Clock oscillators

Clock  oscillators
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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