Digest, Vol 25, Issue 21, Message 4 "HAASOScope"

SVCatITC at aol.com SVCatITC at aol.com
Tue Nov 21 17:34:29 EST 2017

Thanks Very Much Herb for the technical lecture.
Us old farts will agree with you on the analog scope versatility, and I  
also agree
with the functionality of the new "finagled" stuff  out there  now.  The 
latter makes
capturing and documenting the triggered event (for later analysis) easy,  
rather than
the Polaroid coupled cameras to the old Tektronix 548 fire breathing  
monsters' screen
of the day.  This feature is great for documenting screen  shots in reports 
to customers.
I find though that you have to sometimes "help" the new technology by  
the "window" where you might want to look, otherwise trigger thresholds  
lose the event.  You may not know exactly where to find the  "glitch" in 
the "haystack".
>From my experience, analog gives you the "widescreen view" of the  
disturbance and
you can then "zoom" in on the area of interest.
The 20 MHz luggable that I bring into the museum does not have the  
bandwidth to 
check at what frequency the TR20 power supplies are oscillating at (as an  
however as you well noted, you do not necessarily care about the exact  
quantity to 
know you have to kill the "screaming" and make a quiet  supply for analog 
The analog scope sets up easy for a quick GO/NOGO check.
As always, many thanks for the thought provoking insight.
Bill Inderrieden
In a message dated 11/21/2017 12:04:17 P.M. Eastern Standard  Time, 
Herb Johnson writes:

Date:  Mon, 20 Nov 2017 13:45:42 -0500
From: Herb Johnson  <hjohnson at retrotechnology.info>
To: vcf-midatlantic
<vcf-midatlantic at lists.vintagecomputerfederation.org>
Subject:  [vcf-midatlantic] Open Hardware DSO - 100 MHz/250 Msa/sec -
<2320de34-08fc-86ff-33dd-8635edf667cd at retrotechnology.info>
Content-Type:  text/plain; charset=utf-8; format=flowed

I agree with my old friend and  colleague, mostly. But I'd put stronger 
emphasis on detecting "glitches".  I prefer analog scopes, because they 
can show in some fashion glitches  even above their bandwidth. The 
following lecture will be technical. This  matters to me, as this is work 
I've done for decades.

"Bandwidth as  twice sampling rate" is fine for detection of repeated 
events. It's not  fine for reconstructing the signal, or finding one-off 
events. Digital  sampling limits aren't the same as the bandwidth rolloff 
of an analog  oscilloscope amplifier (or probe). Many 20th C. brand-name 
analog  oscilloscopes gave (still give!) good performance well past their  
nameplate bandwidth. also: scope trigger circuits have to respond to  
events far above bandwidth! Triggering is very important.

A  "glitch" is a signal where a logic state changes, not because of a 
change  in logic condition, but because of a timing event such as 
"propagation" or  a "triggering delay". These are very short pulses, on 
the order of several  nanoseconds in TTL circuits. Or, they may manifest 
as nanosecond delays in  signals. Power supply lines carry glitches, 
caused by literal "spikes" in  DC power to logic-switching circuits; 
large currents plus fast changes  produce L di/dt or C dv/dt signals.

These matter, because they can  trigger logic events. If you can't see 
the trigger, you don't know "why"  you have such events. Old TTL designs, 
sometimes generated glitches due to  poor design. Aging components may 
cause glitches, by increasing  capacitance, or sagging DC voltages. Dead 
or absent bypass caps, fail to  filter out power-supply-line glitches.

Look at schematics of the IMSAI  front-panel. They actually CREATE 
"glitches" with a series capacitor to  turn a logic-level change into a 
triggering pulse! Ugly ugly ugly  design.

All that said, any scope beats none. Looking at logic-level  signal 
activity (any activity) is a go / no-go test for  logic chips.  Many 
faults can be diagnosed with a 20Mhz analog oscilloscope and probe.  
(Probe bandwidth matters. Not today's lecture.) But some faults are  
design problems that finally came due, or non-logic component failures,  
or "tired" chips. TTL logic responds to signals and events of several  
nanosecond duration - that's hundreds of megahertz "bandwidth"  
technically. and even a 20Mhz scope, will trigger on much faster events  
- that at least "detects" them. and you can guess if an analog "bump" is  
actually a pulse above your bandwidth.

End of technical  lecture.

In my 20th century view, a $100ish old toolbox-sized HP or Tek  100Mhz 
analog oscilloscope, will show more information about pre 1990's  
computer chips, than most users can appreciate. But a $300 "100Mhz  
sampling" digital oscilloscope you can carry in your pocket, likely  
performs little better than a 30-40Mhz no-brand-I-remember analog  
oscilloscope that you leave behind at a hamfest. Will a 1Ghz modern DSO  
do much better? (shrug) I lack that experience, ask an owner, faster is  
better but.... And a DSO is conveniently tablet-sized, and likely does  
other things too. So the answer for you is: what's in your wallet? Buy  
BOTH if you can afford it, choose otherwise.

Herb "race condition"  Johnson

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