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Model209A
Section IV
SECTION IV
THEORY OF OPERATION
4-1. INTRODUCTION.
4-2. This section contains a description of the basic
principles of circuit operation for the Model 209A.
The information is presented as a discussion of each
block indicated on the Block Diagram, Figure 4-1,
and detailed circuit descriptions which refer to Figure
7-1 and 7-2.
4-3. The Model 209A is basically a Wien bridge
oscillator. The output from the oscillator circuit is
applied to a buffer amplifier and to a sine wave to
square wave converter. These two circuits provide
independent sine wave and square wave outputs,
respectively.
4-4. BLOCK DIAGRAM DESCRIPTION.
furnished
through
the
frequency
determining
network of CIA, R8, CIB, and R16. At the
frequency that the phase of the positive feedback is
0
0
,
Xc
=
R and the maximum ratio of output
voltage is supplied to the amplifier (see Figure 4-2).
The characteristics of the Wien bridge are such that
the output voltage to the
+
input of the amplifier at
F
0
is one third the amplitude of the positive feedback
voltage. Therefore, to maintain unity gain and
oscillation, the negative feedback network (R28, R24
and AGC) was designed with a divider ratio of two to
one, to give the amplifier a gain of three.
4-8. The amplifier itself is a solid-state, high gain
amplifier with the output in phase with the input so
that feedback will produce oscillations.
4-9. PEAK COMPARATOR AND AGC.
-RATIO
FREQUENCY
4-5. BRIDGE AND AMPLIFIER.
4-10. The voltage output from the Wien bridge to the
input of the amplifier is not always one third of the
positive feedback voltage at all operating frequencies,
nor is the amplifier gain constant for all operating
frequencies. One technique used for maintaining
unity gain in the oscillator circuit at all operating
frequencies is to have a dynamic resistance, variable
with changes in gain, in the negative feedback
network. In the Model 209A this is accomplished
with the combination of the peak comparator and
AGC circuits.
4-12. When the oscillator is first turned on, the AGC
gives the amplifier a gain of much greater than three.
Noise in the amplifier is amplified greatly, and the
frequency selective network in the Wien bridge selects
the noise at the tuned frequency. The selected noise
becomes positive feedback to the amplifier, and the
amplifier starts oscillating at the tuned frequency. As
the output amplitude approaches 7.2 volts peak, the
4-11. The peak comparator compares the negative
peak of the oscillator amplifier output to a 7.2 volt
reference. If the output varies above or below the
reference voltage, a difference voltage will be supplied
to the AGC circuit. The "dynamic resistance" of the
AGe circuit is a field-effect transistor with the gate
controlled by the difference signal from the peak
comparator. The oscillator amplifier output is held to
7.2 volts peak amplitude.
LEAD
LAG
-
-
/
-
-
/
/
.".......-
-.........
/
/
-
/
/
//~
/
/
- -
""
-
-
~//
/
~
I
-
- - - - - PHASE
E pf
s
POSITIVE FEEDBACK VOLTAGE TO AMPLIFIER
Eo
" OSCILLATOR CIRCUIT OUTPUT VOLTAGE
F0
"
FREQUENCY WHERE
Xc
11
R
0.4
o.
Figure 4-2. RC Frequency Network Characteristics
4-6. An overall loop gain of at least unity is a
requirement for any amplifier to oscillate. The Model
209A satisfies this requirement with a combination of
positive and negative feedback through the bridge.
4-7. The oscillator bridge is divided into two
networks, the frequency selective network and the
negative feedback network. Positive feedback is
4-1
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