Re: Analog scopes for noise measurements

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@xxxxxxxxxxxx> schrieb im Newsbeitrag
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"Ira Rubinson" <irarubinson@xxxxxxxxx> wrote in message
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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
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