cool logic functions are only
available in SMD packages. The use
of SMD parts was minimized to keep
soldering on the printed circuit board
(PCB) manageable.
To gain experience with different
sizes of SMD components, SOIC and
SOT packages were chosen. SMD
resistors or capacitors were not used
because they are very hard to solder,
and not necessary to effectively
implement the design.
This pulse generator is a
complete design and does not need
any additional circuits to provide
many years of useful service.
It can also be expanded by the
experienced hobbyist who might add
features such as variable output
amplitude or a 50 ohm cable driver.
40 February 2013
The Circuit
The frequency and pulse width
oscillators are classic Schmitt trigger
logic gate oscillators where the
capacitor charges and discharges
between the thresholds of the
Schmitt trigger input. The charge time
is adjusted by the external
potentiometer.
In the frequency section, the
Schmitt trigger oscillator is
multiplexed with a crystal oscillator
and an external input to allow the
user freedom to generate signals as
needed for different kinds of
troubleshooting. The selected
frequency is then applied to the
decade divider string.
■ FIGURE 2.
A specific decade is selected by
grounding the corresponding enable
line on one of the AHC125 tri-state
buffers. Using the crystal oscillator,
an accurate frequency every decade
from 10.00 MHz to 0.10 Hz can be
selected.
The pulse width section is similar
but includes a pair of flip-flops (U9)
and a diode (D1) to start and stop
the oscillator. The pulse width
oscillator is stopped when the input
to the Schmitt trigger (U5) is held low
by grounding the cathode of D1, and
is allowed to run when the cathode
of diode D1 is held high.
When an edge from the
frequency generator clocks the "start"
flip-flop high, the oscillator starts.
When the selected output of the
