Lumped resonance

From:  Bill the arcstarter [SMTP:arcstarter-at-hotmail-dot-com]
Sent:  Thursday, May 28, 1998 8:36 AM
To:  MALCOLM-at-directorate.wnp.ac.nz; tesla-at-pupman-dot-com
Subject:  Lumped resonance

Malcolm, coilers,
 Malcolm wrote:
>From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
>Hi Bill,

>> I placed the cap and inductor in parallel, and drove the cap via my 
>> 600-ohm output freq generator (with an additional 330 ohm in series 
>> current monitoring purposes).  I monitored the cap voltage using my 
>> 543B oscilliboatanchoroscope.
>> At resonance, this LC network acted as a pure resistance of about 38 
>> ohms (measured input current and voltage etc).  This did NOT 
>> match what theory would indicate - ie - net resistance is L/(RC)

>The problem is that you are mostly measuring the ESR of your signal 
>generator in series with the 330 Ohm resistor. A parallel circuit 
>*must* be driven by an infinitely high impedance (current source).
>Try driving the circuit with 1 MOhm in series with the generator and 
>wee what you get. A current transformer can be used in series with 
>the tank with a low value of burden resistor for monitoring 
>circulating currents (presents a very low ESR in the tank).

>> (See my derivation of that result at:
>> http://www.geocities-dot-com/CapeCanaveral/Hangar/6160/resonan1.html )

Malcolm - I have a hard time seeing this.  Why would the net impedance 
of the LRC circuit be affected by the driver's impedance?  I made no 
assumptions about any driver impedance in the derivation...

I would think that in my particular case, my LRC circuit will act like a 
38 ohm resistance (at the specific resonant frequency) independent of 
the driver's impedance.

Perhaps your comments assumed I was trying to measure the "Q"  of the 
circuit by observing a resonant voltage rise, etc.  That wasn't the plan 
- just wanted to check the impedance at resonance.  I'd appreciate any 

PS -  In his book "Troubleshooting Analog Circuits", (c) 1991, Robert 
Pease published a letter by a Mr. Malcolm Watts of Wellington 
Polytechnic, New Zealand.  I assume that is you? :)  Small world - ain't 
it? :)

-Bill Pollack (arcstarter)