What is the proper way of setting up an oscilloscope?

It is normal to see mains 60 Hz pickup when the probes are not connected to anything. It should go away if you touch the probe tip to the earth clip, and also when the probe and the earth are connected to a signal source.

Use the triggering to get a stable display.

Hi, Whatever. First off, it might have helped if you had mentioned how you're making your scope connection to the board you're measuring. If you're using a separate wire from the banana jack on the front or back of the scope panel to the power supply (or Frith forbid, you're just using a BNC-to-alligator clip thingie), you're almost guaranteeing this is going to happen. Use the ground clip on the real scope probe. If it's broken or missing, buy a new one. If you don't have one, get one. Make sure it's rated for the frequency of the scope.

Try triggering off of the calibration signal on the scope (if it has one). I'll bet you don't have this problem with that signal. Actually, most gross ground noise problems of this type are due to the scope probe, plain and simple. You might want to try borrowing a known good probe, and trying it again. They don't last forever.

Good luck Chris

Well, to start, I have the scope and the DC power supply plugged in to the same surge protector. My circuit ground is the negative terminal of one of the DC supply outputs. I connected the ground pin of the oscope probe to this negative terminal.

When I noticed the 60Hz signal, the scope probe was not connected to anything.

Thanks!

GG poster

How is the scope grounded? Signal ground and Power grounds.

Unless the ripple really is present on the signal - you likely as not forgot to ground the scope to your PS or have a ground loop - unlikely with both on the same outlet.

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At the 20 mv sensitivity you might see some ripple on all but the best power supplies,

with the scope probe floating - it has a 10 megohm input impedance so you should expect to see what is in the air - 60 hz is all around. Put a 10 k resistor between the probe and the ground on the probe and it should quiet it out to a flat line.

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Well, the outlet plug for both oscope and power supply are three prongs, and both of them are plugged in the same surge protector.... so I guess the power ground for both oscope and power supply are the same.

The signal ground is referenced to the power supply negative terminal, so this is separate from the power ground. I hope I understood your question correctly.

** Groper alert !

** Too damn lazy to supply a link ??

** Got the scope sitting on top of the PSU ?? ** The probe tip and its ground clip must be shorted to see nothing. ** Sure that surge protector has an earth connection that works ?? ** Use standard earthing and shielding practice & avoid ground loops . ** Noise in the triggering will do that.

Try using the external synch input on the scope linked to an output on the generator that has a fixed level.

Use good co-axial BNC leads.

Maybe de-earth the generator - to do this safely, connect the AC terminals of a 20 amp bridge rectifier ( with + and - linked) in series with its earth wire.

This isolates small hum loop voltages while allowing any dangerous ones to conduct to ground.

....... Phil

Hi, Whatever. That's the real source of your problem -- the external wire GND connection to the power supply. Use the ground clip wire on your scope probe to connect to the local GND at the point you're measuring, and you'll be happy about how much the ground noise is improved. Again, if your ground clip wire is missing or broken, replace it. If you aren't using a real scope probe, get one which is matched to your scope. It's the cost of getting accurate measurements.

To answer the question you're worried about, a working scope has the ability to cancel out the millivolts of difference between local ground for the scope and local ground for whatever's being measured. Typically, you won't have to worry about the device under test and the measurement device using the same power strip, although it certainly can't hurt and is a good idea on principle.

By the way, for safety purposes it's recommended that the scope remain grounded.

Good luck Chris

Sounds like you're doing it right. The scope probe ground is usually the better choice than using the ground on the scope chassis. (more a concern when high frequencies are involved).

Bench (testing) supplies leave the power supply plus and minus floating with respect to ground so a ground loop is unlikely on that type of supply - switching supplies may ground the output, so a ground loop is possible.

You mention seeing the ripple on a 20 mv scope setting. That is quite sensitive, but on a good linear supply I'd expect the regulated output to have 1% or less noise on it - and most of that might be due to measuring technique.

The ripple you are concerned with? Is it sinusoidal or half/full wave pulsing? - a supply falling out of regulation due to poor filtering or a bad diode will show a half or full wave (16/8 millisecond humps) not a sine wave.

Steady display with the sync set to "line"

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Oops.

Nope. The scope and the PSU are away from each other.

I don't know. I always thought that the surge protector had an MOV near the mains cord, so that it shorts out any surge to ground and isolate any of the devices connected to the surge protector. Right?

I'm not familiar with this. I have a bit of introduction to ground loops after reading some PCB layout tips.

Will I be able to minimise this noise?

Thanks!

It's sinusoidal.

Thanks!

Standard shielding practice is to minimize the length of unshielded wire. By using the scope probe ground clip instead of using a separate wire, you'll reduce this length to a couple of inches instead of a meter or more. This is basic stuff.

New scope probes are fairly inexpensive, especially if you go with another vendor besides the original scope manufacturer. You should look at the scope front panel to determine input capacitance, and get a replacement probe with that compensation range. Also, get a X1/X10 probe that's rated for a higher frequency than your scope -- equal doesn't cut it.

One other thing -- this is one of the rare circumstances where buying used is a mistake. Get a new probe and take care of it -- even cheapies last a long time if you're careful with them.

Good luck Chris

The scope is fairly new and came with the manufacturer's probes.

Thanks!

Another thing to check is that the scope shield is really connected. Check from the chassis out to the ground on the probe itself and make sure there's (zero ohms) continuity.

Most scopes will be zero ohms (unless you have the rare bird - true differential inputs)

My own scope probes frequently have a poor connection where the ground connects to the probe at the test end. (small spring clips) I can use the chassis ground with no problems - but then I cut off the ground pin that was on the power cord. Something of a safety hazard.

Of no use to you, but an interesting tidbit: The little ring close to the tip on scope probes is a ground - and is used to minimize the length of wire when the 4" from the normal ground is too long (checking noise on switching supplies is one application).

Grounding practice for instruments isn't 100% the same as circuit boards - but they can both be solved the same way - single point grounding.

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That's like saying a radio antenna isn't connected to anything, so why do we get a signal? There are 60 Hz waves travelling through the space in your home/lab/wherever, and you are simply measuring them on the scope.

Try disconnecting the probes at the oscilloscope input jack. The 60 Hz should be gone, in which case it is not a problem with the 'scope picking up stray 60 Hz or a bad grounding in the 'scope. But if the 60 Hz is still there, then yes there's a problem with the 'scope.

Next, reattach the probes to the oscilliscope, AND connect the probe ends to anything that you know doesn't have 60 Hz. A common battery would work. The 60 Hz should go away when you do this. But if it's still there, you have a real problem.

Hope this helps.

Regards,

Mark