Clock oscillators There are, basically, two categories of oscillators that are of interest to the electronics engineers: Harmonic and relaxation. The former produce sinusoidal waveforms and contains at least one active component that supplies power constantly to the passive components, whereas relaxation oscillators produce non-sinusoidal waveform, such as rectangular pulses. An oscillator is generally an amplifier operating with positive feedback in a manner whereby an output is produced whitout any input signal. To achive the desired frequency every oscillator contains a frecuencydeterminating part, which may be an LC circuit, a phaseshifting RC network or a quartz crystal. Requirements of a clock oscillator Clock oscillators must be reliable, easily reproducible and simple. An example of such an oscillator is shown in figure 1. R1 R2 47n 1-8 MHz N N X C1 Cp 82p N= 7404; 74LS04 figur1. This series-mode circuit suitable for operation between 1 and 8 MHz. Fine adjustment of the frequency is provided by the trimmer C1. If the required frequency accuracy is not important the trimmer may be omitted. Capacitor Cp prevents operation on a harmonic frequency. The value of R1 is 2k2 for operation below 2 MHz. The value of R2 is calculated from R2 = 3000/fc where fc is in MHz. A better oscillator using inverter gates is given in figure 2. 10 M 1-30 MHz N N R X N= 74HC04 Cp 35p 68p figur 2 It operates in parallel mode. Resistor R serves to limit the current through the crystal. If fine frequency adjustment is not required, C1 may be omitted and Cp increadsed to 56pF. This circuit is suitable for use between 1 and 30 MHz. The value of R is calculated from R = 104 /fc –300 where fc is the crystal frequency in MHz.
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