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Model 606A
3-1
OVER-ALL
OPERATION
Refer to the block diagram Figure 3-1.
The level
of the rf oscillator
is stabilized by a feedback
circuit
from the RF Oscillator to the Oscillator
Level Control tube.
In a similar way the rf output
and modulation levels are held constant by a feed-
back loop from the rf output through the Differ-
ential Amplifier to the Modulator.
The rest of the
block
diagram
represents
standard circuitry for
signal generators.
3-2
CIRCUIT DESCRIPTION
The RF Oscillator is a tuned-plate push-pull
oscillator.
The pentode section of V1 acts asa
variable
cathode
resistor
for V2 to control the
oscillator level.
The control grid of this pentode
receives a rectified portion of the oscillator signal.
This
voltage
decreases
the current
through V1
when
the oscillator
level
rises and vice versa.
Since this current is also the cathode current of
the RF Oscillator,
the level of oscillation will be
held constant.
The triode section of Vlis a cathode
follower which provides bias voltage for the grids
of the oscillator and amplifier.
B.
RF AMPLIFIER
The RF Oscillator signal is fed to the control
grids
of the push-pull RF Amplifier stage, V3 and
V4.
The screen grids are connected directly to
+300 volts.
The amplifier cathode current is con-
trolled
by: V6, which acts as a variable cathode
resistor,
See thenext paragraph for an explanation,
C,
MODULATOR
Triode V6 is inserted in the cathode circuit of
the RF Amplifier to obtain cathode modulation. The
internal resistance of the triode changes according
Sect. III] Page 1
SECTION Ill
THEORY OF OPERATION
to the applied modulation signal at the control grid.
Thus cathode current of the RF Amplifier is varied
which,
in turn,
amplitude
modulates
the rf level.
D.
RF FEEDBACK AND LEVEL
CONTROL CIRCUIT
The modulated
rf output signal is sampled at the
secondary winding of the rf output transformerand
rectified by crystal diodes CR2 and CR3.
An rc
filter passes
the de current developed.
The time
constant of the rc filter is determined by the po-
sition of the frequency RANGE selector,
This time
constant is selected to bypass the rf component but
not the modulation.
The demodulated rf signal is
then fed to the control
grid, pin 7, of the Differ-
ential Amplifier.
This demodulated signal is dc-coupled and thushas
a dc component
equal to the cw level of the output
plus an ac component corresponding to the moclu-
lation,
The
demodulated
rf signal also passes
through a second rc filter, C37, C38 and L6, with
a long time constant which removes the modulation.
At the output of this filter a dc current is obtained
which is proportional to the cw rf level. This cur-
rent is then fed to the rf output meter.
A
two-position
attenuator, which
controls
the
a-
mount
of feedback, is inserted between the crystal
detectors
and
the differential amplifier.
In the
1 VOLT and lower output positions the feedback is
obtained through the divider R25 and R36. Only in
the
3 VOLT
position
is R36
(in feedback loop)
shunted by R26. This raises the current through the
RF Amplifier by 10 db, This switching is done au to-
matically by relay K1 whenever the ATTENUATOR
selector
is
switched
to the 3 VOLT
position.
E,
RF ATTENUATOR
The rf output signal is tapped off the seconlary
winding of the output rf transformer and fed tc the
input of the output
attenuator.
This attenw tor
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