Analog scopes for noise measurements

[Ira Rubinson]

Does anybody know why analog rather than digital scopes are preferred for
noise / ripple measurements on power supplies?

Thanks -Ira

 

[jadaha]

"Ira Rubinson" <irarubinson@yahoo.com> wrote in message
news:4umdnSnomP4hY3vfRVn-3g@adelphia.com...

Does anybody know why analog rather than digital scopes are preferred for
noise / ripple measurements on power supplies?

Thanks -Ira

either is OK.
There is no preference, it depends upon the bandwidth of the noise, ripple,
and the bandwidth of your scope(s).
Digital may not sample fast enough to get the noise correctly, but that
means the scopes bandwidth is too low.

 

[legg]

On Tue, 26 Jul 2005 20:33:59 -0700, "Ira Rubinson"
<irarubinson@yahoo.com> wrote:

Does anybody know why analog rather than digital scopes are preferred for
noise / ripple measurements on power supplies?

This is not necessarily the case, however the visible display of an
analog scope includes wideband information that can be more readily
perceived, characterized and usefully interpreted by human beings,
with fewer sources for gross error.

Noise, produced by various effects that are not always coherent, can
be missed by a digital scope, or be displayed too easily in a
misleading or even random manner. Sometimes this can be avoided by
making multiple measurements at differing time scales.

Wide-band digital scopes have traditionally been more expensive, with
difficult or completely useless triggering. This situation is being
corrected. Some of the newer scopes from tektronix and others do a
pretty good job at duplicating analog display persistance effects, and
provide reasonable triggering capability.

By far the greatest problem with power supply noise measurement is
probing methods - common to both scope types.

Knowledge of the limitations and capabilities of any equipment is a
powerful tool in avoiding error. Never buy or use (critically) a
device you have not either proven or seen to be proven adequate for
the job.

RL

 

[Ulrich Bangert]

Hi Ira,

the basic idea behind using a analog scope for noise measurements is: Their
screens supply a three-dimensional information. Beneath x and y there is
beam INTENSITY as the third information. Consider a noisy dc signal. Having
set a analog scope's beam intensity and vertical amplification to the
correct values you will see the dc component as a horizontal line and the
noise as a 'band' centered around the line. The intensity of the band will
resemble pretty well the noise's amplitude probability distribution and for
that reason the band displayed on a analog scope's screen is good measure
for noise.

Note that this effect is due to specific 'after glow' properties of the
phosphor inside the display tube. Due to the after glow the phosphor
performs kind of 'averaging over time' which translates probabilities into
intensity. In contrast to that the raster screens of MOST digital scopes a
basically television like and are optimized to have NO after glow because
they are expected to display a lot of independend pictures per second. You
will never get this 'band' display realized on a NORMAL digital scope.

I said 'MOST' and 'NORMAL', because TEKTRONIX have started to build what
they call DPOs = Digital Phosphor Oscilloscopes. In these scopes intensity
is again available as a third dimension of information, not by means of
analog after glow but with lots of clever digital electronics. I do not know
how well such a thing would perform on noise measurements but if you
consider using a digital scope you should call for a DPO and nothing else.
Because it is done with digital electronics, they can use color in stead of
intensity and also reverse things: Signals having a high probability can be
displayed dark and signals having a low probability can be displayed light.
Pretty well suited to find 'glitches' in a otherwise repeated signal.

Best regards

Ulrich Bangert

"jadaha" <invalid@spamless.com> schrieb im Newsbeitrag
news:42e71755$0$6700$892e7fe2@authen.white.readfreenews.net...

"Ira Rubinson" <irarubinson@yahoo.com> wrote in message
news:4umdnSnomP4hY3vfRVn-3g@adelphia.com...
Does anybody know why analog rather than digital scopes are preferred
for
noise / ripple measurements on power supplies?

Thanks -Ira



either is OK.
There is no preference, it depends upon the bandwidth of the noise,
ripple,
and the bandwidth of your scope(s).
Digital may not sample fast enough to get the noise correctly, but that
means the scopes bandwidth is too low.

 

[Terry Given]

Ulrich Bangert wrote:

Hi Ira,

the basic idea behind using a analog scope for noise measurements is: Their
screens supply a three-dimensional information. Beneath x and y there is
beam INTENSITY as the third information. Consider a noisy dc signal. Having
set a analog scope's beam intensity and vertical amplification to the
correct values you will see the dc component as a horizontal line and the
noise as a 'band' centered around the line. The intensity of the band will
resemble pretty well the noise's amplitude probability distribution and for
that reason the band displayed on a analog scope's screen is good measure
for noise.

Note that this effect is due to specific 'after glow' properties of the
phosphor inside the display tube. Due to the after glow the phosphor
performs kind of 'averaging over time' which translates probabilities into
intensity. In contrast to that the raster screens of MOST digital scopes a
basically television like and are optimized to have NO after glow because
they are expected to display a lot of independend pictures per second. You
will never get this 'band' display realized on a NORMAL digital scope.

I said 'MOST' and 'NORMAL', because TEKTRONIX have started to build what
they call DPOs = Digital Phosphor Oscilloscopes. In these scopes intensity
is again available as a third dimension of information, not by means of
analog after glow but with lots of clever digital electronics. I do not know
how well such a thing would perform on noise measurements but if you
consider using a digital scope you should call for a DPO and nothing else.
Because it is done with digital electronics, they can use color in stead of
intensity and also reverse things: Signals having a high probability can be
displayed dark and signals having a low probability can be displayed light.
Pretty well suited to find 'glitches' in a otherwise repeated signal.

Best regards

Ulrich Bangert

"jadaha" <invalid@spamless.com> schrieb im Newsbeitrag
news:42e71755$0$6700$892e7fe2@authen.white.readfreenews.net...

"Ira Rubinson" <irarubinson@yahoo.com> wrote in message
news:4umdnSnomP4hY3vfRVn-3g@adelphia.com...

Does anybody know why analog rather than digital scopes are preferred

for

noise / ripple measurements on power supplies?

Thanks -Ira



either is OK.
There is no preference, it depends upon the bandwidth of the noise,

ripple,

and the bandwidth of your scope(s).
Digital may not sample fast enough to get the noise correctly, but that
means the scopes bandwidth is too low.

I once spent an afternoon in the lab, with a blanket draped over me and
the 200MHz analogue CRO, intensity cranked right up, looking for a
glitch I suspected was there. Eventually spotted on after a couple of
hours, proving the problem lay with a piece of programmable logic. 10
minutes later we had a slow-scale example demonstrating the behaviour up
and running, and a fix about 20 minutes later (turned out to be
metastability).

A Tek rep came by with a DPO a few weeks later, and using some of the
sexy triggering features we managed to actually trigger on the glitch.
But had we not known it was there, we wouldnt have been able to set up
the triggering.

Cheers
Terry

 

[PeteS]

I've had my share of problems looking at noise (especially wideband
noise) on a digital scope. The major issue I found (for older and not
so old units) was *aliasing* - this is especially an issue especially
when you are trying to find a fast glitch amongst otherwise relatively
low frequency signals, but is always an issue when you deliberately use
the widest bandwidth the [digital] scope has to offer.

Aliasing is a well known issue in digital sampling, although Tek (in
particular) has made great strides to fixing the problem.

I've even had that problem on a LeCroy (what a budget we had
there....), although not as pronounced.

I found I was chasing aliasing ghosts on more than one occasion when
using a (in this case high bandwidth) digital scope. After changing the
timebase a few times, and noticing the ghost signals appear / disappear
depending on timebase, I talked to the vendor (Agilent) who made
triggering suggestions to minimise the issue.

Still, the bottom line is that digital scopes have their foibles that
one should be aware of, especially when looking at wideband low level
analog signals.

Cheers

PeteS

 

[John Larkin]

On Tue, 26 Jul 2005 20:33:59 -0700, "Ira Rubinson"
<irarubinson@yahoo.com> wrote:

Does anybody know why analog rather than digital scopes are preferred for
noise / ripple measurements on power supplies?

Thanks -Ira

Digital scopes are nice because of their variable/infinite persistance
and ability to compute true RMS noise. And nice color displays. And
data export.

Older plugin-type analog scopes (545-series+1A7, 7000-series+7A22) can
have huge common-mode rejection, 10 uv/div sensitivity, and selectable
high/low cutoff frequency, which are all very handy here. Tek does
have nice external amp/isolator boxes that add this capacility to a
digital scope, along with full ground isolation.

John

 

[Chris Carlen]

Terry Given wrote:

Ulrich Bangert wrote:
Hi Ira,

the basic idea behind using a analog scope for noise measurements is:
Their
screens supply a three-dimensional information. Beneath x and y there is
beam INTENSITY as the third information. Consider a noisy dc signal.
Having
set a analog scope's beam intensity and vertical amplification to the
correct values you will see the dc component as a horizontal line and the
noise as a 'band' centered around the line. The intensity of the band
will
resemble pretty well the noise's amplitude probability distribution
and for
that reason the band displayed on a analog scope's screen is good measure
for noise.

Note that this effect is due to specific 'after glow' properties of the
phosphor inside the display tube. Due to the after glow the phosphor
performs kind of 'averaging over time' which translates probabilities
into
intensity. In contrast to that the raster screens of MOST digital
scopes a
basically television like and are optimized to have NO after glow because
they are expected to display a lot of independend pictures per second.
You
will never get this 'band' display realized on a NORMAL digital scope.

I said 'MOST' and 'NORMAL', because TEKTRONIX have started to build what
they call DPOs = Digital Phosphor Oscilloscopes. In these scopes
intensity
is again available as a third dimension of information, not by means of
analog after glow but with lots of clever digital electronics. I do
not know
how well such a thing would perform on noise measurements but if you
consider using a digital scope you should call for a DPO and nothing
else.
Because it is done with digital electronics, they can use color in
stead of
intensity and also reverse things: Signals having a high probability
can be
displayed dark and signals having a low probability can be displayed
light.
Pretty well suited to find 'glitches' in a otherwise repeated signal.

Best regards

Ulrich Bangert

"jadaha" <invalid@spamless.com> schrieb im Newsbeitrag
news:42e71755$0$6700$892e7fe2@authen.white.readfreenews.net...

"Ira Rubinson" <irarubinson@yahoo.com> wrote in message
news:4umdnSnomP4hY3vfRVn-3g@adelphia.com...

Does anybody know why analog rather than digital scopes are preferred

for

noise / ripple measurements on power supplies?

Thanks -Ira



either is OK.
There is no preference, it depends upon the bandwidth of the noise,

ripple,

and the bandwidth of your scope(s).
Digital may not sample fast enough to get the noise correctly, but that
means the scopes bandwidth is too low.

I once spent an afternoon in the lab, with a blanket draped over me and
the 200MHz analogue CRO, intensity cranked right up, looking for a
glitch I suspected was there. Eventually spotted on after a couple of
hours, proving the problem lay with a piece of programmable logic. 10
minutes later we had a slow-scale example demonstrating the behaviour up
and running, and a fix about 20 minutes later (turned out to be
metastability).

A Tek rep came by with a DPO a few weeks later, and using some of the
sexy triggering features we managed to actually trigger on the glitch.
But had we not known it was there, we wouldnt have been able to set up
the triggering.

Then how would you have known to look for it on the analog scope?

What is cool about the DPO and it's sophisticated triggering settings is
that you can set it up to look for a glitch that you hypothesize might
be the problem, and then walk away for 2 hours rather than having to
have your eyes glued to the screen.



Good day!


--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarleRemoveThis@BOGUSsandia.gov
NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.

 

[Chris Carlen]

Ulrich Bangert wrote:

Hi Ira,
[edit]
I said 'MOST' and 'NORMAL', because TEKTRONIX have started to build what
they call DPOs = Digital Phosphor Oscilloscopes. In these scopes intensity
is again available as a third dimension of information, not by means of
analog after glow but with lots of clever digital electronics. I do not know
how well such a thing would perform on noise measurements but if you
consider using a digital scope you should call for a DPO and nothing else.
Because it is done with digital electronics, they can use color in stead of
intensity and also reverse things: Signals having a high probability can be
displayed dark and signals having a low probability can be displayed light.
Pretty well suited to find 'glitches' in a otherwise repeated signal.

The new Agilent 6000 series does digital phosphor display even better
than the Tek.


Good day!



--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarleRemoveThis@BOGUSsandia.gov
NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.

 

[Chris Carlen]

John Larkin wrote:

On Tue, 26 Jul 2005 20:33:59 -0700, "Ira Rubinson"
irarubinson@yahoo.com> wrote:

Does anybody know why analog rather than digital scopes are preferred for
noise / ripple measurements on power supplies?

Thanks -Ira



Digital scopes are nice because of their variable/infinite persistance
and ability to compute true RMS noise. And nice color displays. And
data export.

The RMS thing is the big reason why DSOs are neat for noise. But DSOs
are inherently more noisy. So you can barely do sub-mV RMS noise
measurements.

The new Agilent 6000 series has a 1 sample averaging function which
effectively increases the ENOB of the sampling system for slower
timescales (but works to much faster timescales than their older series).

This makes it possible to get down to about 50uV RMS resolution. The DC
offset can be a pain, but the scope can be user-calibrated which brings
it down, or you can measure RMS and average simultaneously, and correct
out the offset.

Older plugin-type analog scopes (545-series+1A7, 7000-series+7A22) can
have huge common-mode rejection, 10 uv/div sensitivity, and selectable
high/low cutoff frequency, which are all very handy here. Tek does
have nice external amp/isolator boxes that add this capacility to a
digital scope, along with full ground isolation.

John

--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarleRemoveThis@BOGUSsandia.gov
NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.

 

[Joerg]

Hello Terry,

A Tek rep came by with a DPO a few weeks later, and using some of the
sexy triggering features we managed to actually trigger on the glitch.
But had we not known it was there, we wouldnt have been able to set up
the triggering.

That is exactly why fast analog scopes have and will for a long time to
come have their place in the lab.

Regards, Joerg

http://www.analogconsultants.com

 

[Joerg]

Hello Chris,

A Tek rep came by with a DPO a few weeks later, and using some of the
sexy triggering features we managed to actually trigger on the glitch.
But had we not known it was there, we wouldnt have been able to set up
the triggering.

Then how would you have known to look for it on the analog scope?

The nice thing about analog scopes is that you don't have to know. All
you need to know is that something is wrong.

What is cool about the DPO and it's sophisticated triggering settings is
that you can set it up to look for a glitch that you hypothesize might
be the problem, and then walk away for 2 hours rather than having to
have your eyes glued to the screen.

These are nice. However, if you don't know what to set the trigger for
because someone else designed the circuit this may not help. You could
end up with nothing in memory or a memory overflow of screen shots that
don't tell you much.

In the same way I often use a communications receiver to diagnose tough
EMI issues. It can find stuff that even a >50k a spectrum analyzer is
unable to resolve. I used it so often that I wore down a pair of
headphones. Now I have to find out where to get new foam pads :-(

Regards, Joerg

http://www.analogconsultants.com

 

[doug dwyer]

In message <4umdnSnomP4hY3vfRVn-3g@adelphia.com>, Ira Rubinson
<irarubinson@yahoo.com> writes

Does anybody know why analog rather than digital scopes are preferred for
noise / ripple measurements on power supplies?

Thanks -Ira


Digtal scopes digitise with 12 to 16 bits which limits dynamic range.

A good analog scope has an effective 22 bit range .
--
dd

 

[Chris Carlen]

Joerg wrote:

Hello Chris,

A Tek rep came by with a DPO a few weeks later, and using some of the
sexy triggering features we managed to actually trigger on the
glitch. But had we not known it was there, we wouldnt have been able
to set up the triggering.

Then how would you have known to look for it on the analog scope?

The nice thing about analog scopes is that you don't have to know. All
you need to know is that something is wrong.

What is cool about the DPO and it's sophisticated triggering settings
is that you can set it up to look for a glitch that you hypothesize
might be the problem, and then walk away for 2 hours rather than
having to have your eyes glued to the screen.

These are nice. However, if you don't know what to set the trigger for
because someone else designed the circuit this may not help. You could
end up with nothing in memory or a memory overflow of screen shots that
don't tell you much.

In the same way I often use a communications receiver to diagnose tough
EMI issues. It can find stuff that even a >50k a spectrum analyzer is
unable to resolve. I used it so often that I wore down a pair of
headphones. Now I have to find out where to get new foam pads :-(

My wife knitted me a pair!



--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarleRemoveThis@BOGUSsandia.gov
NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.

 

[Chris Carlen]

doug dwyer wrote:

In message <4umdnSnomP4hY3vfRVn-3g@adelphia.com>, Ira Rubinson
irarubinson@yahoo.com> writes
Does anybody know why analog rather than digital scopes are preferred for
noise / ripple measurements on power supplies?

Thanks -Ira


Digtal scopes digitise with 12 to 16 bits which limits dynamic range.
A good analog scope has an effective 22 bit range .

?!?!?!?!?


The effective resolution of an analog scope would be the "fatness" of
the trace divided into the height of the screen.

Hardly 2^22.


And DSOs are usually only 8-9 bits, with some using oversampling
techniques at slower timescales to get effectively up to about 12 bits.


Good day!



--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarleRemoveThis@BOGUSsandia.gov
NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.

 

[Joerg]

Hello Chris,

In the same way I often use a communications receiver to diagnose
tough EMI issues. It can find stuff that even a >50k a spectrum
analyzer is unable to resolve. I used it so often that I wore down a
pair of headphones. Now I have to find out where to get new foam pads :-(

My wife knitted me a pair!

Thanks! Now that is a great idea especially since this headphone is from
Europe and there are no parts available here.

Regards, Joerg

http://www.analogconsultants.com

 

[John Popelish]

Chris Carlen wrote:

doug dwyer wrote:

In message <4umdnSnomP4hY3vfRVn-3g@adelphia.com>, Ira Rubinson
irarubinson@yahoo.com> writes

Does anybody know why analog rather than digital scopes are preferred
for
noise / ripple measurements on power supplies?

Thanks -Ira


Digtal scopes digitise with 12 to 16 bits which limits dynamic range.
A good analog scope has an effective 22 bit range .



?!?!?!?!?


The effective resolution of an analog scope would be the "fatness" of
the trace divided into the height of the screen.

Hardly 2^22.

That depends on how well the vertical amplifiers handle overload. If
you can jack the peaks up to 20 or 50 times the visible part, and
still retain good response for the stuff that is low enough amplitude
to remain on the screen, then you are effectively pushing up the
resolution. This can be a big difference between two scopes that
otherwise have similar specs.

 

[Joerg]

Hello Chris,

Digtal scopes digitise with 12 to 16 bits which limits dynamic range.
A good analog scope has an effective 22 bit range .



?!?!?!?!?


The effective resolution of an analog scope would be the "fatness" of
the trace divided into the height of the screen.

Hardly 2^22.

As John said you can overdrive a good scope to the hilt. Then reduce
brightness, close the curtains or blinds, turn off the lights, put on
the glasses and you might eke out another bit or two.

And DSOs are usually only 8-9 bits, with some using oversampling
techniques at slower timescales to get effectively up to about 12 bits.

The DSOs I worked with including the expensive kind didn't seem to have
anywhere near nine effective bits. Oversampling doesn't help if you are
after a tiny runt pulse that happens once in a blue moon or randomly.
For EMI work I hardly ever use a DSO.

Regards, Joerg

http://www.analogconsultants.com

 

[Terry Given]

Joerg wrote:

Hello Chris,

A Tek rep came by with a DPO a few weeks later, and using some of the
sexy triggering features we managed to actually trigger on the
glitch. But had we not known it was there, we wouldnt have been able
to set up the triggering.


Then how would you have known to look for it on the analog scope?

bingo

The nice thing about analog scopes is that you don't have to know. All
you need to know is that something is wrong.

What is cool about the DPO and it's sophisticated triggering settings
is that you can set it up to look for a glitch that you hypothesize
might be the problem, and then walk away for 2 hours rather than
having to have your eyes glued to the screen.

If you know enough about what you are looking for, sure. In our case, we
didnt know what the problem was, only that there was a problem. There
was no way we could have set up the DPO triggering without this
knowledge, although we could have used the DPO persistence to emulate
what I did with the analogue scope.

These are nice. However, if you don't know what to set the trigger for
because someone else designed the circuit this may not help. You could
end up with nothing in memory or a memory overflow of screen shots that
don't tell you much.

In the same way I often use a communications receiver to diagnose tough
EMI issues. It can find stuff that even a >50k a spectrum analyzer is
unable to resolve. I used it so often that I wore down a pair of
headphones. Now I have to find out where to get new foam pads :-(

Regards, Joerg

nice.

Cheers
Terry

 

[Joerg]

Hello Terry,

In the same way I often use a communications receiver to diagnose
tough EMI issues. It can find stuff that even a >50k a spectrum
analyzer is unable to resolve. I used it so often that I wore down a
pair of headphones. Now I have to find out where to get new foam pads :-(

nice.

Chris' wife knitted him new pads and that seems like a great idea. But I
wore something else down and that's a really nasty problem: The bearings
of the encoder that dials in the frequency.

Regards, Joerg

http://www.analogconsultants.com