"Phil Allison" wrote in message
...

"Arfa Daily"

** When servicing audio, knowing the polarity of the test signal and
if there is significant phase shift is important. A scope triggering
off the incoming wave will it not reveal this in single channel mode -
so AF used both channels.

A much better way is to use the External Synch input on your scope and
link it to a fixed output on your bench audio oscillator ( create on
if you have to). This way, you see instantly if the signal's phase
has reversed or has a large phase shift.

Also, triggering will be rock steady with nearly any kind of
distorted, noisy or contaminated signal on the scope screen.

Try it out for a day or two - you will never go back.



Interesting. Are you squaring up the generator's sine output, to give a
good sharp transition for syncing the scope ?

** FFS - that processing is inside any decent scope.

Supply it with 0.5volt rms sine wave and you are away.


I'll have to look at that on my scope, but off the top of my head, I
thought that the trigger point was still adjustable on external and could
be set anywhere on the trigger signal and if so, I'm not sure that I see
how it helps to trigger the scope externally from the same sine wave as
it would use internally on auto trigger.


** But it is NOT the same sine wave !!!!!!!!!!!!

Various audio devices invert, phase shift, phase modulate and even time
shift sine waves !!!


I can see how it would be helpful to have a fixed trigger point,
virtually at the beginning of a cycle,


** Best set the trigger level to a zero crossing of the generator feed so
the same wave on the scope starts at a zero.

I have mine so the first half cycle is positive when there is no phase
shift.

With external synch, the scope is always triggered and the level on the
screen does not matter.

FFS - TRY IT !!!!!!!!!!!



... Phil



Yeah, ok. I tried it today, and I see what you're saying now. Once you've
set the trigger point on the sine sample from the generator that you're
driving the external trigger input from, then that's it fixed for all
subsequent measurements and you should be able to see any phase shifts or
inversions. When I said about squaring up the sample, I was thinking that by
doing this, not only do you phase lock the scope to the original wave, you
also have an always-known point that the triggering is locked to. If you did
the squaring with a decent comparator, that trigger point could be fixed
within a few degrees of the zero crossing for all cases, eliminating the
need to twiddle the trigger level, and pick some arbitrary point anywhere on
the rising or falling slopes of the sine.

Arfa