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1 .5
THEORY OF OPERATION.
The 75D Capacitance Bridge consists of three basic units:
a signal source, a bridge, and an amplifier/detector with null indicator.
1 .5.1
Signal Source,,
The signal source is a stable, crystal-control led oscillator with
a 6AU6 vacuum tube (V203) in a modified Colpitts circuit.
This oscillator
operates at a frequency of 1 MHz + 100 Hz.
Two outputs are supplied by the oscillator.
One, of 100 volts peak-to-peak,
is used to provide the reference voltage for the phase detector; the second,
of between 1 and 750 millivolts (variable), furnishes the signal source for the
bridge itself.
1 .5.2
Bridge. The bridge network used in the 75D is a modified Young circuit.
Figure 2 shows the basic configuration of the bridge in simplified form.
The 1 MHz oscillator voltage is injected into the bridge by transformer T501 .
The secondary windings of this transformer are accurately balanced and tightly
coupled, ensuring that the signal voltages appearing in both sides of the bridge
are identical in amplitude and opposite in phase.
The tight coupling between
halves of the secondary, and the low impedance level, also serve to minimize
the effects of external capacitance unbalance.
Any unbalance appearing on
one side (as, for instance, a connecting cable) is transferred equally to the
other side.
Thus the 75D is effectively immune to extraneous capacitance
effects.
Referring to Figure 2, the two LO posts (A and B) are the signal-injection
corners of the bridge, and the HI post (D) and ground (C) are the detection
corners. When the bridge is balanced for capacitance and conductance (C
and G), the impedances appearing between each of the LO posts and the HI
post are equal.
The voltages that are summed at the HI post are equal and
opposite in phase; under this condition, the net voltage developed at the
detector corner is zero and the null indicator will show a balance.
The Capacitance dial, calibrated in pF, controls a variable differential air
capacitor of high quality and maximum stability.
The dial mechanism is
carefully designed to hold backlash to negligible proportions, allowing full
advantage to be taken of the high resolution capabilities of the instrument.
Capacitance range multiplication is accomplished by switching multiplier
capacitors in series with the rotor of the main capacitor.
The multiplier
capacitors are precision air variables, carefully adjusted at the factory.
The Conductance control is an infinite-resolution potentiometer connected
between the Standard and the Test arms of the bridge, with its rotor connected
to the HI post.
The range of the Conductance control is varied by switching
multiplier resistors in series with the rotor arm of the potentiometer.
1 .5.3
Amplifier/Detector. Although Figure 2 indicates just a detector, the 75D is
actually fitted with two detectors. When both capacitance and conductance
75D
b-370
7
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