Digital oscilloscope Jittery signal trace - Page 1
Forum section: electronics / repair

author: Theboel
author_rank: Frequent Contributor
timestamp: February 19, 2016, 02:01:47 am
content: Hi All,Friend of mine ask me to repair his DSO its GW-Instek GDS-1062A the repair phase is short its turn to be a cable power intermittent break. but when I try to do some measurement with My reference Oscillator I found something I do not understand. as You can see in the video the waveform is not stable I measure with 4 OCXO and 2 Rb oscillator all show almost same result (10Mhz OCXO, 5MHz OCXO, 10MHz Rb) I try to adjust trigger level there is a slight improvement but the "oscilation" still there but when I measure signal from my audio oscillator with 1Mhz max its show a very stable signal. I also found when I adjust the amplitude there is no difference.Any thought about this ?

author: Theboel
author_rank: Frequent Contributor
timestamp: February 19, 2016, 03:41:09 am
content: Hi,I found this link about a gentleman that have same problem (different scope)Thank You

author: Theboel
author_rank: Frequent Contributor
timestamp: February 21, 2016, 02:19:46 pm
content: after two days and more than 100 view no one give any comment and my Google research only give a little clue about unstable / not accurate trigger circuit / signal but there is no information is this caused by software or hardware.

author: uncle_bob
author_rank: Unknown
timestamp: February 21, 2016, 02:41:02 pm
content: HiIn the video you linked to, the guy shows a very normal noisy signal. It has noise on the display and the same noise causes the trigger to move things around. It could be from:1) Bad scope probe2) Bad probe ground3) Ground loop between scope and signal source4) Bad BNC on front of scope5) Bad input amp on scope4) Bad sampler in scope5) Bad sample clock in scope6) Bad power supply in scope7) Bad connector from scope power to main boardBad ground on main board to case9) Bad ground from power supply to case10) Bad filter on power supply to AC line11) Bad ground on scope to AC line12) Wrong trigger settingThat's the short list of things to check out. Depending on what you find, there is a longer sub list under each of those.Bob

author: Bud
author_rank: Super Contributor
timestamp: February 21, 2016, 11:51:15 pm
content: You want jittery? I will show you jittery !!IMG_0592_ss.aviThis is a quite old Tek 2430, same issue is on both channels, they have separate amplifiers and separate samplers, and I verified the power supply is up to the specs. I also re-sitted all connectors. Next what I am going to do ( and recommend you do it too ) is check the master clock. My Tek has a PLL that multiples the reference 4MHz signal to 200MHz. I checked the reference clock and it looked OK, I will next check the PLL output.It is known that a shitty company called rigol has a problem with the PLL as minimum on their 1000 and 2000 series oscilloscopes. Their "master clock" is a modulated junk , it is not clock as it has to be. So this is not something unheard of in the world of Chinese scopes. If you have a spectrum analyzer , I recommend you check the master clock. You can do it with a loop probe, locate the pll chip and sniff around the chip. At the ADC clock frequency you should see a strong single spectral line, within a span of say 100-200 kHz. If you see multiple spectral lines close together, your PLL would be a suspect. A modulation on the clock degrades the clock jitter, which may manifest itself via the shaky signal screen that you observed.

author: Theboel
author_rank: Frequent Contributor
timestamp: February 22, 2016, 02:23:52 am
content: Hi all,1) Bad scope probealready check NO2) Bad probe groundalready check NO3) Ground loop between scope and signal sourceNeed To check4) Bad BNC on front of scopealready check NO5) Bad input amp on scopeNeed to check4) Bad sampler in scopeNeed to check5) Bad sample clock in scopeNeed to check6) Bad power supply in scopeNeed to check7) Bad connector from scope power to main boardNeed to checkBad ground on main board to caseNeed to check9) Bad ground from power supply to caseNeed to check10) Bad filter on power supply to AC lineNeed to check11) Bad ground on scope to AC lineNeed to check12) Wrong trigger settingNeed to checkFor note: This shaky/jittery only can see with sinusoid waveform I check with 4 square signaleven with point to point CMOS oscillator and divider there is no shaky/jittery at allThis is a quite old Tek 2430, same issue is on both channels,they have separate amplifiers and separate samplers,and I verified the power supply is up to the specs.I also re-sitted all connectors.Next what I am going to do ( and recommend you do it too ) is check the master clock.My Tek has a PLL that multiples the reference 4MHz signal to 200MHz.I checked the reference clock and it looked OK, I will next check the PLL output.It is known that a shitty company called rigol has a problem with thePLL as minimum on their 1000 and 2000 series oscilloscopes.Their "master clock" is a modulated junk , it is not clock as it has to be.So this is not something unheard of in the world of Chinese scopes.If you have a spectrum analyzer , I recommend you check the master clock.You can do it with a loop probe, locate the pll chip and sniff around the chip.At the ADC clock frequency you should see a strong single spectral line,within a span of say 100-200 kHz. If you see multiple spectral lines close together,your PLL would be a suspect. A modulation on the clock degrades the clock jitter,which may manifest itself via the shaky signal screen that you observed.This is interesting Bud, because the owner has give up with this scope and also I am notto interested with this scope maybe do some experiment with the hardware is interesting.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: February 22, 2016, 02:58:43 am
content: Caps,caps and caps.

author: Theboel
author_rank: Frequent Contributor
timestamp: February 22, 2016, 06:26:13 am
content: Quote from: SAUL BRITTO on February 22, 2016, 02:58:43 amWhat caps ? pleaseGot an email from GW-Instek they say :I’m sorry to let you know it’s can’t elimination in the GDS-1000 series.It’s the hardware restriction.However ,it will not affect the measurement result.If you are very care the jitter or shaky.How about test our GDS-1000B or GDS-2000E series.now my only option tear it down see what inside and may be find one or two thing can do.

author: Bud
author_rank: Super Contributor
timestamp: February 22, 2016, 06:34:57 am
content: Of course it is "hardware restriction". Rigol scopes PLL problem is also caused by a wrong hardware - incorrect PLL loop filter component values, complemented by incompetent PLL programming. There is more to it but i will reserve it for a separate post.

author: tautech
author_rank: Super Contributor
timestamp: February 22, 2016, 06:51:06 am
content: @ TheboelI'd take careful notice of what Bud is sharing with you re PLL.He and member MarkL were first to identify the cause of the jitter issue in the 1054Z.GW-Instek's reply revealing a HW limitation as the possible cause points to a similar problem as found in Rigol's. Whether this has been addressed in later builds is unknown.Unless HW mods are implemented the results you are seeing may be as good as it gets.Bud may link you to the posts where the problem is identified in the Rigols and subsequent discussion will maybe give you the info to attempt a fix.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: February 22, 2016, 07:21:53 am
content: Quote from: Theboel on February 22, 2016, 06:26:13 amHave you examined the PS for ripple? I would be very careful with it.

author: Theboel
author_rank: Frequent Contributor
timestamp: February 22, 2016, 12:35:37 pm
content: Hi.@ Bud sound its big operation do you mind to share your experience at least the link discuss about this problem. I get lost in long 1054 thread.@ Saul britto I will test the PSU and maybe do esr test to the cap

author: tautech
author_rank: Super Contributor
timestamp: February 22, 2016, 07:13:12 pm
content: Quote from: Theboel on February 22, 2016, 12:35:37 pmA couple of minutes searching found the posts in the thread where MarkL and Bud identified the PLL problem:https://www.eevblog.com/forum/blog/eevblog-683-rigol-ds1000z-ds2000-oscilloscope-jitter-problems/msg560788/#msg560788Further discussion will point you in the right direction.

author: Theboel
author_rank: Frequent Contributor
timestamp: February 23, 2016, 02:11:10 am
content: Quote from: tautech on February 22, 2016, 07:13:12 pmNever thinking to add jitter in the front of my rigol thread search, I need to read more I can check the PLL area I have enough equipment to do that but for software I am not sure

author: miguelvp
author_rank: Super Contributor
timestamp: February 23, 2016, 02:49:03 am
content: FWIW, the Rigol jitter issue was resolved to even Dave's standards.Bud has issues with Rigol, and it might not be a perfect hardware implementation for a very low end but high featured scope. So take in mind that most of the forum doesn't agree with his views since the software fix is more than adequate.But the OP is not talking about Rigol, so associating both is kind of pointless.We need more details, I'm on an android tablet so I didn't download the zip file to watch the OP noisy signal. But the ground lead will probably pick up a lot of noise and cause the jitter on a sine wave and not as much on the square wave.If the OCXO and Rb ground is shared with the scope, then don't use the ground lead, it might be picking up mains noise causing the jitter.If the ground is not shared, then use a short spring ground on the probe to reduce the noise/jitter.If you are using a coax cable, then proper termination on both ends will clear the noise.Once you eliminate the probing noise, if it still happens its probably the power supply as tautech mentioned, and replacing the caps on the power supply might clean the device capabilities to measure the signal.If you have a second more reliable scope you should be able to see if the signal is noisy using the same measuring techniques but be aware that the video of the similar problem could be improper termination causing the jitter, some scopes have built in 50 ohm terminationwhile others don't so you need a Tee connector and 50 Ohm termination (or a 50 Ohm pass through) for function generator signals and 50 Ohm cable.I think eliminating the ground lead noise would be the first thing to try, then look at the power supply.

author: tautech
author_rank: Super Contributor
timestamp: February 23, 2016, 04:01:10 am
content: @ miguelvpI'll quote the OP's email from GW Instek:QuoteDo you not see similarities with the 1054?By GW's reply it might seem they could not fix the problem without a HW change.I'm sorry but I know of no other well documented example of poor triggering and resultant jitter than in the 1054 thread to guide the OP on his endeavour of a satisfactory repair.If you do, please offer it.

author: miguelvp
author_rank: Super Contributor
timestamp: February 23, 2016, 04:14:38 am
content: Quote from: tautech on February 23, 2016, 04:01:10 amWhat I see from the OP's email is pretty bad communication probably paraphrasing the answer, or translation upon translation.As for well documented example of jitter because of the ground lead picking up noise, I would think the problem would persist on the square wave, but it doesn't, so it's a triggering level fault and it doesn't occur on a fast rise where the signal to noise ratio would make the problem go away.Edit: that of course doesn't rule out power supply noise, which it can be the cause as well.So not a PLL problem, otherwise the jitter would be present on the square wave signal as well.As for the 1054 jitter, it has been resolved by software and it's well within specs.

author: tautech
author_rank: Super Contributor
timestamp: February 23, 2016, 04:30:57 am
content: Quote from: miguelvp on February 23, 2016, 04:14:38 amYes, but it appears to me as a typical questionable English Chinese reply and of course and further explanation from GW would be helpful.Member Theboel might be well advised to ask for further explanation of the "HW restriction" and the suggested component changes that might fix his problem. Even ask if it is to do with PLL implementation.

author: miguelvp
author_rank: Super Contributor
timestamp: February 23, 2016, 04:33:54 am
content: If it was the PLL that would affect the square wave as well wouldn't it?

author: vk6zgo
author_rank: Super Contributor
timestamp: February 23, 2016, 05:02:41 am
content: Is the square wave at 10MHz,too?Have you tried different types of triggering,slope,coupling,etc?Have you tried splitting the signal & using it in the EXT trigger input as well as the vert channel you are using?

author: Bud
author_rank: Super Contributor
timestamp: February 23, 2016, 07:14:00 am
content: Quote from: Theboel on February 22, 2016, 12:35:37 pmYes I will write a detailed post with a guide how to fix DS2072a. The other models would be similar.

author: Bud
author_rank: Super Contributor
timestamp: February 23, 2016, 07:28:00 am
content: Quote from: miguelvp on February 23, 2016, 02:49:03 amPlease allow me to correct you : I have issues with incompetency and people getting fulled by chip junk. Let me assure you I will continue educate people about rigol incompetency. If they still want to buy it is their decision but the decision should be an informed one.

author: tautech
author_rank: Super Contributor
timestamp: February 23, 2016, 07:46:21 am
content: Quote from: Bud on February 23, 2016, 07:28:00 am+1You have no conflict of interest where as I apparently do, my study of a competitor's product and its problems and me sharing them with the forum is apparently bad form for a distributor of a competing product.

author: MarkL
author_rank: Unknown
timestamp: February 23, 2016, 03:55:51 pm
content: Theboel: You could try doing an FFT on the sinusoidal waveform to see if there's another signal that's getting mixed in with the main carrier.Offhand it looks like front end noise to me, or possibly a poor implementation for interpolating the trigger point.Let's have a look at the FFT. And if possible, compare it with any other scope you have that can also do an FFT.

author: Theboel
author_rank: Frequent Contributor
timestamp: February 23, 2016, 05:28:45 pm
content: Hello all,I am sorry for a late respond "a small working accident" hold me to reply.I already tested about grounding, cable termination, trigger setting and nothing changed and in attachment You can find the FFT from 5Mhz OCXO. ( I can take the picture from this OCXO use my SA if its necessary) so far I believe this come from inside the oscilloscope and I have few suspect :1. Ripple in PSU2. PLL Hardware3. PLL software4. Trigger circuit5. Front endTomorrow I try to do more test look at the hardware inside.and thank You for all attentionbtw: no further respond from GW about what kind of hardware limitation they mention in email to me

author: MarkL
author_rank: Unknown
timestamp: February 24, 2016, 05:54:19 pm
content: I don't see the frequency per division scale, or at least it's not clear to me.You'll need to adjust you sweep in the time domain to improve your resolution. And you'll want to set the maximum numbers of points, if it can be adjusted.Take a look in vicinity of your carrier up to the digitizing rate. If you can see it in the time domain, any interfering signal with a specific frequency will show up quite readily in the frequency domain.You're looking for any peaks besides the main carrier if it's something besides noise.It wouldn't hurt to verify your signal source on the SA also. Make sure you have a single clean tone.

author: uncle_bob
author_rank: Unknown
timestamp: February 25, 2016, 12:14:29 am
content: Quote from: Theboel on February 22, 2016, 02:23:52 amHiA sine wave has a slow moving edge as it crosses the trigger point. A square wave is moving very quickly.Example:Sine wave takes 2 us to go 0.1VSquare wave takes 0.1 ns to go 0.1 VIf your scope is set to 2 us / division, you will never see any trigger issue on the square wave. It goes through the trigger region long before any of the noise in your environment has a chance to act.Bob

author: Theboel
author_rank: Frequent Contributor
timestamp: February 25, 2016, 02:39:01 am
content: Quote from: MarkL on February 24, 2016, 05:54:19 pmIf hope this set of pic can help

author: Theboel
author_rank: Frequent Contributor
timestamp: February 25, 2016, 02:44:55 am
content: Quote from: uncle_bob on February 25, 2016, 12:14:29 amHi Bob,Bud also said a similar thing with You and the problem are its a hardware and software issue. I do not have capabilities to check the software issue but the hardware sure I can deal with it.I will open soon the PLL section and run it in simulator I really hope can do a thing or two not to eliminated but at least reducing this problem.Btw : I also check the PSU section and there is enough garbage came from this PSU to cause a lot of problem. I will try to filtering the PSU first there is nothing wrong have a clean PSU in this Oscilloscope, Right ?

author: uncle_bob
author_rank: Unknown
timestamp: February 25, 2016, 02:55:00 am
content: Quote from: Theboel on February 25, 2016, 02:44:55 amHiSince you started out fixing a power supply / cabling issue .... maybe that damaged something in the supply? Not knowing if the problem was internal or external to the scope, it's a bit tough to guess at some of this.Bob

author: Theboel
author_rank: Frequent Contributor
timestamp: February 25, 2016, 03:40:18 am
content: Quote from: uncle_bob on February 25, 2016, 02:55:00 amHi,I am sure this problem came from internal of the scope,What I wanna do with PSU is adding a few filtering thats all from the DC voltage measurement (without schematic) seem all voltage is reasonable to me.