RE: In search of a good trigger point

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Gary.Lau-at-compaq-dot-com>

Hi Finn:

I think the problem you describe is due to the fact that the waveform you
want to see is not truly periodic, due to jitter in the gap timing.  Each
occurrence (sweep) is slightly different than the previous one.  I don't
think it's possible to get a stable, multi-sweep display capturing both
mains and tank frequency events.  This is the reason for having storage and
even better, digitizing scopes.  You only want to capture one sweep, and who
cares if the next one is slightly different.

But it's not clear why you're concerned with both mains and tank frequency
events.  The time scales are so different that if you can see one, the other
will be lost.  I would think you would just trigger on the mains for RSG
timing, or on the indictor voltage for ringdown.  Just beware with the fiber
voltage probe near the tank, I've found that it picks up the tank field and
results in an invalid reading.

Regards, Gary Lau
Waltham, MA USA

 -----Original Message-----
From: 	Tesla list [mailto:tesla-at-pupman-dot-com] 
Sent:	Thursday, December 14, 2000 6:40 PM
To:	tesla-at-pupman-dot-com
Subject:	In search of a good trigger point

Original poster: "Finn Hammer by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<f-hammer-at-post5.tele.dk>

Scope jockeys!

While I am modifying my new RAT (Realtime Adjustable Teslacoil) to 1500
RPM SRSG for greater overlap and tuning range of the fixed electrodes, I
am haunted by the experience of trying to document the results of
different tuning and coupling issues.

I seem to have a problem selecting a good trigger source. When I set the
electrodes for equal bangsize, I have 2 inputs:
1: the incoming 240 VAC
2: Voltage across the primary capacitor.
The horizontal sweep is set to 2 mS/div, and the trigger is the Channel
that handles the incoming AC. This way I can see the relation btwn. the
AC and the capacitor discharging, which is what I want. I have found
that by adding HF-Reject to the trigget source, and a bit of Trigger
holdoff, I can get a nice and steady display this way.

However, when I want to watch the ringdown itself, things start to get
nasty:
The scope is a Tektronix 485, and it has the ability to add one more
sweep to the display, called the B-sweep, or the B-delayed sweep. This
sweep can run at a faster horizontal rate than the main sweep that it is
based upon, so that a small portion of the main sweep can be scrutinized
with greater detail. If the main sweep is set to 2 mS/div, and the
B-Delayed sweep set to 50µS/div then the Main sweep covers the full AC
cycle with all 6 ringdowns, (at 300 BPS) and the B-delayed sweep then
covers the ringdown nicely proportioned within 2-3 divisions. 

But due to what I expect is slightly differeft breakdown voltages of the
gap from bang to bang, this display that zooms in on the ringdown, jumps
back and forth on the screen, making it difficult to capture on the
digicam, as well as counting ringdown cycles, etc.

One remedy would be to make one more fiberprobe channel, hook it up
across the primary inductor, and run it into the external trigger, to
trigger on the nice sharp rise at each gap firing, but that way the
relation to the 50Hz cycle would be lost. Perhaps Trigger holdoff could
solve that problem, but I hope there is another more elegant solution.

So what do you other scope jockeys use to trigger your scopes?
Would a storage scope make this problem go away 
Remember that I have 3 ringdowns on each 1/2 phase.

Cheers, Finn Hammer