What am I looking at here? - Page 1
Forum section: electronics / beginners

author: S185149
author_rank: Contributor
timestamp: September 29, 2017, 12:59:54 am
content: Learning to use my DS1000Z scope so I put this simple RC circuit consisting of a 33k resistor and .01u cap. Driving it with the infamous FY3200 signal generator. Using the standard probes that came with the scope. Channel 1 ground clip is connected to the negative side of the sig gen. The probe is on the input and it looks fine. Channel 2 ground lead is disconnected and the probe is on the top of the cap and it looks like a mess.So what is going on and why am I seeing multiple waves on channel 2?Thanks,

author: Johnny10
author_rank: Frequent Contributor
timestamp: September 29, 2017, 02:20:12 am
content: Why is the ground unconnected?You aren't going to get a stable waveform with just 1 point of measurement.For more info you might try Alan's site on YoutubeLots of good basic scope use information.

author: S185149
author_rank: Contributor
timestamp: September 29, 2017, 02:47:08 am
content: I have spent many hours watching his videos. Enjoy his presentation style.The whole subject of grounding is taking some time to understand. I've learned from his video that the shorter the ground lead the better. I did not appreciate that the consideration is relative to the probe. After trying a few more things, I've realized that I need to think in terms of the distance from the probe.Thanks for the reply. Onwards to the discovery of whatelse I don't understand.

author: technogeeky
author_rank: Frequent Contributor
timestamp: September 29, 2017, 08:06:39 pm
content: Quote from: S185149 on September 29, 2017, 02:47:08 am1. Generally (thought notalways, I guess) you only ever need to connect exactly one of your ground clips (one of the four channels' ground clips). This results in two connections being made:The ground of the oscilloscope itself is connected to the Earth ground (unless you are floating your oscilloscope -- which you should not unnecessarily do).The ground clip connects this node (again, which ought to be 0.0V if the scope is not floating) to some place in your circuit.I can't explicitly think of a reason you would connect two ground clips to two different potentials. This can be very dangerous. If there is a potential difference (say, for example, 120V) then the two connections above present a very unfortunate picture: the resistance from one ground clip, through the scope, back through the other ground clip (or it could go through the Earth pin on the device under test, if there is one) is essentially zero. So let's say 1 ohm. 120 volts over 1 ohm is 120 amps. You'll be lucky if your oscilloscope probe leads, oscilloscope probes, the oscilloscope itself, and you survive.2. You actually are dealing with signals high enough in frequency that a shorter ground lead (or even a grounding spring) could help a little bit, but it's probably not the source of your problems here.3. I would guess that your problem is that you're triggering on Channel 1, and Channel 2 is out of phase with channel 1. There are two ways to check this:Trigger on CH2.Take a single shot capture.You certainly will not see multiple waveforms overlapping with a single shot capture. You may have the same problem with CH1 appearing to be multiple waves if you are triggering on CH2. But really, if you are triggering on the faster waveform the slower waveform should not appear multiple times.As an extra bit of information (as if the above wasn't enough), there are some oscilloscopes designed to tackle exactly this problem (from the days before digital scopes were invented, and you could use a long single shot capture): adual time baseoscilloscope.Of course, the legendaryw2aew(already referenced above) has made a video showing one of these in use:This Tektronix 2467 has adualdelayedtime base. This means it can trigger on two totally independent signals and keep both of them stable. (Your rigol oscilloscope has a delayed time base, but not a dual time base).

author: Johnny10
author_rank: Frequent Contributor
timestamp: September 29, 2017, 08:39:26 pm
content: Good Point !I should have stated tying grounds to same point.I have found that waveforms can be wonky with ground clip not connected.

author: S185149
author_rank: Contributor
timestamp: September 30, 2017, 01:30:02 am
content: If I learned nothing else after watching all the videos is that I MUST clip the grounds to the same point. If the equipment under test is earth grounded, then I MUST clip to earth ground.My original thought was that I had one ground lead clipped in and both channels are measuring relative to that ground so I should be able to do without the ground clip from probe two to the same ground as probe one. After getting your responses, I tried with the ground clip attached to probe two and clipped it to the same ground as probe one. Channel two trace came in significantly better. No more multiple traces on top of each other. I then tried it with the spring attached to the barrel. Improvement was slight but it was still noticable.I also tried single shot and that allowed me to capture just one trace for channel 2.To avoid issues in the future, my lessons learned here is that I should use the ground for each probe and as the frequency goes up, the ground lead need to be shorter.

author: ebastler
author_rank: Super Contributor
timestamp: October 01, 2017, 07:36:57 am
content: Quote from: S185149 on September 29, 2017, 12:59:54 amHmm; independend of potential grounding issues, I am not sure what you have measured. I understand that you have built a low-pass filter, which -- with the component values you mention -- should kick in at a few kHz. You are connecting a 13 MHz signal, and still a significant part of that makes it to the output of the low pass filter?!I am wondering whether the tip of the channel 2 probe was properly connected at all. Maybe you were picking up some stray signal from the circuit's input, which was capacitively coupled directly to the open channel 2 tip?

author: exe
author_rank: Unknown
timestamp: October 01, 2017, 11:04:10 pm
content: I'm not sure what you really asked, but multiple traces are because of multiple captures displayed simultaneously. The signal drifted a bit in frequency, that's why they are "misaligned".

author: S185149
author_rank: Contributor
timestamp: October 01, 2017, 11:16:02 pm
content: That's a good point so I breadboard it back up and setup channel 2 vertical scale to be 1/2 of channel 1. The output should be down by half by 480Hz but I got closer to 800 hz.I can't explain why I was able to get the traces that I showed. I am noting that the scope will give me something that doesn't look right and I have to cycle power. Have not seen enough of this to say what it may be related to.

author: S185149
author_rank: Contributor
timestamp: October 01, 2017, 11:18:57 pm
content: Quote from: exe on October 01, 2017, 11:04:10 pmSo what you are saying is that although I have locked on channel 1 for the input, the signal at the top of the cap is shifting phase slightly with each sweep?

author: exe
author_rank: Unknown
timestamp: October 02, 2017, 05:58:14 pm
content: Quote from: S185149 on October 01, 2017, 11:18:57 pmYes, as it seen by the _scope_. But quite often this indicates problems with measurement (most often too long wires or bad grounding). As you noticed, each channel has to be grounded for measurements to be correct.At frequencies above 2-3MHz be sure to use 10 probes. Beyond a few 10's of MHz probe's ground lead needs to be removed as it creates too much inductance.

author: S185149
author_rank: Contributor
timestamp: October 02, 2017, 07:25:17 pm
content: Quote from: exe on October 02, 2017, 05:58:14 pmI've used the probe in the x1 position once by accident. Appreciate the guidelines.Thanks