20 Section
Paragraphs 4-1 to 4-10
SECTION IV
PRINCIPLES OF OPERATION
4-1. INTRODUCTION. 4-6. The attenuator is a bridged-T attenuator which
provides continuous control of the oscillator output
voltage while maintaining constant output impedance.
4-2. The Model 204B consists of an RC bridge oscil-
lator circuit, a peak detector circuit, and an output
4-7. OSCILLATOR CIRCUIT.
attenuator (bridged-T type). These circuits and the
front panel controls associated with them are shown
4-8. The RC bridge in the oscillator circuit consists
in the block diagram, figure 4-1.
of an RC frequency-selective networkand a resistive
voltage divider network. The frequency-selective
4-3. The RC bridge oscillator consists of an RC network is similar to one leg of a Wien bridge; the
bridge, a two-stage amplifier, and two emitter follow - resistive voltage divider, the other leg.
ers. The output of this circuit is a sine-wave signal
4-9. As in any oscillator, an in-phase feedback volt-
which is 1) returned to the RC bridge as feedback,
age (from the oscillator circuit output) is necessary
and 2) applied to the output attenuator.
to maintain oscillations. The proper phase relation-
ship at the desired frequency is maintained by the RC
4-4. The RC bridge consists of an RC frequency-
components in the bridge.
selective networkand a resistive voltage divider net -
work. The RC frequency-selective network supplies 4-10. The frequency-selective network consists of a
positive feedback to the amplifier and determines the series branch, C1 and R1, and a parallel branch, C2
frequency of oscillation. The resistive voltage divid- and R2, as shown in figure4-2. For the frequency at
er network supplies negative feedback to the amplifier. which X = R in the series andparallel branches, the
The output of the amplifier is proportional to the positive feedback voltage to the amplifier is maximum
difference between the feedback signals. and is in phase with the oscillator circuit output vol-
tage (figure 4-3). Only that frequency at which X =
4-5. The peak detector detects changes in the RC Rwill be amplified; at frequencies where Xc does got
bridge oscillator output voltage and changes the divi- equal R, the positive feedback voltage is not of the
sion ratio of the resistive voltage divider network, right phase and is insufficient in amplitude to sustain
thereby changing the amount of negative feedback. oscillation. Figure 4-3 shows the positive feedback
The peak detector with the divider networkmaintains curve and phase relationship for frequencies above
the RC oscillator output at a constant level. and below the frequency where X = R.
C
\
\
-
FEEDBACK AMPLIFIERS EMITTER ATTENUATOR
RC
, -- I
BRIDGE FOLLOWERS I
(RIOA,B,C2,C71 QI , Q2 03, 04 R40A,Bh
I
I
I
I
FEEDBACK
Figure 4-1. Block Diagram
01369-2 4-1 |