Re: Variation of secondary Q

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Paul,

At 04:42 PM 3/17/2002 +0000, you wrote:
>Terry wrote:
>> Ok, If the equation "Vin x Q = Vtop" is adequate, I can do a
>> lot of things ;-) 
>
>But you lose the advantages of the method proposed, in which
>F and Q are computed from a captured ringdown waveform. 
>The advantages are:
>
>a) Insensitive to scope gain variation, and variation of field
>pickup:  Doesn't matter how much the Y gain on the scope varies
>from shot to shot,  and you should be able to move the field pickup
>around without affecting the F and Q readings. 

True, but I think I can stabilize the read back signal to about 0.01% with
a different scheme.  Reading a few hundred top volts at 80kHz is easy ;-)

>b) Doesn't require you to measure anything on the coil, you can keep
>well away, so that the experiment will be more stable.

The coil is always monitored remotely.  Too cold outside ;-)

>c) Doesn't require you to manually tune a sig gen to Fres, so you
>don't need to be in attendance at each reading.

Do you think Fo will vary?  The Fo on my first test at this seems rock
solid.  However if Fo is expected to change too.  I would have to think more...

>d) Very accurate - the only calibration required is the scope
>timebase, and this could be done by inserting a marker signal that
>the software would pick up and align to.  

Digital scopes have crystal controlled time bases.  200PPM Max error.

>Use a search coil eg
>ferrite rod for the field pickup and tune it to some convenient
>24hr long wave transmission so that a little of it appears in the
>scope trace along with the TC signal.
>
>If you don't want to tie up your scope, maybe one of those cheap
>PC-based scopes would do.  You don't need stability, accuracy,
>sensitivity, or high bandwidth, so a cheap bit of kit would do fine.
>This also solves the problem of getting the data into the PC to be
>filed in an un-attended way.

I could probably buy the RS-232 interface for the 3012 scope for the same
price.

>
>And you don't need to use the HP 33120a, just fire the coil base
>from a 555.  Stability of the pulse amplitude is not required.
>
>[current profile measurements]
>
>> Is it ok if the secondary is slightly cone shaped for that test?
>
>That's fine, the modeling software can do pretty much any profile
>now.  Go for about h/d=1.0 with the winding - smaller h/d's can
>be tested simply by leaving out the shorting links higher up. For
>modeling purposes a thin-walled former, eg a light duty poly bucket,
>would be best, because I still haven't got those dielectrics sorted.
>If you wind an unbroken coil and then cut into it for the taps,
>measure the first three resonances before and after cutting in the
>taps.

Ok Cool!  I got a bucket so I'll get started on that


=======New Stuff=====

I tried the square wave input tonight and picked up the ring with a field
probe very close to the coil.

This is the raw signal:

http://hot-streamer-dot-com/temp/Qvar/Tek00000.gif

It is picking up electrical noise and you can see the plane antenna's
frequency response effects (~50Hz cutoff).  So the signal has a varying DC
level component as well as a lot of odd noise on it.  Here it is up close:

http://hot-streamer-dot-com/temp/Qvar/Tek00001.gif

The singal level is still very low so noise is a problem.  I can filter
that by averaging it 128 times to get these:

http://hot-streamer-dot-com/temp/Qvar/Tek00004.gif

http://hot-streamer-dot-com/temp/Qvar/Tek00001.gif

I pulled the data for the last on and put it here:

http://hot-streamer-dot-com/temp/Qvar/TEK00000.CSV

Here is a LabCam pic of the setup.

http://hot-streamer-dot-com/temp/Qvar/Image1.jpg

Note: Terry's new LabCam I set up just today.  Maybe catch Terry toiling
over his experiments :o))

http://hot-streamer-dot-com:8080/


All things considered, I think running a sine wave into the coil and taking
the signal from a calibrated antenna and using Vtop = Q x Vin is an easier
way to go.  A computer can easily data log the Q directly then with time
stamps.  A few op-amps could do the signal conditioning.  something like this:

http://hot-streamer-dot-com/temp/Qvar/NewSystem.gif

Cheers,

	Terry

>--
>Paul Nicholson
>--
>