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Neon Protection - More Scope Pictures and More Testing


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From:  terryf-at-verinet-dot-com [SMTP:terryf-at-verinet-dot-com]
Sent:  Thursday, March 05, 1998 2:13 PM
To:  Tesla List
Subject:  Re: Neon Protection - More Scope Pictures and More Testing


>From:  Antonio C. M. de Queiroz [SMTP:acmq-at-compuland-dot-com.br]

>Subject:  Re: Neon Protection - More Scope Pictures and More Testing
>
>Terry wrote:
>
>>         I have been doing more testing with a RLC transformer protection
>> network useing the VI probe to record scope pictures.  RLC networks seem to
>> have problems!  Perhaps I used poor values or did something wrong?  Will
>> have to check results with spice, when I have time, to be sure the actual
>> measured results are correct! :-))
>
>Your voltage probe appears to not have a compensation for the poor frequency
>response that results if you connect a high resistance resistor to a load
>that has some capacitance, even parasitic (with that construction, possibly
>you can add a small capacitance in parallel with the resistors by placing
>an adjustable metal plate in the other side of the circuit board). 
>You don't say also what are those operational amplifiers, that may be not 
>fast enough for accurate measurements. 
>
>Antonio Carlos M. de Queiroz
>http://www.coe.ufrj.br/~acmq
>

Hi Antonio,

        The original voltage probe was modified to give much better
high-frequency response in this test (the photo may have made it look
simpler than it is).  I have since ordered parts to create a much better
probe.  However, I believe this probe was giving accurate results.
Oscilloscope probes use RC matching to compensate for the input impedance of
the oscilloscope input circuits.  The transducers I use are substantially
different in that they are constant-voltage current sensors (in this case).
I will investigate this further to be sure there is no problem.  The design
of these high voltage and high resistance probes is not simple and must be
done very carefully indeed.  In my case, parasitics and radiated noise are
the main concerns that need to be addressed.  I am much better able to
verify their function now that everything is operational and known circuits
can be measured with them.
        I use LM837 op-amps, their GBP is 25MHz and I use them near unity
gain at ~500mV levels.  The circuits have been tested for frequency response
and they could easily run at 10MHz if needed.  I add some frequency limiting
to insure stability and noise immunity but I have tested the
transducer-fiber-receiver-scope path to insure adequate frequency response,
delay, and phase characteristics.  The IR LEDs and photo diodes are, by far,
the limiting components.  The probe uses shielded and bypassed surface-mount
components with minimal spacing.

        Terry