HCT crystal oscillator: The wide frequency range, low power consumption and well-defined switching levels of HCMOS inverters make these devices eminently suited to building quartz crystal oscillators with 171. compatible outputs. Here, the six cares in a 74HCT04 package are used to build three crystal oscillators. The only difference between the 2 MHz, -16 MHz, and 24 MHz oscillators is the capacitance around the quartz crystal., which in all cases must be a type that resonates at the fundamental frequency: overtone crystals cannot be used here.
For output frequencies, ƒo, other than the ones used here, use the following design data:
C2 = 723 / ƒo;
C1= C2 / 4.
where ƒo is in MHz and C1, C2 in pF. For 1-MHz crystals with a high impedance:
C1 = C2 / 10.
When one of the oscillators is not built, make sure to fit a wire link in PCB position C 1, C3 or C5. This ensures a low level at the input of the first oscillator gate, preventing high current consumption and spurious oscillation of the HCT04.
Parts list
Resistors:
R1, R3, R5 = 10 M Ω
R2, R4, R6 = 220 Ω
Capacitors:
C1 = 82 pF
C2 = 330 pF
C3 = 12 pF
C4 = 47 pF
C5 = 5.6 pF
C6 = 22 pF
C7= 100 nF
Integrated circuits:
IC 1 = 74HCT04
Miscellaneous:
X1 = crystal 2 MHz
X2 = crystal 16 MHz
X3 = crystal 24 MHz
There are different types of crystal oscillator circuits such as the Colpitts Crystal Oscillator which normally uses a common collector amplifier, Pierce Crystal Oscillator which uses transistors and FETs, the Butler oscillator which is a crystal-controlled oscillator and the Tri-tet oscillator which is a crystal-controlled vacuum tube oscillator circuit. The Pierce oscillator configuration is commonly used because of cost, size, stability, and easy implementation. It includes a resistor, two capacitors, a single digital inverter, along with a quartz crystal.
