Mode of Oscillation, Hartley and Clapp Oscillators
A quartz crystal is designed to vibrate on its fundamental frequency or one of its overtones. Sometimes quartz crystal can be made of frequencies ranging from 70 kHz to 200MHz. Looking at the Radio Frequency Spectrum, the range is from Low Frequency (LF) to Very High Frequency (VHF). The quartz crystal is operated in its fundamental frequency to its other overtones. The odd harmonic intervals could from (1, 3, 5, 7, etc.).
In order to understand this, let's make an example, if we have a 90 MHz frequency, its 3rd overtone crystal will look just like a 30 MHz crystal in its fundamental frequency. Therefore, a 30 MHz quartz crystal can be made to oscillate at in the odd harmonic intervals multiplied by its fundamental frequency. Take note, never attempt to use a fundamental mode of crystal unit operating at an overtone frequency.
In order to understand this, let's make an example, if we have a 90 MHz frequency, its 3rd overtone crystal will look just like a 30 MHz crystal in its fundamental frequency. Therefore, a 30 MHz quartz crystal can be made to oscillate at in the odd harmonic intervals multiplied by its fundamental frequency. Take note, never attempt to use a fundamental mode of crystal unit operating at an overtone frequency.
Additional Information:
The Hartley oscillator is an improvement over the Armstrong oscillator. Although its frequency stability is not the best possible of all the oscillators, the Hartley oscillator can generate a wide range of frequencies and is very easy to tune. The Hartley will operate class C with self-bias for ordinary operation. It will operate class A when the output waveform must be of a constant voltage level or of a linear wave shape. The two versions of this oscillator are the series-fed and the shunt-fed. The main difference between the Armstrong and the Hartley oscillators lies in the design of the feedback (tickler) coil. A separate coil is not used. Instead, in the Hartley oscillator, the coil in the tank circuit is a split inductor. Current flow through one section induces a voltage in the other section to develop a feedback
Collector current and voltage waveforms of a class C oscillator
a) The Colpitts oscillator is very similar to the Hartley oscillator, but instead of a tapped grid coil, it has tapped capacitance.
The tap between the two capacitors is grounded and the feedback is obtained from the coupling capacitor, C1. The amount of feedback depends on the ratio of C2 to C3. The capacitor part of the LC circuit consists of both C2 and C3, which determines the oscillating frequency. This oscillator has more frequency stabilities than the Hartley oscillator.
b) The Clapp oscillator is an electronic oscillator which is constructed from a transistor and from a positive feedback network. The clapp oscillator uses the combination of an inductor (L) and a capacitor (C) in order to generate the desired output frequency. The clapp oscillator sometimes referred as the LC oscillator.
Clapp Oscillator (direct-current biasing network not shown)
The clapp oscillator network comprises a single inductor and three capacitors. The function of the two capacitors (C1 and C2) form a voltage divider that determines the amount of feedback voltage applied to the transistor. The capacitor (C0) makes the feedback voltage of the circuit stable, making it more easy to achieve the desired output frequency.. The oscillation frequency in hertz (cycles per second) for the circuit in the figure, which uses a field-effect transistor (FET), is
REFERENCES
http://www.allaboutcircuits.com/vol_3/chpt_8/12.html
http://www.visionics.a.se/html/curriculum/Experiments/RC%20Phase%20Shift%20Oscillator/RC%20Phase%20Shift%20Oscillator1.html
http://www.electronixandmore.com/articles/oscillators.html
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