Applications - BWD MINI-LAB 603B Manual

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APPLICATIONS (continued)
5.3
•S
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METHODS OF OBTAINING FREQUENCY
SWEEP
IN
CONJUNCTION
WITH
AN
OSCILLOSCOPE
TO
VIEW
THE
RESPONSE
OF VARIOUS
CIRCUITS
(Cont'd)
The values of
R1
and R2
are set to
give
less
than
lOV
pp out of
the
Voltage
Amplifier
set
to unit gain.
Variation of the Bi-Polar Power Supply
adjusts
the
DC
level
of the output, and
is
used
to set
the
start
of the
sweep
at
zero
volts.
If
the
sweep
output from the oscilloscope
starts
at
zero
volts,
the
-fve
terminal
of the amplifier
may
be grounded. The
gain
control
will
adjust the
peak
to
peak
value of the output
signal.
Having produced
the appropriate
signal for
modulating
the Function Generator,
the
frequency
dial
of the generator
is
set to
0.1
and
the output of the
Voltage
Amplifier
is
connected
directly to the
F.M.
input socket.
The output frequency
of the
Function Generator
will
now
vary over approximately
two decades,
starting
at
the
frequency
shown by
the dial
and
increasing linearly
to
the
maximum
frequency
for
the frequency range selected.
If
a
smaller
range
of
frequency modulation
is
required,
e.g.
200Hz
to
300Hz
the
procedure
is
as
follows;-
Set the
minimum
frequency
required
on
the
dial
(200Hz).
Adjust the amplitude of the modulating
signal, either
by
varying the gain
of the amplifier or
by adjusting the
values of RI
and
R2, so
that the pp amplitude,
in volts,
is
equal
to
the
number
of major
scale divisions
over
which
the
frequency
is
to
change. (From 200
to
300Hz
is
one
major
division).
Then check
that the
start
of the
sweep
is
at zero
volts
and
re-adjust
if
necessary.
(b)
Logarithmic Frequency Sweep;
Whereas
the linear
frequency
sweep
covers only two decades,
changing
the shape
of
the
sweep
voltage
and allowing
the
start
to
be below zero
volts
enables a
sweep
of
up
to
four
decades
to
be achieved
in
a
manner
which
is
usable.
Both the
Power
Amplifier
and
the
Voltage
Amplifier
are used;
one
to
amplify the oscilloscope
sweep
signal
and
the other
to
provide a
non-linear input/output
characteristic.
Several external
components
are used:-
(
1
)
(
2
)
lx
2Q
1
Watt
Resistor
lx
47KQ
2
Watt
Resistor
lx
Voltage dependent
resistor
3Watt
Philips
No. 2322-
555-01161
lx 20Kfi Variable
resistor.
603B