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CONSTRUCTION
Only construct this project if you are
competently experienced at constructing
mains powered circuits, or are supervised by someone who is suitably qualified. Readers are reminded that the
voltages involved can KILL.
Most of the components for the 200kHz
Function Generator are mounted on a
single-sided printed circuit board. The topside component layout and full-size underside copper foil master is shown in Fig.2.
This board is available from the EPE PCB
Service, code 385.
The author prefers to use solder pins for
external connections to the board as these
are then more robust and may be made
from the component side of the board.
Where these pins are used it is generally
advisable to fit them first as some insertion
force is usually necessary. There are 22 of
them in total.
Following this the two wire links should
be fitted, then the resistors and diode D2,
the eleven 100n capacitors, the i.c. sockets
and bridge rectifier REC1 which should
have its positive output situated at top
right. After this, the six presets should be
fitted followed by the four electrolytic
capacitors all of which have their positive
sides towards the top of the board. None of
the i.c.s should be fitted yet as these will be
added during testing.
FIRST TESTS
Always remember to disconnect
the unit from the mains before making any adjustments. Do not come
into contact with the 230V a.c. input
to the transformer.
Normally the author recommends
initial testing with a current-limited
bench power supply but this is a bit difficult with a dual supply rail design so
this project was just connected to its
transformer (T1) and powered up, testing each part of the circuit in turn. It is
assumed that a DVM and an oscilloscope will be available as a function
generator is usually used in conjunction with a ’scope anyway.
The first check is to connect the
transformer and power it up. Around
22V d.c. should be present with respect
to ground or 0V, positive at the top of
capacitor C4 and negative at the bottom
of C2.
If these voltages appear correct IC1
can be fitted and on powering the board
again the regulated negative 12V
should appear at the bottom of C5. It
should also appear at pin 4 of the sockets for IC4 and IC5, and pin 11 of the
socket for IC3.
Next IC2 should be fitted. This time
the regulated positive voltage should be
present at the top of capacitor C6 and
on pin 8 of IC4’s socket, pin 7 for IC5,
and pin 6 for IC3. It probably won’t be
exactly 12V, but it can now be adjusted
to approximately this value with preset
VR1.
Layout of components on the completed circuit board.
Fig.3. IC3 should now be inserted into its
socket.
When the circuit is now powered up, the
sinewave (around 5V pk-pk) and triangle
wave (8V pk-pk) should be present at their
connection points for switch S3. The 22nF
capacitor is the value for 200Hz to 2kHz,
so Frequency control VR3 should give a
range somewhere around this, though it
may not be accurate until the presets VR2
and VR4 have been adjusted.
VR5
IC2
IN
S3 C4 OUT
ADJ
C8
+
+
W
VR7
C7
+
b
c
e
C5
IC4
C
11
TR4
e
c
b
R
5
S3
SK2
b
c
e
TR3
IC5
C
21
R
33
R R
e
27 30
c
b
TR5
R R
20 19
C
22
R
32
TR2
R
21
SK1
C
19 R
R
12 D2 18
R
29
W
TR1
S3
C1
VR4
R
28
e
c
b
S2
R R
13 15
C
20
R
3
R
4
VR3
S3
+
COM
IN
C2 OUT
R
16
R
14
R
17
REC1
W
W
IC3
R R
11 10
C
10 R
8
R
9
W
R
7
R R
C6
1 2
VR1
W
R
6
k
a
D1
VR2
S3
C9
+
C3
VR6
Next IC4 can be fitted. Both outputs
of this, pins 1 and 7, should produce
squarewaves with an amplitude of about
20V pk-pk. If transistor TR1 is now fitted,
the 5V pk-pk squarewave output should
appear from the connections for socket
SK1. Note that this swings between ground
(0V) and +5V, not symmetrically about 0V
as the other waveforms will.
If IC5 is now fitted and an oscilloscope
used to view its output (pin 6, but helpfully
VR8
/S4
R
31
IC1
122mm (4·8in.) x 65mm (2·6in.)
385
WAVEFORM CHECK
Frequency control VR3 should now
be temporarily connected along with a
22nF 1% capacitor across the two connection points for switch S2. All of
these points can be readily identified
from the connections diagram shown in
200
Fig.2. Printed circuit board topside components layout and full-size underside copper
foil master for the 200kHz Function Generator.
Everyday Practical Electronics, March 2003