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THEORY OF
4-1. GENERAL DESCRIPTION.
4-2. The Model 654A Test Oscillator (see Block Diagram,
Figure 7-1) contains a Wien Bridge Frequency Adjustable
Oscillator (10Hz to 10 MHz) followed by a Buffer
Amplifier and a Balanced Amplifier with a single ended
input and balanced output. The output of the Balanced
Amplifier is a leveled, sinusoidal signal; this signal passes
through Balanced Attenuators and a Balance and Unbalance
Impedance Switch (output impedance switching network)
to the front panel output connectors.
4-3
.
An
Average Responding Detector monitors the output
from the Balanced Amplifier to provide two dc currents
(proportional to the signal level); one current flows to the
meter circuits and the other to the Amplitude Control
Integrator. Au tomatic leveling of the 654A signal
is
achieved by means of the Amplitude Control Integrator
which compares the current from the Average Detector
with the current from an Amplitude Current Reference to
regulate the current through the lamp of a photosensitive
control device (A2DSVI). The lamp controls the imped-
ance of a resistive divider at the inpu t of the Buffer
Amplifier
so
as to maintain a constant output level from
the Balanced Amplifier. The output level attenuators
provide attenuation in 10 dB and I dB steps at the output
connectors and a front panel AMPLITUDE control gives
2 dB of continuous output level adjustment by varying the
current from the Amplitude Current Reference.
4-4. The
current
from the Average Detector which flows to
the meter circuits is divided into two parts
:
a fixed amount
of
current
(approximately 1.25 rna) flows into the Meter
Offset Current Reference and the remainder flows to the
meter. In this way the meter is offset so that it indicates
only over the range of -I dBm to
+
I dBm. The current
flowing into the Meter Offset Current Reference is held
con~tan't
by the Meter Differential Amplifier which clamps
the Input of the current reference to a virtual ground.
4-5. A Counter
Emitter
Follower provides isolation
between
the
oscillator circuit and the rear panel COUNTER
OUTPUT. Regulated Power
Supplies
provide the
+
31
V
and -
26
V reqUired to operate the 654A.
4-6. CIRCUIT DESCRIPTION_
4-7. OSCILLATOR CIRCUIT (Schematic No
.
1, Figure
7-2)
4-8. The frequency adjustable Oscillator drives the Buffer
Model 654A
Section IV
SECTION IV
OPERATION
Amplifier with a stable sine wave at a frequency determined
by the setting of the FREQUENCY RANGE switch and the
FREQUENCY dial. The circuit is a Wien Bridge Oscillator
which has a standard, frequency selective, RC leg and a
resistance leg modified by the addition of a variable
impedance (A2CR I and A2CR2). A2Q26 and A2Q I
through A2Q6 comprises the amplifier section and A2Q7 is
a peak detector which provides negative feedback to the
bridge for
leveling.
Two types of feedback are used; positive
feedback from the frequency selective network drives the
base of A2Q3 through the source follower A2QI, and
negative feedback from the resistive side of the bridge
drives the base of A2Q2. A2Q2 and A2Q3 form a
differential amplifier. Only at the
selected
frequency does
the positive feedback overcome the negative feedback to
sustain
oscillations.
4-9. The six frequency ranges are selected by means of the
RC networks mounted on the FREQUENCY RANGE
switch (S2); continuous adjustment of the frequency on
each range is accomplished by rotating the FREQUENCY
dial, which controls the
setting
of the tuner capacitors CI A,
C
I
Band C
I
C. The signal from the amplifier out put (from
A2Q5 and A2Q6) is developed across the RC network of
the bridge; at the selected frequency, where Xc
=
R (Figure
4-1), the positive feedback to the base of A2Q3 has the
correct
phase
and
sufficient
amplitude
to sustain
oscillations. The high input impedance of the field effect
transistor (A2QI) prevents the amplifier from loading the
frequency determining leg of the bridge; the feedback
prOVided by A2Q26 prevents any
changes
in the parameters
of A2QI from affecting the frequency response of the
amplifier. The difference between the positive and negative
FREQUENCY
0.5
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7 /
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LAG
0.4
0.3
_F_
p_
PHASE
F
0.1
LEAD
F
p;
POSITIVE FEEDBACK
TO AMPLIFIER
RATIO--
PHASE -----
F ; FEEDBACK TO RC NETWORK
6!54A-A-187
9
Figure 4-1
.
RC Network Characteristics
4-1
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