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Model 651B
Section IV
4-1. GENERAL DESCRIPTION.
4-2.
The Model 651B Test Oscillator includes an
oscillator, power amplifier, peak detector, attenua¬
tor, and monitor circuit. A block diagram of the in¬
strument is shown in Figure 6-1.
The oscillator
circuit uses a modified Wein bridge network to gen¬
erate a stable, distortionless sine wave signal which
is applied to the power amplifier circuit.
The peak
detector circuit provides a
degenerative feedback
voltage to the oscillator circuit to stabilize the signal
applied to the power amplifier.
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
provides a means of attenuating the signal at the out¬
put connectors in nine steps of 10 dB each.
The mon¬
itor circuit continuously monitors the signal level at
the input to the attenim.tor. The regulated power supply
provides all voltages required by the 651B circuits.
4-3. CIRCUIT DESCRIPTION.
4-4.
Refer to Figures 6-2 and 6-3 for the follow¬
ing 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 Wein bridge circuit,
consisting of an RC frequency selective network and
a resistive voltage divider network.
The Weinbridge
in the Model 651B Test Oscillator differs from the
conventional Wein bridge circuit in the design of the
resistive voltage divider network.
The resistor in
the conventiorml Wein bridge is replaced with imped¬
ance Zl, which consists of A2CR6 and A2CR7.
4-7.
Oscillation at the selected frequency is made
possible by the use of both positive and negative feed¬
back.
Positive feedlmck is provided through a fre¬
quency sensitive RC network to the differential amp¬
lifier A2Q2 and A2Q3; negative feedback is provided
to the differential amplifier through a network insen¬
sitive to frequency.
Only at the selected frequency
will the positive feedback exceed the negative feedback
voltage to sustain oscillation.
4-8.
The RANGE switch, SI, selects combinations
of resistors and capacitors (SlRl through S1R24, and
SlCl through S1C14) to establish the frequency sen¬
sitive RC networks for the she frequency ranges of
the instrument.
The FREQUENCY Dial varies the
main frequency tuning elements CIA, CIB, and CIC.
The RC components maintain the proper phase rela¬
tionship of the positive feedback voltage.
At frequen¬
cies where Xc = R, the positive feedte-ck voltage is in
phase with the oscillator output voltage (refer to
Figure 4-l)and exceeds the negativefeedbackvoltage.
At frequencies other than where
Xq=
R, the positive
feedback voltage is neither of the right phase nor of
sufficient amplitude to maintain oscillations.
4-9. A field effect transistor, A2Q1, is used as the
impedance converter because of its extremely high
input impedance and low noise characteristics.
It
provides a high impedance in series with the input
impedance of the differential amplifier on the lower
four frequency ranges (XIO - XlOK).
The high im¬
pedance added prevents the RC bridge circuit from
being loaded by the low input impedance of the differ¬
ential amplifier, A2Q2 and A2Q3, on the lower fre¬
quency ranges.
The impedance converter is bypassed
on the XIOOK and XIM 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 com¬
plementary symmetry circuit A2Q5 and A2Q6, throi^h
emitter follower A2Q4.
A positive feedback voltage
from the output of the complementary symmetry cir¬
cuit is applied between resistors A2R8 and A2R9, in
the collector circuit of A2Q2, on the first four fre¬
quency ranges.
The application of the feedback volt¬
age at this point is used to make the effective resis¬
tance of the collector l<»d higher tte.n the input
impedance of the emitter follower A2Q4, forcing the
collector current into the base of the emitter follower.
The increase in the Imse current results inan increase
in the loop gain of the oscil^tor circuit.
The feed¬
back voltage is removed on the XIOOK and XIM fre¬
quency ranges due to the value of resistors A2R8 and
A2R9 exceeding the input impedance of the emitter
follower at the higher frequencies.
01810-2
4-1
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