There are many simple
available to the hobbyist.
something more is
needed when the
waves just aren’t
adequate. For these
generator is necessary.
instruments are very
Digital ARB Basics
However, for about
$20 you can build a
(ARB) that will cover
the audio range.
Most ARBs are computer
controlled instruments that require a
significant amount of hardware and
software. Figure 1 shows the block
diagram of a typical ARB. Basically,
there is a memory circuit that holds a
digitized representation of one full
cycle of the desired waveform. This
memory is accessed sequentially with
a counter circuit to output each step
in the waveform. This digital value is
converted to an analog value with a
digital-to-analog (D/A) converter and
filtered to remove the digital edges.
There are typically several
thousand steps (memory locations)
possible for one cycle of the
arbitrary waveform. Getting the
values into the memory to begin with
is not trivial and usually requires a
significant software interface.
It can be seen that more waveform steps equates to a higher clock
speed for a given waveform
frequency. For example, a 1 kHz
waveform of 1,000 steps requires a
step-speed (or clock rate) of 1 MHz.
A 1 kHz wave of 5,000 steps needs
a 5 MHz clock. If you want to go to
1 MHz for your waves of 1,000
steps, you will need a clock rate of
1 GHz with memory and a D/A that
can handle that speed. So, you can
understand why traditional ARBs
are expensive instruments.
Analog ARB Theory
■ FIGURE 1. The block diagram of a typical
digital arbitrary waveform generator. It requires
high-speed memory and a fast digital-to-analog converter, as well as significant software
for inputting and modifying the waveshape.
The design presented here is an
analog ARB (see Figure 2). It is far
simpler than the digital ARB, so it has
less capabilities than the digital ARB.
However, with a basic parts cost of
under $20, the price-to-performance
ratio is extremely good. It is suitable
for many applications where nonstandard waves are needed.
The 25 step “analog memory”
of the analog ARB is made up of 25
potentiometers (pots) configured as
voltage dividers. Each of the 25
voltage steps of the waveform is
generated by the setting of one pot.
(These analog voltages eliminate the
need for a D/A.) A simple digital
circuit is used to sequentially select
one analog voltage and present it to
the output buffer and filter circuit.
There are a number of design
approaches that will work to select
the analog voltages in sequence. Most
people would probably first think of
using CMOS analog switches (CD4053