Author Topic: Scope trace interference  (Read 2569 times)

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Offline StillTrying

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Scope trace interference
« on: July 06, 2017, 01:53:47 am »
Is it just me that can sometimes get this amount of interference - on a good day. :)

Both probes are shorted gnd clip to tip. Switching the probes to X1 or or X10 makes very little change to the 50MHz to 80MHz waveform.
After ending up in the dark with just the scope on, PC in standby, and a torch, I think it must be coming from next door.  :wtf:
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Online BrianHG

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Re: Scope trace interference
« Reply #1 on: July 06, 2017, 02:13:12 am »
Cell phone broadcasts?
WiFi packets?
Radio transmitter?
AM Radio broadcasts?
Dying Fluorescent lamp ballast on the AC mains, anywhere within you and your neighbors apartments?
CB Radio broadcast?
Light Switches, or really heavy load switching like Air-conditioner or heater or refrigerator?
Sensitive or dying switching supply, not necessarily your scopes, but, someone else PC or TV power supply?
Dying LCD flat-screen TV electro-flourescent HV power inverter?

Just because some of these broadcast out of your scope's bandwidth, some short packet bursts of the right length of time at these frequencies can look like a lower frequency signal due to your scope's ground clip wire length acting as a tuned antenna at a sub-harmonic.
« Last Edit: July 06, 2017, 02:16:43 am by BrianHG »
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Offline tautech

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Re: Scope trace interference
« Reply #2 on: July 06, 2017, 02:16:29 am »
Is it just me that can sometimes get this amount of interference - on a good day. :)

Both probes are shorted gnd clip to tip.
Which is a nice little dipole antenna.  ;)

Have you any 50 \$\Omega\$ terminators ? What does the apparent interference look like with them on the inputs ?

With a little bit of fiddling around you might be able to knock up something directional and home in on the source.
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Online BrianHG

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Re: Scope trace interference
« Reply #3 on: July 06, 2017, 02:18:22 am »
Yes, disconnect your probes and turn on your scope's 50 ohm terminator, see if the interference is still there, even if it is at a reduced amplitude.
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Offline StillTrying

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Re: Scope trace interference
« Reply #4 on: July 06, 2017, 03:14:14 am »
Of course I still had the PC on(but not the monitor) across the room, but I often do and the interference's amplitude reduces quite a bit if I put the scope on the same desk right near the PC.

If I take the scope to the kitchen about 15 10 meters away the interference is just about completely gone, just left with the small normal 90-100MHz wobble.

It somehow looks like SMPS to me.

I've just remembered that one of the buildings next door is actually empty, could it possibly be a radar (not PIR) landlords alarm just behind that particular wall.

I've put the scope back by 'the wall' for testing purposes. No 50R here, but if I remove one of the probes it stops the pickup on that channel as shown.

So I still don't know what it is ATM, other than it's strongest near that wall.
« Last Edit: July 23, 2017, 11:06:28 am by StillTrying »
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Offline StillTrying

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Re: Scope trace interference
« Reply #5 on: July 06, 2017, 05:51:30 am »
It was gone next time I looked, after spending 15 minutes trying to reproduce it, I noticed it was starting to get light outside 4am-ish, maybe it was a street light outside ? :-//  I might find out 10pm-ish.  :)

Peak-Detect same as in 1st post 1st pic. but no interference now.
« Last Edit: July 06, 2017, 05:55:47 am by StillTrying »
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Offline CJay

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Re: Scope trace interference
« Reply #6 on: July 06, 2017, 07:15:48 am »
As others have said, there are many potential sources of interference, I had a 12V PSU for some LED strips fitted under the wall mounted kitchen cupboards, when it was on my 'scope was unusable, a street light or a neighbour's dawn-dusk porch light sounds likely though.
 

Offline vealmike

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Re: Scope trace interference
« Reply #7 on: July 06, 2017, 02:06:20 pm »
Looks like you have something at a medium freq and everytime that couples in there is some hf ringing.
Do you have FFT? If so, turn it on and hunt down the frequency of what ever it is at a medium freq. a SMPS should could be running anywhere from a few hundred KHz to very low MHz.


Once you've tried a 50 Ohm terminator on the scope (and therefore eliminated pickup inside the 'scope), concentrate on probing technique.

As has been said, grounding your probe tip isn't always effective. Many probes have a flying GND lead that attaches a long way from the tip. Using this to ground the tip can create a really big loop that does act as an aerial.

If you take the hook off the end of the probe, you should see that there is a GND ring very close to the tip. Try winding some tinned Cu wire around this and grounding the tip with as short a connection as possible.
 

Offline StillTrying

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Re: Scope trace interference
« Reply #8 on: July 07, 2017, 01:02:16 am »
The spikes came back tonight at ~10:40pm, about half an hour after it gets dark, despite leaving the scope on a very slow timebase waiting for them I still managed to miss the actual start. >:D

As others have said, I think they're just very narrow spikes and any frequencies I see are just ringing.

If I trigger off the mains instead of the spikes, then the timing of the spikes is very mains related, but I'd be surprised if on the same mains phase as shown.

It's something that automagically switches on and off, and is near the front of the building somewhere.

« Last Edit: July 07, 2017, 01:06:09 am by StillTrying »
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Online BrianHG

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Re: Scope trace interference
« Reply #9 on: July 07, 2017, 03:11:14 am »
That's might be electronic lighting ballasts used in sodium lamps, either in your building on on the street.  Decades ago, in my old house, we had outdoor sodium lamps on each side of the house which made a 2 thin static looking bars slowly scroll through some of our CATV channels when the lights were on.  You might be out of luck to fix this.  I can only recommend trying a quality power bar with noise filter for your scope and hope it's getting this noise through your mains just as much as it is coming from you probe leads.  You may lower the problem, but, I doubt you can get rid of it unless you turn your lab room into a Faraday cage, with a good mains filtered to a separate earthing to the ground.
« Last Edit: July 07, 2017, 03:17:38 am by BrianHG »
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Offline tautech

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Re: Scope trace interference
« Reply #10 on: July 07, 2017, 03:55:13 am »
Hmmm, what do your power providers use for HW heating control ?
Over here some still use ripple control superimposed over the mains, sometimes you can hear it on audio appliances but it normally only lasts a short while, maybe just a few seconds.
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Offline StillTrying

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Re: Scope trace interference
« Reply #11 on: July 08, 2017, 02:27:21 pm »
Thanks for all the suggestions, I'm reading them and seeing if any fit.

A difficulty of course that the probes little gnd loops don't seem to act usefully as an antenna. If I shield the loop completely with my hand/body the amplitude of the spikes on that channel actually increase. It almost always seems as if the mains lead is acting as the antenna, and the probes as the gnds, and it's more difficult to position or wave the mains lead around.

Quick quoted answers, but they'll do!

"Hmmm, what do your power providers use for HW heating control?"

AFAIK there isn't any encoding on UK mains, in most areas the mains is very reliable. I think there's an economy provider or 2 encoded on to Radio4's 198kHz, together with time/date. For time/date signals we can use MSF 60kHz, R4 198kHz, DCF 77.5kHz, TDF 160.2kHz, GPS.  :)

"That might be electronic lighting ballasts used in sodium lamps, either in your building on on the street."

Yes, I was thinking it might be the nearest(~9 meters) outside lamp especially as I've seen it switch to some dim mode, but for some reason this nearest lamp hasn't lit up at all during the last 2 nights, it was on for night1, but no difference to the level of spikes whether it's lit or not.  :-//

"thin static looking bars slowly scroll through some of our CATV channels"

Yes, I can see it as intermittent thin horizontal lines on a composite-type video source, but luckily nothing at all shows on RGB.

"I can only recommend trying a quality power bar with noise filter"

I forgot to try a filtered mains bar, I'll try that tonight.


On night 3 after only 12-15 minutes waiting I managed to capture the start of the spikes as shown. A clean switch on, with the density of the spikes slowly reducing over time, eventually the gaps between the spikes can reach 3 or 4 seconds as in 1st post,1st pic. this is giving me a possible idea of what it might be, which I'll test when I can.
« Last Edit: July 08, 2017, 02:47:37 pm by StillTrying »
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Online BrianHG

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Re: Scope trace interference
« Reply #12 on: July 09, 2017, 03:14:34 am »
"That might be electronic lighting ballasts used in sodium lamps, either in your building on on the street."

Yes, I was thinking it might be the nearest(~9 meters) outside lamp especially as I've seen it switch to some dim mode, but for some reason this nearest lamp hasn't lit up at all during the last 2 nights, it was on for night1, but no difference to the level of spikes whether it's lit or not.  :-//

Yes, a dying sodium light ballast, or a ballast trying to ignite a dead/dying lamp will spike RF in tune with the mains.  Also, if water or humidity has gotten into the ballast.  Replacing 1 of the 2 outdoor sodium lamps with new one at my old house fixed the problem as it was half as bright as the one on the other side of the house.

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Offline CJay

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Re: Scope trace interference
« Reply #13 on: July 09, 2017, 09:42:47 am »
found the pics I took of the interference from the LED PSU..

Didn't and still don't have a digital 'scope to capture from
 
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Offline StillTrying

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Re: Scope trace interference
« Reply #14 on: July 09, 2017, 07:28:27 pm »
"giving me a possible idea of what it might be, which I'll test when I can."

I thought it might be some nearby newish emergency lights trying to turn on because it's dark, or to SMPS top up their internal batteries, but after 3 or 4 tests it's definitely not them.

I tried a filtered mains bar, - no difference.
I moved the scope upwards, into the roof really, well away from any TV/SAT and most mains cables, up there the spikes were as bad as ever. So the only thing I'd got a bit closer to there would be the street lamp bulb, about the same height as the bulb and no brick wall shielding between the scope and bulb. Luckily the lamp rarely lights up now, so hopefully it's be fixed this week. |O
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Offline StillTrying

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Re: Scope trace interference
« Reply #15 on: July 23, 2017, 11:33:35 am »
The spikes were the outside street lamp, after they eventually replaced the dead HP sodium bulb the nite-spikes stopped!

I've tried to detect a switch on spike in the seconds just as the lamp starts, not seen one yet.
I'm still amazed by how much EMI a street lamp was emitting over some distance.
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Online BrianHG

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Re: Scope trace interference
« Reply #16 on: July 23, 2017, 02:53:09 pm »
The spikes were the outside street lamp, after they eventually replaced the dead HP sodium bulb the nite-spikes stopped!

I've tried to detect a switch on spike in the seconds just as the lamp starts, not seen one yet.
I'm still amazed by how much EMI a street lamp was emitting over some distance.

I won!  ;D
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