Re: In search of a good trigger point

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

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



Tesla list wrote:
> 
> 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.

Yes, some sort of jitter must be at play. This could be the gap breaking
down at slightly different voltages, or the line voltage (which I
trigger on) fluctuating a bit.
I have never seen a digitizing scope in operation, but your description
shows that it is what is wanted for easy documentation: just one of many
sweeps is recorded.
> 
> 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.

The reason that I would like to remain triggering on 50Hz is this: Since
I have 3 HF_events_ per 1/2 phase, and these are bound to happen at
slightly different initial potential (although the purpose of this
exersize is to make them equal), If I trigger on the onset of the
ringdown, then I`l get those 3 ringdowns slapped on top of each other on
the scope. Since they don`t start out from the same polarity, it will 
be hard to count transfer cycles and such. (on second thought I`l get
all 6 ringdowns on top of each other).
I would like to examine only one of these ringdowns, as it happens on
one individual gap. There are some strange things happening, like the
gaps don`t quench in the same way. Out of 3 ringdowns, one leads to the
capacitor starting to recharge at 1 kV, whereas the other 2 start from
0kV.
Running at 100 BPS is much simpler scopewise.
Of course this is a problem arising from the fact that I have litle
experience with scopes, and I`l just have to get more of it.

Cheers, Finn Hammer 
> 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