BWD MINI-LAB 603B Manual page 41

6
.
CIRCUIT
DESCRIPTION
(continued)
6.5
Amplitude Modulator;
(Drg.
No. 1357
UlOl
is
connected
as
a variable gain amplifier. The
carrier input
is
taken
via buffer
U105
to Pins
3
and
6.
The modulating
signal
is
taken
via
RV112
and U106
to
Pin 2.
The output appears
at Pin
1,
is
DC
level shifted
by
RV102, and then taken via amplifier U108
to switch
SI
03b.
6.6
Function Generator Output
Amplifier
;
(Drg.
No.
1357)
The
required input
waveform
is
selected
by
SI
00,
SI 01
and
SI
02.
S103A
and
B redirect the signal
through the amplitude modulator
when
required.
RVllO Output
amplitude control adjusts the input to U501
,
the output
of
which
is
fed
to
Q
100 base.
The output appearing
at the collectors of
Q102
and
Q103
is
fed
back
to the base of
QlOl
via
R506, providing negative
feedback and a gain
of
approximately 20.
DC
offset
is
taken from
RV104
via
S104 and R505
to the
base
of
QlOl.
6.7
Ramp
Generator
;
(Drg.
No.
1357)
C402
is
alternately
charged and discharged by;-
a.
The
constant current source
provided by
Q401
and U401A.
and
b.
The negative going output
of
U401B
via
R404 and D401
Assume
the output of
U401B
is
positive
(approx. +10V).
U401B
functions as
a comparator allowing C402
to
charge from
the constant current source to
a
DC
level
of approximately 40. 6V.
At
this
point the voltage
across
C402
(and therefore
U402
output)
becomes
slightly greater than the voltage at the
NON-INV
input of
U401B whose
output switches
to
-lOV, and
via
R404
and D401
discharges
C402
to
zero
volts
in
approximately 5mSec. The
NON-
INV
input of
U401B
falls
to
zero
volts,
so
that
when
the
voltage
across
C402
reaches zero, U401B switches
to
a positive output,
allowing C402
to
charge
from the constant current source.
U401A
is
wired
as
a unity gain
amplifier,
providing low loading of the
frequency
control
(which also functions as
a
DC
offset control for
the
Function Generator).
U401D
is
the output amplifier
with RV401 providing variable gain,
and hence
an output voltage
control.
U401C
senses
when
the output
frequency of the
Function Generator exceeds
its
upper
limit for
whatever range
is
selected.
When
this
occurs
it
resets
the
Ramp
to
zero
volts via
U401B,
as
described
above
This circuit
prevents
ambiguous
frequency
sweep
operation
when
the
Ramp
attempts
to
produce a
higher frequency than
the
Function
Generator
will
allow.
603B
6-3