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Model 654A
Section IV
SECTION IV
THEORY OF
OPERATION
4-1. GENERAL DESCRIPTION.
4-2. The Model 654A Test Oscillator (see Block Diagram,
Figure 7-1) contains a Wien Bridge Frequency Adjustable
Oscillator ( 10 Hz 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.
Automatic 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 (A2DSV1). The lamp controls the imped-
ance of a resistive divider at the input 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 1 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 ma) 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 - 1 dBm to + 1 dBm. The current
flowing into the Meter Offset Current Reference is held
constant 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
7-21
4-8. The frequency adjustable Oscillator drives the Buffer
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 (A2CR1
and A2CR2). A2Q26 and A2Q1
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 A2Q1, 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 C 1 A,
CI B and C1C. The signal from the amplifier output (from
A2Q5 and A2Q6) is developed across the RC network of
the bridge; at the selected frequency, where X
c
= 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 (A2Q1) prevents the amplifier from loading the
frequency determining leg of the bridge; the feedback
provided by A2Q26 prevents any changes in the parameters
of A2Q1 from affecting the frequency response of the
amplifier. The difference between the positive and negative
0.5
0.4
0.3
0.2
FREQUENCY
0.1
F
0
X,
R
F = POSITIVE FEEDBACK
TO AMPLIFIER
F = FEEDBACK
TO RC NETWORK
LAG
PHASE
LEAD
RATIO
PHASE
654A- A-1879
Figure 4-1. RC Network Characteristics
4-1
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