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SECTION 1 1 1
CIRCUIT DESCRIPTION
3-1
GENERAL
The c i r c u i t of the Model 206A i s shown schematically in a diagram
a t the back of this manual and in block-diagram f o r m in Fig. 2.
The instrument includes an oscillator, a selective amplifier, and
a regulated power supply. In the generator output path there i s a
rectifier -type voltmeter, a three -section attenuator, and an i m -
pedance -matching transformer.
The oscillator i s variable, and may be tuned for the production of
any signal voltage in the 20 to 20,000 c p s range. The output f r o m
the oscillator p a s s e s to the selective amplifier, the tuning section
of which i s tracked with that of the oscillator. Similar components
a r e used i n 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
c i r c u i t r y reduces to a fraction of a percent the harmonic voltages
present in the wave generated by the oscillator. The low-distortion
output f r o m the selective amplifier i s passed to the output ampli-
f i e r , the input of which may be adjusted by means of the AMPLI-
TUDE control. The level of the signal voltage f r o m the output
amplifier i s indicated by the voltmeter. An attenuator, which may
be s e t for up to 111 db in attenuation, provides a facility for r e -
ducing the output voltage a s low a s approximately 12 microvolts
a c r o s s 600 ohms. The signal voltage may be coupled to the output
terminals via an impedance-matching t r a n s f o r m e r , o r passed
directly to a single -ended termination which presents an impedance
of 600 ohms. The t r a n s f o r m e r i s arranged to present a n impedance
of 50, 150, o r 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 i s adjustable
a s 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
a r r a n g e d a s a two-stage amplifier with both positive and negative
feedback loops. The frequency-selec tive network i s in the positive
feedback loop. The network i s a resistance -capacity combination
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