30MHz Spark Gap Testing - Is this real??

From:  terryf-at-verinet-dot-com [SMTP:terryf-at-verinet-dot-com]
Sent:  Wednesday, April 08, 1998 8:18 AM
To:  Tesla List
Subject:  Re: 30MHz Spark Gap Testing - Is this real??

Hi Jim,

>From:  Jim Fosse [SMTP:jim.fosse-at-bjt-dot-net]
>Sent:  Tuesday, April 07, 1998 2:07 AM
>To:  Tesla List
>Subject:  Re: 30MHz Spark Gap Testing - Is this real??
>Hi Terry,

Many snips.....

>First, you are hitting any LC circuit (distributed elements or not)
>with what amounts to a Dirac delta function. If it can ring, it will.
>I've seen "RFI" (using a 3' wire antennae) into the 100s of MHz when
>just running my rotary spark gap from my PIG with 6' of connecting
>wire. I did not have the primary or secondary of my TC connected in
>this test. I was just testing out my rotary gap.

I have good square wave resopnse at 40MHz so it should all be stable.  I
worry about the frequency responce of the shunt.  Perhaps usinging the light
from the gap would solve this worry.  It is supposed to be proportional to
current.  My fragil fiber cable is too expensive to try this with right now
but others are working on this.

>Secondly, the power levels are high enough that unintentional pickup
>becomes a real problem. 
>Put some opaque tape over your fiber optic cable and then run your
>system. Do you still see the signal? If so, it's from a parasitic
>antennae. Don't forget that black plastic IS conductive. I've measured
>1 megohm per foot on black 1/4" vacuum hose.

        If I unpluge the fiber cable and disconnect the antenna the signals
dissappear.  If I turn up the scope gain to max I can see noise there but
the S/N ration is very good.  The Tek 210 is designed for all the world's
EMI standards and all that.  Everything is well shielded internally despite
the funny plastic case.  I use die-cast enclosures and such for the
electornics and good independant graounds.  Everything is battery operated
and such.  The noise is very low on the scope.

>I've included 3 private posts to Malcolm Watts regarding my
>experiments with a photodiode looking at my static spark gap. I had
>some very read problems with unintentional pickup.


I have also include a letter I wrote this morning to Malcolm on this
subject.  You should use the very high speed buffered receivers to see the
effects.  The other fiber optics and bare PIN diodes cannot be buffered at
the frequencies needed by adding op-amps and such.  I am quickly relearning
all those RF tricks again too...

At 07:22 PM 4/8/98 +1200, you wrote:
>Hi Terry,
>            I have located a piece of cheap plastic optical fibre. It 
>has been my intention for a while to use light from the gap itself 
>for various purposes, for instance, for controlling a DC switchmode 
>charger to shut it off so that charging currents can have no effect 
>on gap operation. It would also reduce losses and improve the power 
>factor slightly. But now it seems that a high frequency examination of 
>the gap would be interesting in itself. Compare optical output with 
>the various current and voltage waveforms for example.
>    Do you have up to date info on probe construction on your site? I 
>would like to follow in your foosteps and conduct some investigations 

Hi Malcolm,

Nothing has been updated.  However it is much simpler now.  Try and find a
fiber optic receiver module that has a built in buffer amp.  Just hook it up
as the data sheet suggests (use good RF wiring, all the bypass stuff they
recommend and put it in a well shielded box) and feed it to a scope input.
The output of mine really are very low-Z even at high frequency.  Be sure to
get a fast 125MHz or better one.  Avoid the ones that are designed for pure
digital signals.  They don't have a good linear outputs (Schmidt triggers).
The good ones have a rise time of about 2nS or even less.  The cheap ones
clock in at 1000nS.  I used an AMP 269111-1 module from the new digi-key
catalog but there are all kinds (~$20 each :-().  Don't get straight PIN
diodes because you won't be able to buffer them at 500MHz bandwidth.
        I tried to look at the spark once with the old probe fiber and had
trouble getting it positioned so that the gap would not be too bright or too
dim.  However that was just a matter of getting it set.  It should work fine.
        It is also easy to get a small two-way radio antenna (designed to
really be 50 ohms at some frequency) from anywhere (CB radio, cell phone,
marine, scanner....) and terminate it into a 50 ohm surface mount resistor
or real 50 ohm termination to see the spikes well.  One bad thing is when I
try to see this on my cheap analog scope the triggering is hard to set and
it jumps around.  A digital storage scope is definitely a big advantage if
you have one.
        My present fiber cable is expensive and I don't want to try this and
risk damaging its fragil ceramic ends.  The cheap plastic fiber will work
fine.  You may have to be creative hooking it the the receiver which is
designed for a fancy connector and fiber.  Should be easy.
        Someone said that the light from the gap is proportional to the
current.  I am not sure I beleive that but it would solve the current shunt
bandwidth problem.  If you have the antenna and long fiber cable in each
channel of the scope, you can also measure the speed of light easily.

Let me know if you have any questions on the details.