SECTION 111
CIRCUIT DESCRIPTION
3-1 GENERAL
The circuit of the Model 206A is shown schematically in a diagram
at the back of this manual and in block-diagram form in Fig. 2.
The instrument includes an oscillator, a selective amplifier, and
a regulated power supply. In the generator output path there is a
rectifier -type voltmeter, a three -section attenuator, an im- and
pedance -matching transformer.
The oscillator is variable, and may be tuned for the production of
any signal voltage in the 20 to 20,000 cps range. The output from
the oscillator passes to the selective amplifier, the tuning section
of which is tracked with that of the oscillator. Similar components
are used in the frequency-determining networks of the two circuits,
and setting the FREQUENCY controls and RANGE switch for a
particular frequency tunes both circuits. The selective -amplifier
circuitry reduces to a fraction of a percent the harmonic voltages
present in the wave generated by the oscillator. The low-distortion
output from the selective amplifier is passed to the output ampli-
fier, the input of which may be adjusted by means of the AMPLI-
TUDE control. The level of the signal voltage from the output
amplifier is indicated by the voltmeter. An attenuator, which may
be set for up to 111 db in attenuation, provides a facility for re-
ducing the output voltage as low as approximately 12 microvolts
across 600 ohms. The signal voltage may be coupled to the output
terminals via an impedance-matching transformer, or passed
directly to a single -ended termination which presents an impedance
of 600 ohms. The transformer is arranged to present an impedance
of 50, 150, or 600 ohms, depending upon the position of the IM-
PEDANCE switch.
The facilities of the Model 206A and design of the circuit provide
a stable low-dis tortion audio-frequency voltage which is adjustable
as to frequency, voltage level, and output impedance. A built-in
regulated power supply furnishes voltages for the vacuum tube
circuits.
3 -2 OSCILLATOR
The Model 206A oscillator circuit includes pentodes V1 and V2
arranged as a two-stage amplifier with both positive negative and
feedback loops. The frequency-selec tive network is in the positive
feedback loop. The network is a resistance -capacity combination |
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