In a message dated 2/15/00 4:52:43 AM Pacific Standard Time, [EMAIL PROTECTED]
writes:
<< Can anybody explain what a "choke" is? >>
-----------
A choke is a Ping-Pong size toy our infants often try to swallow and get
stuck (choke) half way pass its mouth. :-)
A choke is a metal-alloy part of a protection system on the rock face to
prevent a 100% fall in rock climbing sport.:-)
A choke is a move or flex part of in a check vale system.:-)
OK I am serious ....
An RF choke is a low loss inductor with inductance chosen at a value no so
sensitive to frequency change. It has impedance about 20X larger than the
network impedance it serves. Its main purpose is to by pass low frequency
signal from DC to sub-MHz and reject higher MHz frequencies. If it can reject
RC band noise, it WILL reject RC band signal equally well.
An RF dissipative filter most commonly used has ferrite core (often
misunderstood as a low loss RF choke) is a RF choke look alike. A dissipative
filter is not intended to reflect (bounce) unwanted signal back to its source
(...so that it will return bouncing off at the source to the filter again
like most reflective RF filter do. An open wire section associated with
filter like that is given a name, called an antenna) The dissipative filter
may be view as a HIGH LOSS inductor defined by its ac resistance
characteristic relative to its (residual) inductance. The lower cut off
frequency of such filter is defined as when ac resistance has drop to a value
equal to its inductive reactance. or R / XL = 1.0. Generally its operating
frequency range has ac resistance ALWAYS multiple times (> 20X) larger than
its inductance. The impedance of this filter will peak as operating
frequency increase and will roll off (down) beyond a peak point. The upper
frequency limit is defined at a point passing its peak and when the current
value is half that peak value.
We think of a problem, we think of a cause, we try fit solutions, we find the
right fit, we solve the problem, we think we really understand the problem
... not. This happen as often as lightening strike on earth every day. But
who cares. :-)
YK Chan
Seattle area.
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