DS1054Z - Beginner issue with crosstalk? - Page 1
Forum section: electronics / testgear

author: AndyCap123
author_rank: Newbie
timestamp: April 22, 2017, 12:30:38 pm
content: Hi Guys,This is my first oscilloscope so I'm not sure if this is expected behaviour or if there is something wrong with the scope.In this example I am looking at two signals from an ESP32 development board.When I have the lower signal probe disconnected at from the ESP32 board the top signal looks ok.When I connect the lower signal probe to the ESP32 the signal seems to interfere with the top signal.When I disconnect the lower signal probe from the oscilloscope but not from the ESP32 board I still see this interference.Could someone please explain what might be going on here as I'm a bit confused!All the bestAndy

author: frozenfrogz
author_rank: Frequent Contributor
timestamp: April 22, 2017, 12:44:40 pm
content: Hello Andy and welcome to the forumAre you using the bare probe tips, or the hook attachments?The hook attachments can act as an antenna.Frederik

author: AndyCap123
author_rank: Newbie
timestamp: April 22, 2017, 12:51:06 pm
content: Hi Frederik,Thanks very much for the reply and the welcome.I was using the hook attachments.I will try using the probes.....CheersAndy

author: AndyCap123
author_rank: Newbie
timestamp: April 22, 2017, 12:57:58 pm
content: Hi,I just tried with the bare probe tips and it is exactly the same.CheersAndy

author: wraper
author_rank: Unknown
timestamp: April 22, 2017, 01:30:34 pm
content: Quote from: frozenfrogz on April 22, 2017, 12:44:40 pmAs well as a ground alligator clip with wire.There is a good reason why people use such things:

author: AndyCap123
author_rank: Newbie
timestamp: April 22, 2017, 01:33:39 pm
content: Hi,Thanks for the info, I will try with those spring clips.I have a bit more info as well, the "Interference" seems to be inverted.The attached image has the blue lower signal probe connected to the earth on the oscilloscope.

author: frozenfrogz
author_rank: Frequent Contributor
timestamp: April 22, 2017, 01:42:19 pm
content: Just a side-note:You can take clean screen grabs directly to USB thumb drive by pressing the green printer button (below "Help").So you do not have to fiddle around with a camera while probing etc.The thumb drive needs to be FAT32 or FAT16 formatted, there are reports on some drives that just don’t want to work, it is recommended to use a drive of 8Gb or smaller (my unit has no problems with 128Gb, but who knows).

author: Fungus
author_rank: Super Contributor
timestamp: April 22, 2017, 06:34:51 pm
content: Looks like a probing/grounding problem to me.How are you connecting the probes? Can you connect both ground clips to the exact same point on the device under test? If the ground clips are at different points on a PCB then that sort of signal is very likely.It could also be radio noise from switching. Using the spring clips can help with that. If the peaks are reduced in size when you use a spring then that's probably the explanation.

author: ataradov
author_rank: Super Contributor
timestamp: April 22, 2017, 06:36:19 pm
content: That's more or less normal. You can deal with this by carefully thinking about your setup, or just forget about it, as long as you know the reason.Also, see that "BW Limit: OFF"? Enable this if you are looking at slow signals, it will clean things up a lot.

author: Electro Fan
author_rank: Super Contributor
timestamp: April 23, 2017, 05:53:50 am
content: Quote from: AndyCap123 on April 22, 2017, 12:30:38 pmTry just connecting the yellow signal by itself and work on improving the probe's ground connection until you get a cleaner signal without the overshoot spikes. (The shorter and more secure the ground connection between the probe and the DUT the better.)Likewise, better probe grounding might help produce a cleaner waveform for the blue signal (i.e., minimize the spikes).As for the blue signal showing interference even when the yellow probe is disconnected from the scope, if that is occurring when the yellow probe remains attached to the DUT, it would seem to indicate the yellow probe is picking up RF from something or otherwise contributing to the interference. (Presumably, the interference on the blue signal goes away when you disconnect the yellow probe not only from the scope but also from the DUT?)It looks like these might be two separate issues, although perhaps if the probe ground connection is better made for the yellow signal (and maybe also for the blue signal) it might help reduce the interference impacting the blue signal. Fixing the yellow ground issue should be relatively easy, fixing the blue interference issue might take more work.

author: Electro Fan
author_rank: Super Contributor
timestamp: April 23, 2017, 06:09:27 am
content: Excellent video on probe ground connections.

author: AndyCap123
author_rank: Newbie
timestamp: April 23, 2017, 06:33:28 am
content: Hi Guys,Thanks very much for the interest and replies.@frozenfrogz - thanks for the tip I will do that from now on.@Fungus - everything being grounded from the same pin, using the grounding springs attenuated the interference but it is still there.@ataradov - as long as it's normal I'm happy. I just want to check that the scope is not faulty and from everyones responses it looks like it isn't the scope.@Electro Fan - With just the yellow signal attached to the board there are no spikes. If I have the blue signal attached and remove the yellow probe from the DUT the blue signal is perfect.One thing I have noticed is that I get the least interference if I just ground the yellow probe and leave the blue probe floating, does this make any sense?Also I have noticed that if I connect a single probes ground and probe to the ground tag on the scope I get quite a bit of noise on the scope is that normal as well?

author: AndyCap123
author_rank: Newbie
timestamp: April 23, 2017, 08:49:50 am
content: Hi Guys,I have just done some more experimentation and you are all absolutely correct it is a grounding problem.Even with just one probe connected to ground via the long ground lead I could see the high frequency signal leaking through onto the probed signal.With the spring attached and grounded properly there is no leak at all. I guess last time I tried I was doing something not quite right!Thanks to all your help I have learnt a lot here, thanks very much.Andy

author: AndyCap123
author_rank: Newbie
timestamp: April 23, 2017, 09:08:45 am
content: Hi Guys,With my better understanding of grounds I did the same test from the oscillator ground but this time using the spring clip.Noise is much better.

author: Electro Fan
author_rank: Super Contributor
timestamp: April 23, 2017, 10:03:37 am
content: maybe something of interest in here:https://m.youtube.com/watch?v=eVB_n4t57Yc&feature=youtu.be

author: Electro Fan
author_rank: Super Contributor
timestamp: April 23, 2017, 10:44:54 am
content: maybe even further off topic but maybe slightly relevant with respect to viewing, measuring, and interpreting noise:

author: Jeroen3
author_rank: Super Contributor
timestamp: April 23, 2017, 11:07:00 am
content: Quote from: frozenfrogz on April 22, 2017, 01:42:19 pmOr just use this:http://peter.dreisiebner.at/rigol-bildschirmkopie-lan/My rule of thumb is that you have to enable BW Limit when you are using the ground clip.

author: AndyCap123
author_rank: Newbie
timestamp: April 23, 2017, 12:08:09 pm
content: Thanks guys.Those videos were very interesting, looks like it's my weekend to learn things!Thanks for the link for that software, I will give it a go...CheersAndy

author: TK
author_rank: Super Contributor
timestamp: April 23, 2017, 02:49:57 pm
content: Quote from: AndyCap123 on April 22, 2017, 12:30:38 pmIt looks more like a grounding / power supply problem on the DUT (ESP32 board) than a noise problem on the scope. There is a lot of ringing on the signal. What are the signals being probed? Maybe the crosstalk is already present on the ESP32 board and the scope is capturing exactly what is happening on the DUT.

author: Electro Fan
author_rank: Super Contributor
timestamp: April 23, 2017, 05:33:38 pm
content: Quote from: TK on April 23, 2017, 02:49:57 pmAgreed. I think the issue was amplified/masked by the probe grounding technique; now that Andy has that pretty well squared away, the noise level is relatively lower. The videos with information on scope settings were just to show that it's sometimes possible to fine tune the view into the noise. Whatever noise is left might either be a non-issue for the DUT's performance, or if it is, it might be due to a further grounding and/or power supply problem on the DUT. It's a matter of peeling things back depending on the level of subtleties that need to be examined. To me this is the part of the intrinsic enjoyment of test equipment - test equipment not only helps people measure, design, build, test, debug, and repair but in the process it helps people learn by visualizing relatively small, subtle, and otherwise difficult to observe phenomena.

author: AndyCap123
author_rank: Newbie
timestamp: April 23, 2017, 05:55:24 pm
content: Hi Guys,The original images I posted were probed from a breadboard connected by 6 inch jumper cables to the ESP32 board with the probe grounds also connected to another 6 inches of jumper wire so 12 inches in total.I now know if you want more accurate results this isn't a good idea.If I probe directly off the pins on the board using the spring ground everything cleans up well.Originally I was using it to look at 4 pins of 20Mhz SPI and the interference was causing problems with the decoding, I have since worked out how to change the logic trigger levels of the SPI decoder on the scope to get over this problem.I haven't checked if using the spring ground directly to the board pins will clean up the ringing, I will have a look in the morning.The help and videos posted here have helped me understand a lot more about what is going on, now I know there is nothing wrong with the scope, it was only my ignorance getting in the way!What would be quite useful is some attachment for the end of the probe that split out the probe and ground onto short wires with pincers, I'm on the lookout for some if I can't find anything I guess I can knock something up. This would get over the problem with the springs of needing an earth near what you want to probe.CheersAndy

author: Fungus
author_rank: Super Contributor
timestamp: April 23, 2017, 05:58:36 pm
content: Quote from: AndyCap123 on April 23, 2017, 05:55:24 pmDefinitely bad.Quote from: AndyCap123 on April 23, 2017, 05:55:24 pmQuote from: AndyCap123 on April 23, 2017, 05:55:24 pmI'm sure there'll be something out there.

author: TK
author_rank: Super Contributor
timestamp: April 23, 2017, 06:48:07 pm
content: Quote from: AndyCap123 on April 23, 2017, 05:55:24 pmI also use breadboards for early prototypes / tests, but try to use male pin headers to create scope/logic analyzer hookup points and it gives better results than flexible jumper wires.

author: Electro Fan
author_rank: Super Contributor
timestamp: April 23, 2017, 07:13:45 pm
content: Quote from: AndyCap123 on April 23, 2017, 05:55:24 pmYour idea might make a very useful product or tool if it could be designed/made. Let us know what you come up with.Here is a thread that advocates making test points with MMCX connectors. Not sure if this will always be practical, but it's an approach.https://electronics.stackexchange.com/questions/136123/how-do-you-attach-an-oscilloscope-ground-spring"I tend to try to scatter MMCX footprints around my board layouts, so I can get easy probe access to any nets I'm interested in. Plus, they make decent pads for probing with a spring ground clip if you don't want to solder connectors down."Glad to see that the DS1054Z is doing a good job for you with SPI. That scope packs a lot of functionality for the price.