Theory Of Operation; General Description; Circuit Description; Oscillator Circuit - HP 652A Operating And Service Manual

Model 652A

4-1. GENERAL DESCRIPTION.

4-2. The Model 652A Test Oscillator includes an oscillator,
power amplifier, peak detector, attenuator, normal monitor
circuit and expand monitor circuit. A block diagram of the
instrument is shown in Fignre 6 - 1 . The oscillator circuit uses
a modified Wien bridge network to generate a stable,
distortionless sine wave signal which is applied to the power
amplifi e r circuit. The automatic gain control utilizes a
feedback voltage from a peak detector to the oscillator
circuit to stabilize the output signal. The power amplifier
circuit is used to increase the output power available at the
50-ohm and 600-ohm output connectors and to improve the
frequency stability of the output signal with changing output
loads. The output attenuator providcs a means of attenuating
the signal at the output connectors in nine steps of
each. The normal monitor circuit monitors the signal level at
the input to the attenuator. The expand monitor circuit
allows the top 10% of the monitor scale to be expanded, so
that a reference can be set. The regulated power supply
provides all voltages required by the 652A circuits.
4-3.

CIRCUIT DESCRIPTION.

4- 4 . Refer to Figures 6-2 thru 6-4 for the following
discussion.

4-5. OSCILLATOR CIRCUIT.

4-6. The oscillator circuit generates a sinusoidal signal at the
frequency selected by the RANGE switch and FREQUENCY
Dial located on the front panel. The RC bridge network is a
modified Wien bridge circuit, consisting of an RC frequency
selective network and a resistive voltage divider network. The
Wien bridge in the Model 652A Test Oscillator differs from
the conventional Wien bridge circuit in the design of the
resistive voltage divider network. The resistor in the
conventional Wien bridge is replaced with a variable
impedance (Zion Figure 6-1) consisting of A2CR6 and
A2CR7.
4-7. Oscillation at the selected frequency is made possible by
the use of both positive and negative feedback. Positive
feedback is provided through a frequency sensitive RC
network to the differential amplifier A2Q2 and A2Q3;
negative feedback is provided to the differential amplifier
through a network insensitive to frequency. Only at the
selectcd frequency will the positive feedback exceed the
negative feedback voltage to sustain oscillation.
4-S. The RANGE switch, S I , selects combinations of
resistors and capacitors (S l RI through S l R24, and SI C l
through S I C I 5) to establish the frequency sensitive RC
networks for the six frequency ranges of the Test Oscillator.
SECTION IV

THEORY OF OPERATION

The FREQUENCY Dial varies the main frequency tuning
elements C 1 A,
the proper phase relationship of the positive feedback
voltage. At freqv.encies where X C ; R, the positive feedback
voltage is in phase with the oscillator output voltage (refer to
Fignre 4-1) and exceeds the negative feedback voltage. At
frequencies other than where Xc
voltage is neither of the right phase nor of sufficient
amplitude to maintain oscillations.
4-9. The impedance converter transistor, A2Q1 , provides a
high impedance in series with the input impedance of the
differential amplifier on the first four frequency ranges (XIO
- X IOK). The high impedance added prevents the RC bridge
circuit from being loaded by the low input impedance of the
10
differential amplifier, A2Q2 and A2Q3, on the lower
d
B
frequency ranges. The impedance converter is bypassed on
the XIOOK and X1M ranges due to lower resistor values in
the RC bridge.
4-10. The difference between the feedback voltages from the
bridge circuit is amplified by differential amplifier A2Q2 and
A2Q3, and is applied to the complementary symmetry
circuit A2Q5 and A2Q6, through emitter follower A2Q4. A
positive
complementary symmetry circuit is applied between resistors
A2RS and A2R9, in the collector circuit of A2Q2, on the
first four frequency ranges. The application of the feedback
voltage at this point is used to make the effective resistance
of the collector load higher than the input impedance of the
emitter follower A2Q4, forcing the collector current into the
base of the emitter follower. The increase in the signal level
results in an increase in the loop gain of the oscillator circuit.
The feedback voltage is removed on the XIOOK and XIM
F
F
and C I C. The RC components maintain
CIB,
feedback voltage from
FREQUENCY
0. 5
0. 4
/
/
/
0.1
'"
P
POSITIVE FEEDBACK
TO AMPLIFIER
'" FEEDBACK TO RC NETWORK
4-1.
Figure

RC Network Characteristics

Section IV
R, the positive feedback
=
Lhe output of the
LAG
/ /
PHASE
LEAD
RATIO-­
PHASE -----
6SIA-S-IOA
4-1