Bird Technologies 8890-400 Series - Manuale (Pagina 15)

Chapter 2
Theory of Operation
The Series 8890-400 equipment consists essentially of a
film-on ceramic resistor immersed in a dielectric coolant.
The resistor, which is individually selected for its accuracy
is enclosed in an exponentially tapered housing, providing a
reduction in surge impedance directly proportional to the
distance along the resistor. When the resistor unit is im-
mersed in the dielectric liquid, the characteristic impedance
will be 50 ohms at the connector end and graduate down
from there to zero ohms at the rear, where the resistor joins
the housing to form the return conductor of the coaxial cir-
cuit. This produces a uniform, practically reflectionless line
termination over the stated frequencies of the load resistor.
Element
Socket
An accurately positioned socket for inserting a radio fre-
quency coupling device, called a Thruline Plug-In Element,
is mounted on the outer conductor. The socket is precision
bored to hold the rotatable element in its calibrated posi-
tion, with a spring-loaded clasp to keep the element firmly
seated. The machined step on the top face of the socket en-
gages a stop-pin on the element. Rotary movement of the
element is thereby restricted to 180° and is stopped on the
axial center line.
The measuring socket has a hole bored through the wall
through which an insulated phosphor bronze contact finger
projects. The plug-in element has terminals on diametrically
opposite sides of its body, so that pick-up can be made from
either side. Refer to figure 2. A small silvered button tip can
contact the element only in the precise forward and re-
flected measuring position (against the end stops.) A spe-
cially designed jack mounted on the side of the socket mates
with the plug on the dc cable furnished with the read-out
meter. The dc jack assembly has a built-in filter capacitor
shunted across the meter circuit. This more fully protects
meter readings against the adverse effects of any stray RF
energy generated in the plug-in element.
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