MC74HC4060A Datasheet, PDF (8/12 Pages) ON Semiconductor – 14-Stage Binary Ripple Counter With Oscillator

MC74HC4060A
DESIGN PROCEDURES
The following procedure applies for oscillators operating below 2MHz where Z is a resistor R1. Above 2MHz, additional
impedance elements should be considered: Cout and Ca of the amp, feedback resistor Rf, and amplifier phase shift error from
180°C.
Step 1: Calculate the equivalent series circuit of the crystal at the frequency of oscillation.
+ _{** )} ^{)) **} + ) Ze
jXCo(Rs jXLs jXCs)
jXCo Rs jXLs jXCs
Re
jXe
Reactance jXe should be positive, indicating that the crystal is operating as an inductive reactance at the oscillation frequency.
The maximum Rs for the crystal should be used in the equation.
Step 2: Determine β, the attenuation, of the feedback network. For a closed-loop gain of 2,A_νβ = 2,β = 2/A_ν where A_ν is
the gain of the HC4060A amplifier.
Step 3: Determine the manufacturer’s loading capacitance. For example: A manufacturer may specify an external load
capacitance of 32pF at the required frequency.
Step 4: Determine the required Q of the system, and calculate Rload, For example, a manufacturer specifies a crystal Q of
100,000. In-circuit Q is arbitrarily set at 20% below crystal Q or 80,000. Then Rload = (2πfoLS/Q) – Rs where Ls and Rs are
crystal parameters.
Step 5: Simultaneously solve, using a computer,
+ ^@ b
XC XC2
(with feedback phase shift = 180°)
@ ) * R Re XC2 (Xe XC)
( Eq 1 )
+ ) ) + Xe
XC2
XC
ReXC2
R
XCload (where the loading capacitor is an external load, not including Co)
+ ⁾⁾ ₎₎ ^*) ₎₎ ⁾ Rload
RXCoXC2 [(XC XC2)(XC XCo) XC(XC XCo XC2)]
X2C2(XC XCo)2 R2(XC XCo XC2)2
( Eq 2 )
( Eq 3 )
Here R = Rout + R1. Rout is amp output resistance, R1 is Z. The C corresponding to XC is given by C = C1 + Cin.
Alternately, pick a value for R1 (i.e, let R1 = RS). Solve Equations 1 and 2 for C1 and C2. Use Equation 3 and the fact that
Q = 2πfoLs/(Rs + Rload) to find in-circuit Q. If Q is not satisfactory pick another value for R1 and repeat the procedure.
CHOOSING R1
Power is dissipated in the effective series resistance of the
the first overtone. Rf must be large enough so as to not affect
the phase of the feedback network in an appreciable manner.
crystal. The drive level specified by the crystal manufacturer
ACKNOWLEDGEMENTS AND RECOMMENDED
is the maximum stress that a crystal can withstand without
REFERENCES
damage or excessive shift in frequency. R1 limits the drive
The following publications were used in preparing this
level.
data sheet and are hereby acknowledged and recommended
To verify that the maximum dc supply voltage does not for reading:
overdrive the crystal, monitor the output frequency as a
Technical Note TN-24, Statek Corp.
function of voltage at Osc Out 2 (Pin 9). The frequency
Technical Note TN-7, Statek Corp.
should increase very slightly as the dc supply voltage is
D. Babin, “Designing Crystal Oscillators”, Machine
increased. An overdriven crystal will decrease in frequency Design, March 7, 1985.
or become unstable with an increase in supply voltage. The
D. Babin, “Guidelines for Crystal Oscillator Design”,
operating supply voltage must be reduced or R1 must be Machine Design, April 25, 1985.
increased in value if the overdriven condition exists. The
user should note that the oscillator start-up time is
proportional to the value of R1.
ALSO RECOMMENDED FOR READING:
E. Hafner, “The Piezoelectric Crystal Unit-Definitions
and Method of Measurement”, Proc. IEEE, Vol. 57, No. 2,
SELECTING Rf
The feedback resistor, Rf, typically ranges up to 20MΩ. Rf
determines the gain and bandwidth of the amplifier. Proper
Feb., 1969.
D. Kemper, L. Rosine, “Quartz Crystals for Frequency
Control”, Electro-Technology, June, 1969.
bandwidth insures oscillation at the correct frequency plus
P. J. Ottowitz, “A Guide to Crystal Selection”, Electronic
roll-off to minimize gain at undesirable frequencies, such as Design, May, 1966.
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