Model 201C Section IV
SECTION IV
THEORY OF OPERATION
4-1. GENERAL.
BO
04
4-2. The Model 201C consists of an oscillator section, an
amplifier section, an output attenuator and a power supply
as shown in the block diagam, Figure 4.1.
4-3. OSCILLATOR SECTION.
44. The oscillator section consists of V1 and V2 as a
resistance coupled amplifier containing two feedback loops.
The positive feedback loop sets up oscillation while the
negative feedback loop reduces distortion and maintains a
constant amplitude of oscillation. The positive feedback
network contains fixed resistances (established by the FREQUENCY
-
RANGE switch) and a variable capacitance. A simplified
-
schematic diagam is shown in Figure 4-2. The network is
Figure 4-3. Oscillator Network Characteristics
designedsothat Rl,ClAandB=R8,ClCandD.
I I T I I 4-6. The cathode by-pass capacitors in the oscillator
section C5, C7 correct phase shift at higher frequencies.
4-7. The negative feedback network minimizes change of
output amplitude with change in frequency. The
incandescent lamp, used as a cathode bias resistor for V1, is
part of the negative feedback voltage divider. It has a
temperature resistance characteristic such that its resistance
increases in direct proportion to the voltage applied to it.
Thus, changes in its resistance will change the amount of
negative feedback in the oscillator output. The thermal
inertia of the lamp is great enough to be unaffected by sine
Figure 4-2. Simplified Schematic Diagram of
wave voltages at the lowest frequencies involved.
Oscillator Section
4-5. The oscillator output is coupled to the input stage
4-8. AMPLIFIER SECTION.
through C8, and the input voltage is derived from this
signal. Oscillation will occur when there is zero phase shift
between the voltage applied to the network and the voltage 4-9. The amplifier section of the instrument consists of a
applied to the grid of V1. The zero phase shift point is also voltage amplifier V3A direct coupled to a phase inverter
the point of minimum loss through the network as shown
V3B, and a push-pull output stage V4 and VS. The output
in Figure 4-3. The frequency of oscillation (relative
transformer contains a tertiary winding for overall negative
frequency in Figure 4-3) is given by the expression:
feedback around the amplifier. As a result of negative
1
feedback in excess of 30 dB, very little distortion is
Fr = 2nd (R1 . ClA,B) (R8. ClC,D) introduced by the amplifier section of the instrument.
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