HEWLETT PACKARD (HP) 201C
Service Manual

Model 201C

Section IV

SECTION IV THEORY O F OPERATION
4-1. GENERAL.
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.
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4-3. OSCILLATOR SECTION.
4 4 . 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 RANGE switch) and a variable capacitance. A simplified schematic diagam is shown in Figure 4-2. The network is designedsothat R l , C l A a n d B = R 8 , C l C a n d D .

FREQUENCY

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Figure 4-3. Oscillator Network Characteristics

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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 t o 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 wave voltages at the lowest frequencies involved.

Figure 4-2. Simplified Schematic Diagram of Oscillator Section 4-5. The oscillator output is coupled to the input stage 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 applied to the grid of V1. The zero phase shift point is also the point of minimum loss through the network as shown in Figure 4-3. The frequency of oscillation (relative frequency in Figure 4-3) is given by the expression:

4-8.AMPLIFIER SECTION.
4-9. The amplifier section of the instrument consists of a

voltage amplifier V3A direct coupled to a phase inverter V3B, and a push-pull output stage V4 and VS. The output transformer contains a tertiary winding for overall negative feedback around the amplifier. A a result of negative s feedback in excess of 30 dB, very little distortion is introduced by the amplifier section of the instrument.

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Fr = 2 n d (R1 . ClA,B) (R8.ClC,D)

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