BWD MINI-LAB 603B Manual page 40

6
.
CIRCUIT
DESCRIPTION
(continued)
6.2 Triangle
and Square
Wave
Generators (Drg.
No,
1358) (Cont'd)
Looking at Circuit Drg.
No.
1358, the current source
control
is
derived
in
the following way:-
RV2, the
Frequency Vernier
produces at
the
-ve
input of
UlA
a
variable
voltage.
This
voltage
is
inverted
by
UlA
and becomes
the input voltage
for
both
UIB and UlC.
UIB voltage follower reproduces the
input
voltage
at R115, whilst
UlC,
acting as
an
inverter
with unity gain, reproduces
an
inverted replica of
the input
voltage
at R118,
Q2
and
Q4
are the
+ve and -ve
current sources
with
their emitters
fed
by R115 and R118
respectively.
Q1
and
Q3
emitter
followers
produce
fixed
base voltages
for
Q2
and
Q4, so that
changes
in
control
voltage
via
UIB and
UlC
produce a change
in
current
through R115
and
R118,
and hence
a
change
in
current
from
the collectors of
Q2
and Q4.
D104, D105, D106 and
D107
form the current switch,
steering either
one
or
the other current to the timing capacitors
C14, C16, C23 and
C102-110.
The frequency range switch S3
to
S9
selects the particular
capacitor required,
with S3B
and
S4B together selecting the
low frequency range (XO.OlHz)
capacitors
C109 and
Cl
10.
The
FET
follower, U5,
has
a
high input
impedance
and a low output
impedance. The
Triangle output
is
taken from
this
point via
a
buffer amplifier comprising Q5,
6,
9.
Integrated circuit
U2
functions as the comparator, with
its
output taken
to
the current switch via emitter follower Q8, with diodes
D115
and
D116
for
level shifting.
Hysteresis of the
comparator
is
caused by feeding back
a portion of the output
signal to pin 5.
The output
of
the
comparator
is
clipped by
D117
and D118
to
produce a
±0.6V square
wave
which
is
used
as
the comparator
reference.
The comparator
output
is
also
clipped by D119 and D120
to
produce
the
square
wave
output.
Because
of
its
fast rise
time, portion of the switching
signal capacitively bypasses
the
current switch.
This
Is
compensated by
applying
an
equal
and
opposite signal via
C24.
6.3
T.T.L. Output
;
(Drg.
No.
1358)
The inverted output of the
Comparator
is
taken
via R121
to
Q7
base,
Q7
amplifies the square
wave
and provides a
OV
to 4-5V output at
its
collector.
6.4
Sine
Wave
Shaper
;
(Drg.
No.
1358)
Because
of
its
S-shaped
transfer
characteristic,
UlOO
converts the triangle
input
to
a
sine
wave
output.
RV103
adjusts the
symmetry
of
the resultant
sine
wave, while RV106
adjusts the
degree
of flattening of
the peaks.
RV107 allows
the
DC
level to
be
set to
zero,
and RV109
adjusts
the output
amplitude.
6-2
603B