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
with the functionality of the new "finagled" stuff out there now. The
capturing and documenting the triggered event (for later analysis) easy,
the Polaroid coupled cameras to the old Tektronix 548 fire breathing
of the day. This feature is great for documenting screen shots in reports
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
>From my experience, analog gives you the "widescreen view" of the
you can then "zoom" in on the area of interest.
The 20 MHz luggable that I bring into the museum does not have the
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
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.
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>
<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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