FIGURE 4. A better
without any tank
circuit (i.e., R = 0
and C = parasitic
pF) — will free run at
roughly 40 MHz,
and we should
prudently limit our
design to 5 MHz or
less to avoid anomalous operation and
Note the following constraints for
the tank circuit. The tolerance on Vcc
should be strictly observed, while the
limits on R and C are approximate.
ately gives you the required RC time
constant. This done, values of R and C
can then be selected.
Example: What RC time constant
is required to produce a nominal
frequency of oscillation of 100 kHz
when Vcc = 5.0 volts?
Solution: R x C = numerator / 1 x 105
= 1.194 / 1 x 105 = 1.194 x 10-5
seconds = 11. 94 microseconds.
74HC14 hex Schmitt inverter would
work equally well. A subsidiary benefit
of using a NAND gate is that the
remaining input can be used as a
strobe for the oscillator.
The design liability of feeding the
output back through a capacitor — as
in our “bad” example — has been
avoided here. The inverter contributes
180 degrees of phase shift, and then a
much safer RC low pass tank element
has been used to achieve the additional 180 degrees of digital shifting.
This shift can be observed by considering that when the output changes
state, the capacitor “remembers” and
applies the previous output state,
lagging 180 degrees, to the inverter’s
input. Note that output noise is now
rejected by both the RC filter and the
input Schmitt dead band.
Since the propagation delay for the
74HC132 is about 15 to 25 nanoseconds, we can see from our previous
example in Figure 2 that this device —
2 volts ≤ Vcc ≤ 6 volts.
2K ohm ≤ R ≤ 2M ohms. Use 1%
tolerance metal film units, leaded or
100 pF ≤ C ≤ 1 microfarad. Use 2% or
5% tolerance, NPO or X7R dielectrics
for surface mount capacitors.
Polyester or polycarbonate film are
good choices for leaded units. See
text for C > 1 microfarad.
A Schmitt trigger involves positive
feedback within the gate itself and produces a middle input dead band where
no output transitions are allowed. The
upper and lower thresholds of this
device’s input dead band reside at
roughly Vcc/3 and 2/3 Vcc. These
thresholds can vary greatly from unit to
unit, and between the four Schmitt
NAND gates within a given unit.
The dead band and duty cycle
also varies with Vcc, but not strongly
so, being partially ratiometric to it. The
charging of the RC tank is exponential, but a simple table incorporating
all of these factors can be computed
to simplify the design (see Table 1)
which gives the value of a lumped
numerator in an equation to determine the required RC time constant of
the tank circuit in Figure 4. Simply
dividing the desired frequency of
oscillation into this numerator immedi-
Now we want to select an appropriate resistor and capacitor to produce this RC time constant. To sharpen
your design sense in these matters,
note that resistors over two megohms
will start showing sensitivity to humidity, and capacitors over one microfarad
will typically require tantalum or aluminum electrolytic implementations,
where leakage may start to be comparable to the charging currents when
the selected resistor increases in value.
Small capacitors, by contrast,
approach the circuit parasitic lead
capacities and the input capacities of
the IC inputs, producing frequencies
that some remove from computed
values. Thus, the lower limit of 100 pF
Accordingly, good design choices
for our example above would be: R =
11.8K ohms (MIL decade) and C = 1
nF. A less optimal design choice would
be: R = 118K ohms and C = 100 pF.
Other Design Precautions
+Vcc [volts] Numerator
3. 5 1.181
4. 5 1.191
5. 5 1.197
Tabular accuracy can vary from (-49% to
+60% for Vcc = 5 volts based on the specified
wide variations in the actual dead band relative to the nominal dead band. TABLE 1. Required RC time constant = Numerator/frequency.
70 March 2008
One of the extra NAND gates is
used as an output buffer. As a general
rule, always buffer oscillator outputs.
The tank circuit should be the only
significant load that the tank-driving
output ever sees. Do not employ this
circuit with a switched capacitor or
resistor, since if the feedback goes open
loop — even briefly — the NAND gate is
so fast that the output may hang at high
frequency when feedback is restored.
Connect the decoupling capacitor pair directly and closely to pin 14,
Vcc, of the integrated circuit as
shown. This will help shunt AC rail