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Re: A Question About A Simple LRC circuit



At 16:24 3/5/01 -0500, Robert Zannelli wrote:
In a message dated 3/5/01 3:35:40 PM Eastern Standard Time,
haar@PHYSICS.ARIZONA.EDU writes:

<< The underdamped solution is a sine in an
exponentially decaying envelope. The mechanical
version is a damped spring and mass system.
>>
Thank you Roger for this explanation. I should have seen right off that the
capacitor and inductor would form a "flywheel " type circuit whose damping
would be a function of the circuit resistance. This is of course the basis
for LC resonant frequency oscillators which prevent damping by providing
energy to the circuit.

Bob Zannelli

Long ago, when plan position indicators were the latest thing in radar
display technology, it was desired to provide range rings - as an example,
5 mile radius rings were used on a 25 mile radius display.
One method was to hold a parallel tuned circuit quiescent with a
conducting "cathode follower" i.e the tube equivalent of an emitter
follower, during the display flyback and latency period until the next
transmitter pulse, which in some cases measured one microsecond at
1000 pulses per second.
This arrangement has the characteristic of a low impedance conductive
state, and high impedance off state between cathode or emitter and
ground rail.
When the magnetron fired, the CRT spot began deflecting from the center
outwards. The follower meanwhile, was turned smartly to the off state,
and the LC resonated on a long exponential decay - while the peaks
were used to bright up the spot.
What was the resonant frequency used to depict 5, 10, 15, 20 sm radius
range rings?
The periodic time of that circuit was about 54 microseconds per
5 mile range bright-up. The spot was traveling at a scaled 0.5c over the
ground depiction. I'm sure you can see why that would be.

'Q' is an interesting thing. Some long cantilevered rods will vibrate
for a minute or two when plucked, where others go still in a few cycles.
Certainly air provides some damping - but there is a mechanical equivalent
for high inductance and low capacitance and low resistance
(which is one recipe for high-Q). Could it be high mass and low
spring stiffness?




brian whatcott <inet@intellisys.net> Altus OK
Eureka!