Hello there!

I have an Instek GDS-2202 digital storage oscilloscope. I am trying to
figure out how to setup my scope to minimize the pick up of stray 60Hz
mains signal. If I change my scope's vertical sensitivity to 20mV/div,
then I can see the 60Hz signal. The signal's amplitude increases even
more when I turn on my DC power supply. I can pick this signal up
without even connecting the probes to anything.

Should I connect the oscilloscope to another outlet instead of putting
it in the same surge protector? How do I minimize the 60Hz pickup?

Also, I have a programmable signal generator that is pretty accurate. I
put out a 1 kHz sine wave test signal with a low amplitude (about 200
mV), and it seems like I have some display issues. On a Tektronix
scope, this signal looks pretty stable. But on my GDS-2202 the signal
drifts a lot, so instead of seeing one sine wave, I see a non-inverted
and inverted sine wave on top of each other.

I am new to this and would like to learn from you about the proper way
of setting up instrumentation equipment.


Thanks!

"MRW" <m...@gmail.com> wrote in message 
news:1...@45g2000cws.googlegroups.com...
> Hello there!
>
> I have an Instek GDS-2202 digital storage oscilloscope. I am trying to
> figure out how to setup my scope to minimize the pick up of stray 60Hz
> mains signal. If I change my scope's vertical sensitivity to 20mV/div,
> then I can see the 60Hz signal. The signal's amplitude increases even
> more when I turn on my DC power supply. I can pick this signal up
> without even connecting the probes to anything.
>
> Should I connect the oscilloscope to another outlet instead of putting
> it in the same surge protector? How do I minimize the 60Hz pickup?
>
> Also, I have a programmable signal generator that is pretty accurate. I
> put out a 1 kHz sine wave test signal with a low amplitude (about 200
> mV), and it seems like I have some display issues. On a Tektronix
> scope, this signal looks pretty stable. But on my GDS-2202 the signal
> drifts a lot, so instead of seeing one sine wave, I see a non-inverted
> and inverted sine wave on top of each other.
>
> I am new to this and would like to learn from you about the proper way
> of setting up instrumentation equipment.
>
>
> Thanks!
>

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. 

MRW wrote:
> Hello there!
>
> I have an Instek GDS-2202 digital storage oscilloscope. I am trying to
> figure out how to setup my scope to minimize the pick up of stray 60Hz
> mains signal. If I change my scope's vertical sensitivity to 20mV/div,
> then I can see the 60Hz signal. The signal's amplitude increases even
> more when I turn on my DC power supply. I can pick this signal up
> without even connecting the probes to anything.
>
> Should I connect the oscilloscope to another outlet instead of putting
> it in the same surge protector? How do I minimize the 60Hz pickup?
>
> Also, I have a programmable signal generator that is pretty accurate. I
> put out a 1 kHz sine wave test signal with a low amplitude (about 200
> mV), and it seems like I have some display issues. On a Tektronix
> scope, this signal looks pretty stable. But on my GDS-2202 the signal
> drifts a lot, so instead of seeing one sine wave, I see a non-inverted
> and inverted sine wave on top of each other.
>
> I am new to this and would like to learn from you about the proper way
> of setting up instrumentation equipment.
>
>
> Thanks!

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

Thanks all!

Chris wrote:
>First off, it might have helped if you had mentioned how
> you're making your scope connection to the board you're measuring.

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!

On 28 Nov 2006 14:40:24 -0800, "MRW" <m...@gmail.com> wrote:

>Hello there!
>
>I have an Instek GDS-2202 digital storage oscilloscope. I am trying to
>figure out how to setup my scope to minimize the pick up of stray 60Hz
>mains signal. If I change my scope's vertical sensitivity to 20mV/div,
>then I can see the 60Hz signal. The signal's amplitude increases even
>more when I turn on my DC power supply. I can pick this signal up
>without even connecting the probes to anything.
>
>Should I connect the oscilloscope to another outlet instead of putting
>it in the same surge protector? How do I minimize the 60Hz pickup?
>
>Also, I have a programmable signal generator that is pretty accurate. I
>put out a 1 kHz sine wave test signal with a low amplitude (about 200
>mV), and it seems like I have some display issues. On a Tektronix
>scope, this signal looks pretty stable. But on my GDS-2202 the signal
>drifts a lot, so instead of seeing one sine wave, I see a non-inverted
>and inverted sine wave on top of each other.
>
>I am new to this and would like to learn from you about the proper way
>of setting up instrumentation equipment.
>
>
>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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On 29 Nov 2006 12:16:38 -0800, "MRW" <m...@gmail.com> wrote:

>Thanks all!
>
>Chris wrote:
>>First off, it might have helped if you had mentioned how
>> you're making your scope connection to the board you're measuring.
>
>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!

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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Hello!

default wrote:
> How is the scope grounded?  Signal ground and Power grounds.

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.

"MRW" <m...@gmail.com> wrote in message 
news:1...@45g2000cws.googlegroups.com...


**  Groper alert  !


> I have an Instek GDS-2202 digital storage oscilloscope


**  Too damn lazy to supply a link  ??

http://exphil.com/pdf/Instek/GDS-2202.pdf


> I am trying to
> figure out how to setup my scope to minimize the pick up of stray 60Hz
> mains signal. If I change my scope's vertical sensitivity to 20mV/div,
> then I can see the 60Hz signal. The signal's amplitude increases even
> more when I turn on my DC power supply.


**  Got the scope sitting on top of the PSU  ??


> I can pick this signal up
> without even connecting the probes to anything.


**  The probe tip and its ground clip must be shorted to see nothing.


> Should I connect the oscilloscope to another outlet instead of putting
> it in the same surge protector?


** Sure that surge protector has an earth connection that works ??


> How do I minimize the 60Hz pickup?


**  Use standard earthing and shielding practice & avoid ground loops .



> Also, I have a programmable signal generator that is pretty accurate. I
> put out a 1 kHz sine wave test signal with a low amplitude (about 200
> mV), and it seems like I have some display issues. On a Tektronix
> scope, this signal looks pretty stable. But on my GDS-2202 the signal
> drifts a lot, so instead of seeing one sine wave, I see a non-inverted
> and inverted sine wave on top of each other.


**  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

MRW wrote:
> Thanks all!
>
> Chris wrote:
> >First off, it might have helped if you had mentioned how
> > you're making your scope connection to the board you're measuring.
>
> 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!

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

On 29 Nov 2006 13:54:41 -0800, "MRW" <m...@gmail.com> wrote:

>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.

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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