Table of Contents
Advertisement
Section
IV
Paragraphs 4-9
to 4-14
4-9.
SCHEMATIC
THEORY.
4-10.
FREQUENCY
CONTROL
NETWORK
4-11.
The FREQUENCY dial (R2) in conjunction with
the MULTIPLIER
switch (S5) provide the frequency
control.
The basic frequency equation can be
expressed as:
1
1
o SCe
hd
Where
i, and
izare currents from two current
sources,
C is the ramp capacitor and
e is the peak
to peak voltage of the triangular waveform across the
ramp
capacitor.
The position of the MULTIPLIER
switch selects the
ramp
capacitor and the FREQUENCY
dial determines
the sum of the currents
i, and
i,. The frequency
control voltage is applied to the base of current con-
trol transistor Q 11 which establishes the amount of
current available to the ramp capacitor from the
current sources @ 12 and Q 13.
4-12.
CURRENT
SOURCES.
Current source transistors Q 12 and Q 13 provide a
constant current for charging the ramp capacitor,
the
ratio of one current to the other is divided between
the two sources by the SYMMETRY
control.
This
enables the charge and discharge rate of the ramp
capacitor to be controlled hence permitting the duty
cycle to be varied with no change in the frequency.
(reference Figure 4-2).
Model
211B
4-13.
SWITCHING CIRCUIT.
Assume
Q20 is conducting
and Q21 is cut off.
This
establishes a base
voltage,
on the switching
circuit
transistors, that turns on Q14and turns off Q15. This
action creates two current
paths:
Current
flows
through switching transistor Q14 and current
source
Q12 to the -20 V supply and the ramp
capacitor
lin-
early discharges through CR19 and current source
Q13.
As the voltage at the ramp capacitor is going
positive,
the base of Q20 is going negative,
until the
Schmitt trigger changes state.
This reverses
the
operation of the switching circuit, 14
turns off and
Q15 turns on.
Current now flows through current
source Q13 and switching transistor Q15 to the -20 V
supply and current source Q12 linearly charges
the
ramp capacitor through CR18.
The base voltage of
@20 will go positive until the Schmitt trigger switches
again.
4-14.
RAMP CAPACITOR AMPLIFIER.
On the upper four ranges the ramp capacitors C29-C32
are connected directly to ground. On the lower three
ranges C26-C28 and the ramp capacitor amplifier
form the ramp capacitor circuit.
@ 16 inverts the
phase and amplifies the ramp capacitor signal present
at the emitter of
919. Q17 and Q 18 provide a low
impedance path between the ramp capacitor and ground.
The inverted ramp signal applied to the ramp
capacitor
increases the charging time as shown in Figure 4-3.
[
a.
+—— PERIOD ——»
Figure 4-2.
Simplified Current Source Operation.
4-2
Advertisement
Table of Contents
