Bird Technologies 6730B Series - Manual (Page 13)

Chapter 2
Principles of Operation
The Termaline Wattmeter circuit is basically a voltage
capacitive divider with one of the capacitors being a
probe whose distance from the center conductor deter-
mines its capacitive value. This probe spacing is adjusted
at the factory in the calibration procedure to produce the
required value and is then locked in place. The small
voltage developed across the fixed capacitor is rectified by
a diode. A simple RC filter eliminates any RF present
and the rectified dc current is fed to the meter, see the
schematic diagrams on the following pages.
The wattmeter measures three separate power ranges by
selectively switching various values of resistance in series
with the meter. One of the two resistors in each range is
adjustable and is set and sealed at the factory during the
calibration procedure. These resistors are mounted on the
back of the meter itself and the range selector switch is
located on the front panel of the meter housing.
Mechanically, the wattmeter section consists of a 50 ohm
line section mounted on the load. For the Models 6732B,
6734B, 6735-300A, 6736A and 6737A, a socket is used to
hold the voltmeter cartridge that contains the crystal di-
ode. The meter cable is screwed directly on top of the
voltmeter cartridge to complete the assembly. The Models
6734B030, 6736-030A and 6737-030A utilize a line section
with an attached voltmeter block. The meter cable atta-
ches directly on top of the voltmeter block.
The Termaline load consists essentially of a carbon film
resistor on a ceramic substrate immersed in a dielectric
coolant. The resistor, individually selected for its accu-
racy, is enclosed in a special exponentially tapered hous-
ing. This provides a linear reduction in surge impedance
directly proportional to the distance along the resistor.
When surrounded by the dielectric coolant, the character-
istic impedance is therefore; 50 ohms at the front (connec-
tor) end, 25 ohms at the mid-point, to compensate for the
resistance already passed over, and zero ohms at the rear
where the resistor joins the housing, forming the return
conductor of the coaxial circuit. This produces a uniform,
practically reflectionless line termination over the stated
frequencies of the load resistor.
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