CIRCUIT DESCRIPTION
Refer to the Circuit Diagrams on pages
11
and
12.
Power Supply
The incoming mains supply
is
switched
by
S
15
to the
stepdown transformer
T1
.
F1
provides protection
against fault currents.
021
to
024
rectify the trans-
former secondary voltage.
C:20
and
C21
smooth the
d.c. output
of
the rectifier. TR22,
-TR24
and ½IC10
form the
+ 15V regulator. TR23, TR25 and ½IC10
form the - 15V regulator.
ZD1
is a stable reference
voltage for the
+ 15V power supply. The
-15V
regu-
lator tracks the
+ 15V output by taking its reference
voltage from the
+ 15V
output
via the voltage divider
R98
and
R99.
The + 10V and
-10V
supplies are deve-
loped from the
+ 15V and - ·15V by ZD2 and
Z05.
ZD3, TR26 and D20 form a current limited
shunt
regu- ·
lator
to
produce
the
5Vttl supply.
Note
that D10
is
also
the front panel power indicator. ZD4 forms a 5Vaux
supply
for
the sine converter circuit.
Triangle
Generator
IC1
and
TR1
form
a voltage controlled current source,
the
output
current being determined
by
the
sum
of
the
voltages
at
the sweep
input
and the frequency control
VR1. VR2 and VR4 are used
to
calibrate
the
front
panel frequency scale. The
output
current
of
TR1 is
mirrored
by
the
complementary current source IC2
and TR2 by monitoring
the
voltage across R
13.
The actual:current flowing in these current sources
is
determined
by
the
voltage across R13 and R16 and
ranges from 400uA
with
the
front
panel control set
to
0.2 to 4mA
with
the
front
panel control set to 2.0. The
current flowing
in
these current sources causes a
control voltage
to
be developed across R12 and
R17.
This voltage controls the current in the
two
current
sources formed
by
IC3, TR3 and IC4, TR4. These
current sources have three ranges set
by
R18,
R
19
and
R20
for the positive current source (IC3, TR3) and
R21,
R22
and R23
for
the negative current source
(IC4, TR4).
The
three current ranges are:
4µA
to
40µA, 40µA to 400µA and 400µA
to
4mA.
The current
from these
two
current sources
is
steered
into
the
range capacitors
by
the diode gate
D1
to
04
under the
control
of
the comparator output.
The symmetry function is performed
by
unbalancing
the currents in the current sources. This
is
achieved by
switching in the components
R14,
VR6 and R15 and
adjusting the symmetry control VR6. Note that
when
the symmetry function
is
selected the effective resi
s-
tance in the emitter circuits
of
TR2 and TR3 is
increased to
~
3K.Q
, (with the symmetry control set
to
its centre position), this decreases the frequency
of
oscillation
by
a factor
of
approximately
10.
S 1 to
S7
is
the frequency range switch bank.
Capacitors
C1
to
C4 are the range capacitors. The
2H
z
to 2KHz frequency ranges use capacitor
C1
and the
current range resistors
R18,
R19,
R20 and R21, R22,
R23
set the operating frequency.
When
the 2Hz range
is selected the capacitance multiplier circuit IC5
incr
eases
the effective capacitance
of
C1
to 10µF. The
multiplication
factor
of
the multiplier circuit is:
4
1 +
'jR25/R24I.
To
ensure
that
a linear triangle wave is produced the
range capacitors
must
be
buffered by a high
impedance amplifier
with
a very
low
input
bias
current. The triangle amplifier consists
of
the mono-
lithic dual FET
TR5
connected
as
a source follower.
The
unity
gain amplifier formed
by
the transistor array
IC6 buffers
the
output
of
the source follower and
ensures a
low
impedance drive for the comparator and
sine converter circuits.
VR9
is
used to adjust
out
any
m1tial
offset in this stage.
The comparator stage formed by IC7,
05
to
08
and its
associated components acts
as
a
window
comparator
and determines the triangle wave amp
lit
ude. The
triangle wave is attenuated
by
R37
and R38 and sent
to the comparator
input
(pin 3
of
IC7),
C9
provides
high frequency compensation for the attenuator. TR6
level shifts the
TTL
output
of
IC7
to
drive the diode
gate
05
to
08.
VR
10
and
VR
11
are
used
to
adjust the
positive and negative threshold voltages
of
the com-
parator.
The comparator circuit operates
as
follows:-
As
the
triangle wave
at
the buffer amplifier
output
(TP9)
reaches
+ 0.5V the comparator
output
switches the
diode gate
low
and sets the comparator threshold
voltage
to
-0.25V,
the triangle wave · then ramps
down
to
- 0.
5V
at
which
point the diode gate output
switches high and sets the comparator threshold
to
+ 0.25V and the triangle wave then ramps
up
again
towards the positive threshold.
TR7 and TR8 buffer the
output
of
the level shifter TR6
to provide a
low
impedance drive
for
the diode gate
01
to
04.
Sine
Shaper
T9e sine shaping circuit utilizes the non-linear, logar-
i
thm
ic relationship between Vbe and collector current
in transistors
to
smooth the triangle wave into a close
approximation
of
a sinewave. IC8 performs the sine
shaping function. The
two
transistor differential ampli-
fier operating
off
the 5Vaux supply converts the
triangle wave
into
a sinusoidal current which
flows
in
R52.
The three remaining transistors in IC8 along
with
TR9 and TR
10
form a current to voltage converter to
produce a sinusoidal voltage at TP10. VR12 adjusts
the bias voltage
at
the input
of
the sine converter and
dii°termines the symmetry
of
the resultant sinewave.
VR13 attenuates the triangle wave to the
optimum
amplitude for the sine shaper,
VR
16
is
used to trim out
any offset and VR14 adjusts the
ga
in
of
the current to
voltage converter
to
produce
an
output
sinewave
amplitude
at
TP10
of
1Vp-p.
Squarewave
Shaper and
TTL
Output
The
TTL
output
from pin
11
of
IC7
(the comparator),
is
level shifted
by
TR11
and fed to the diode gate
012
to
015,
current
is
steered into and
out
of
the load
(R4
+
VR
15)
to
produce a squarewave
with
contro
ll
ed
rise and fall times. VR15
is
adjusted to give a square-
wave amplitude
of
1Vp-p. IC9 buffers the
output
of
the comparator to provide the TTL output.
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