The Arbitrary Waveform Generator
■ PHOTO 1. The hardest part of the
project is mounting the 25 pots. I cut
the front piece in half so I could use
a band-saw to cut the slots for the
controls. I made a custom triangular
case from plastic. You can choose to buy
or build your own case as you prefer.
Nothing is very critical or difficult
about the construction. The primary
concern is obtaining the slide pots at a
reasonable price. I got mine at the
Electronic Goldmine (
www.goldmine-elec.com) for four for $1. The actual
value is not critical and any resistance
from 5K to 100K should work fine.
(Note: If you get the same pots I did,
you will find that you need 2 mm
diameter screws to mount them. These
metric screws are not usually stocked
at the local hardware store but
Digi-Key has them.) You don’t have to
use slide pots. It is possible to use
ordinary rotary parts, but visualizing
the waveform is much more difficult.
Mounting the pots is the most
challenging part of the project. I built
a triangular-shaped custom case from
clear plastic (see Photo 1). I cut the
face-piece in two so that I could easily
use a band-saw to cut the slots for the
pots. All the mounting screws hold the
two pieces together tightly. Or you
can simply cut out a large square
section to permit the movement of
the sliders, if desired.
I used two small PC breadboards
for the electronics. The first board
held the digital circuits (see Photo 2).
The diodes are placed on end and can
be seen in a row at the top. Note that
this photograph was taken before I
had to add the resistor/transistor
switch. I mounted the additional
resistors between the
diodes and the counter
chips. The transistors were connected
directly to the pots. Photo 3 provides
an overall view of the inside of the
ARB. (I painted the inside of the case
white.) The transistors are mounted to
the pots and are difficult to see.
The analog PC board is sparsely
populated as seen in Photo 4. I kept
the analog circuit physically isolated
from the digital board to reduce
possible noise pickup. Resistor R12
and capacitors C12 and C13 also help
to reduce the noise by isolating the
digital and analog power lines.
The various switches and controls
can be mounted to suit your preference. The final result is seen in Photo 5.
crank the Offset control all the way
positive or negative, the whole waveshape will be clipped and only a flat line
at ground or VCC will be output. A
regulated DC supply is recommended
for best operation. Typical unregulated
AC adapters can introduce 60 Hz noise.
The Filter controls can eliminate
the glitches at the switching points.
Photo 6 illustrates a typical sawtooth
wave with a deliberate negative spike
placed in the middle. It is seen that
there are glitches at the edges of the
steps. These glitches can be removed
by filtering as shown in Photo 7.
If you plan to use this without an
oscilloscope, you will still have to calibrate the unit and label the settings.
This will require an oscilloscope to
determine the operating frequency
and measure the effects of the filter.
The operation is very simple especially when used in conjunction with an
oscilloscope. Just adjust the Frequency
controls for the frequency you need
and adjust the pots for the proper waveshape. If you don’t want the full voltage,
you can use the Level control to reduce
it. The Offset control adds a DC
component to the output, if desired.
Note that it is possible to adjust the ■ PHOTO 3. A view that shows the inside
Offset control so that the wave is of the ARB. The transistors for the
clipped at either VCC or ground (the step-circuits are mounted to the pots
amplifier obviously can’t produce a (difficult to see). I used #30 wire-wrap
signal that exceeds these limits). If you wire, but any small gauge wire will work.
(The inside has been painted white.)
There are a number of options
you can add to improve the
performance. The first is to use a more
■ PHOTO 2. The digital
board is not complicated.
was used. Note that the
step circuit resistors are
not shown (they were
August 2007 45