform clipping, see details below.Quote from: george7941 on June 11, 2025, 03:16:57 pmYour screenshot clearly shows that the waveform is clipped, indicating ADC overload. In such cases, the oscilloscope may distort the waveform, as it takes some time for internal amplifiers and other circuits to recover once the signal returns to the visible range. To avoid this, you should ensure that the entire waveform remains within the vertical limits of the screen - from left to right, without exceeding the top or bottom edges.As for 12-bit oscilloscopes (e.g., from Siglent or Rigol), they don't always offer better visible detail. In fact, many of them use higher ADC resolution to simplify the design of their programmable gain amplifiers (PGAs), which can result in lower analog front-end precision. Consequently, the visual detail may be worse compared to a properly used 8-bit oscilloscope that employs high-resolution analog amplifiers, allowing finer control and better signal visibility.

author: george7941
author_rank: Contributor
timestamp: June 11, 2025, 06:07:23 pm
content: The latest capture I posted was not zoomed in later, it was recorded at 2ms/div and 20mv/div. It does not show much stair-step effect.I should have used 50mv/div. I think that caused the wave clipping.

author: radiolistener
author_rank: Super Contributor
timestamp: June 11, 2025, 07:39:50 pm
content: Quote from: george7941 on June 11, 2025, 06:07:23 pmbut it has clipping which is not good, you cannot believe to waveform when it happens