noise — inbound or outbound — to digital ground. Keep the
leads of R and C short and dressed close to the gate. Ideally,
terminate C in a ground plane, rather than a trace.
Tolerances and Stability of the
Allowing for variations in the broad Schmitt thresholds,
remember, we are using digital logic as an analog
component. Computer runs show a maximum unit to unit
(UTU) variation in frequency from -49% to +60%, relative to
Table 1. To these tolerances, we must also add component
tolerances for R and C. If this is not to be a production
oscillator, consider replacing R with an adjustable rheostat,
and trim the frequency to the desired value. Hand trimming
in production scenarios is very expensive, and if you need
inherently accurate frequencies in a production setting, a
quartz crystal oscillator may be more appropriate.
Duty cycle will vary between about . 38 and . 53,
depending on the gates and Vcc. The temperature
coefficient of drift will not be less than several hundred
parts per million per degree Celsius.
Test Your Circuit
The First Commandment of all good design is that you
can’t get enough testing. Historical episodes like the
disastrously under-tested Mark 14 torpedo of World War II
(Ref. 2) show that a little sober, logical testing under the
conditions of usage (and beyond) would have made all of the
difference between success and failure, even life and death.
Test your circuit for stability and temperature
coefficient beyond the range of ambient temperatures to
which it will be exposed in use. Vary Vcc from six volts to
two volts; go below two volts and watch the circuit’s
interesting decay dynamics. Cycle the power on and off at
random and observe the start-up waveforms. In all cases,
note the stability of frequency, amplitude, and shape of
the waveforms under test variations and over time. Test the
output with actual loads.
I would wish you good luck with your first RC
relaxation oscillator design, but luck has nothing to do
with it — this is engineering, and no oversight on your part
will go unpunished by Mother Nature. It is important to
understand what you are doing, and it is difficult to be
too careful. NV
(1) Datasheets for the gates shown can be obtained online at
www.fairchildsemi.com. Search by part number.
(2) Inviting Disaster, by James R. Chiles. HarperBusiness. ISBN
0-06-662082-1. Chapter 4, “Testing is Such a Bother,” pages 95 and 96.