Tektronix Type 130, Instruction Manual

Page: 13 / 53

AA
3-1
Variable Oscillator
V4 is the variable-oscillator tube, with T1, C2, C3, C4, C5
and the unknown in the tuned circuit. Feedback from the
plate of the pentode section of V4 is coupled to its grid
through the triode section, which is connected as a cathode
follower. The output signal of the pentode is such as to
drive the cathode follower below cutoff except during posi-
tive peaks, so that the cathode current consists of pulses.
The pulses are fed back to a winding on the oscillator
transformer, T1, through C10.
Screen current for the pentode is the cathode current for
the cathode follower, filtered by means of R10, C11. This
arrangement stabilizes the operating point of the
pentode plate, which in turn determines the average cur-
rent of the cathode follower. For example, if the average
plate voltage becomes too high it will raise the cathode-
follower grid and cathode voltage, which will raise the
pentode screen. The increased pentode plate current re-
turns the pentode plate back down toward its original level.
The phase of the pentode plate voltage can be adjusted
by means of C7 so that the feed-back signal will be in
phase with the tuned-circuit voltage. When this adjustment
is properly made, reduction in Q of the tuned circuit, caused
by resistance components in the unknown, will not appreci-
ably affect the oscillator frequency for effective shunt re-
sistance of 100 kilohms or higher. (10 ohms series resistance
for inductance.)
When the selector switch is set for inductance measure-
ment and no coil is connected, the grid of the oscillator is
held toward positive by R6 and R14, which are connected
to the plate supply, so that grid current flows. The result -
ing low grid-input impedance reduces the voltages coupled
in to the grid through stray capacitances on the grid lead,
and so reduces any tendency for the circuit to oscillate
spuriously. Oscillator input to the mixer, V60, is coupled
through buffer V15A.
The selector switch, sections A and B, arranges the oscil-
lator tuned circuit so that the UNKNOWN terminal either
shunts the tuned circuit for capacitance measurements, or
is in series with the tuned circuit inductance for inductance
measurements.
Guard-Voltage Circuit
V110 is a cathode follower whose gain is slightly less
than one. The voltage at its grid is increased over the
voltage at the UNKNOWN terminal by a small additive
winding on T1. The additional voltage is just enough to
make up for the slight voltage loss in the cathode follower,
so that the GUARD VOLTAGE output voltage is equal to
the UNKNOWN terminal voltage. Voltage divider R122, R113,
sets the dc grid voltage at about +50 volts so that about 5
ma of cathode current flows. The output impedance with
this amount of cathode current is about 250 ohms.
Fixed Oscillator
V30 is the fixed oscillator that can be adjusted to 140 kc
by means of the powered-iron tuning slug in T30. The cir-
cuit is similar to the variable oscillator circuit, but without
the feedback phase adjustment. V45A is the buffer ampli-
fier.
Buffer Amplifiers
V15A and V45A are the buffer amplifiers that reduce the
coupling between the two oscillators. When two oscillators
couple to each other, they tend to pull together to a com-
mon frequency when their natural frequencies are nearly the
same, and actually lock together at the same frequency with
enough coupling. The two buffer amplifiers reduce the
coupling so that there is no lock-in above about one cycle
per second, and the pull-in produces no error above about
three cycles per second. Output from the buffers has ap-
proximately sawtooth waveform because of the high-resis-
tance plate loads.
Mixer
V60 is the mixer. The purpose of the mixer is to produce
an output at the frequency difference between the two
oscillators. Higher frequency output components of the
mixer are reduced by a low-pass rc filter with C61, C62,
c^3, and R61, R62. The 124- to 140-kc carrier components
are additionally reduced by a bridge-T rejection filter
with R64, C64, and C65. The output dc level of the filter is
adjustable by means of R68, labelled ADJ. 1.
Multivibrator
V70 is a multivibrator that generates a square pulse for
each cycle of the difference frequency. The square wave