Checking PS for ripple, trouble with noise patterns on scope. Newbie question.

[Chris Wilson]

I am having trouble getting any sensible waveforms when trying to check an old Tektronix scope power supply lines. I suspect the electrolytics in the power supply are iffy, and am using my good scope to see if there's a lot of ripple in the line voltages, which are various, from 5 volts to 110 volts.  I only have cheap Ebay x1 / x10 switchable probes, the scopes that are working properly are a 100 meg Philips / Fluke Combiscope, that is switchable analogue to digital mode, and a 150 meg good quality Dataman 520 USB scope. I am setting the scopes to AC, and using the ground tag on the PCB board of the scope under test and probing the various voltage test tags. I get no discernible ripple like waveform, but I do get what looks like random noise. I am setting to read in millivolts and using various timebase settings. I don't think either scope has a setting to reduce bandwidth, which I have read is desirable for these sorts of measurements. I think the probe leads may be picking up general noise, possibly off the ground lead? Is this an art form, or am I doing something wrong? Is there another, easier way to check these supplies for ripple? Thanks.

[ModemHead]

Here are three different views of the exact same test point on a Tek 465 (the main +55V supply.) Using a 10X probe, grounded to the nearest labeled ground test point.

All set to measure ripple: AC-coupled, max sensitivity at 20mV/div, 5ms/div sweep speed, line triggering. All I see is this fuzz.



Let's see if we can find out what the fuzz is all about. Sweep speed up to 10ns/div, as fast as this scope will go. Trigger off Ch. 1, signal itself, instead of line triggering. A big fat signal at about 96MHz appears. Stands to reason, since I have a very powerful FM broadcast tower nearby with a station broadcasting at 96.1MHz. This noise permeates everything in my workspace. I can pick it up out of the air by grounding the probe tip with it's little 10cm ground clip and then dangling it about.



Back to the original settings, except turn the 20MHz bandwidth limiter on. Now I can just barely see some bumps every 16.7 ms, corresponding to US mains frequency. Looks good to me.



I'm sure your Combiscope has a bandwidth limiter somewhere, probably on a softkey. Look in the vertical set-up menus.

[w2aew]

And also keep in mind that the electrolytics that you suspect would be reasonably large values - thus suitable for soaking up the relatively lower frequency ripples (line ripple, switching fundamentals, etc.).  So, you don't need to be hunting for really high frequency supply noise.  That's why ModemHed is unconcerned about the 96MHz wiggle - this is well above where the electrolytics are going to be effective due to their lower self-resonant properties.  If you are looking for artifacts due to bad electrolytics, concentrate on looking well below 1MHz.

[Chris Wilson]

Thanks for the rep;es, I will have a look at the manual for the Combiscope (if in doubt RTFM... ) It seems to be pretty comprehensive in most regards, I am probably just not seeing it yet. I have just had a look at the manual for my USB scope, that certainly does not appear to have a way to reduce its bandwidth. I'll hopefully have some time over this weekend to have another go, I'll let you know if I have more success now I have your good advice to follow, thanks again.

[alm]

The Combiscope will very likely have at least a fixed 20 MHz bandwidth limit, probably not a variable one. No idea about the unnamed USB scope, I wouldn't expect much from it.

[Chris Wilson]

So really, in some ways, you can have a scope that's "too good" for some applications and needs throttling back in order not to see irrelevant data? I had never really thought about this aspect before. I now have an elderly, but actually brand new and unused  GW Instek 10 meg analogue scope someone gave me for sorting something out on their engine the other day, but I suspect that may lack timebase ability for this? http://www.testequipmentdepot.com/instek/pdf/GOS-310.pdf

[ModemHead]

Actually, it's the vertical sensitivity that counts most for this measurement. You're looking for really small, low-frequency signals.

The specs for the power supplies are in the Tek 475 Service Manual, Fig 5-5 in the long Calibration section.  That should give you an idea what you're looking for.



I have one more scope photo that may interest you. This is the +110V supply on my 465, and it's the only supply on this unit that exhibited the "classic" ripple pattern at twice the mains frequency. The others were barely measurable, even with a 1X probe. This one has about 8mV p-p ripple, the 465 manual says that 20mV is allowable, so it's good.  Interestingly, the 475 spec allows up to 1V ripple on that supply.

[ModemHead]

So really, in some ways, you can have a scope that's "too good" for some applications and needs throttling back in order not to see irrelevant data?
<snip>

Yep, it all depends on what you're looking for. That's why wideband scopes have bandwidth limiters, and why we're all pretty sure the Combiscope has one.

The Instek analog should work, with a 1X probe.

[Chris Wilson]

OK, I know it works as I tried feeding my signal generator into it, the screen size is tiny, but ey, it's unused and a gift. I will try it as well as read up on the Combiscopes set up parameters in more depth.

I found a none working Thurlby 1503 digital bench multimeter on UK Ebay last week and put in a low bid as it will also run off internal batteries, which is usseful to me. It arrived today so I have been having a poke about inside tonight and have found all that was wrong with it was the external power supply chassis mount socket was mechanically damaged, so neither the internal battery tray (un-corroded!!) nor external 12 volt supply was feeding the PCB board. I have now done a temporary fix and ordered a new socket from RS. So I now have a decent 'ish bench multimeter. When the calibrated voltage source I got via a tip on these forums arrives from the US I can check its calibration and tweak it if needed. At least I have managed to repair one thing

Thanks!

[ModemHead]

<snip>
So I now have a decent 'ish bench multimeter. When the calibrated voltage source I got via a tip on these forums arrives from the US I can check its calibration and tweak it if needed. At least I have managed to repair one thing

Excellent.  Feel free to gloat, with pictures. We love pictures of everyone's latest eBay acquisitions.