Re: High frequency impedance of a neon sign transformer

Jim Fosse wrote:

	How are you determining the impedance? I'm beginning to
question my method of using a pot in series with the inductor. i.e. I
is 90 degrees out of phase with V. therefor the V across the resistor
is out of phase with the applied voltage.
Hi Jim,

[excuse me being pedantic, it's just that I want others to see
what I'm doing if they're interested. I'm well aware that
you're a highly trained engineer.]

Actually I have used two methods with consistent results:

Both methods use a signal generator connected in series with both 
the neon secondary and a current monitoring resistor.
The total impedance is Z=Vs/Is where Vs and Is are the neon
secondary voltage and current, respectively. The current Is is 
obtained as Is=Vr/R where Vr is the voltage obtained across
the current monitor resistance, R.

(1) Use a HP1200B Differential input scope to simultaneously
measure the voltage across the neon secondary and the series-
connected current monitor resistor. This measurement allows
one to estimate the phase angle. 

Note: If you try this with a 
grounded input scope and grounded ouput signal generator, 
something will get shorted out!

(2) Use a HP3478A Multimeter to independently measure the
two voltages without regard to phase angle. This method
is sufficent if one just wants the total impedance. And,
since the two voltage measurements are done with the same
meter, any frequency response problems are canceled when
the impedance is obtained from dividing the 2 voltages.

Hope this helps,
-Ed Harris