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A tuned oscillator uses a LC (inductor-capacitor) tank-circuit, a frequency-selective RC (resistor-capacitor) circuit or a quartz crystal circuit in its feedback path. Generally, the output waveform shape from a tuned oscillator circuit is sinusoidal and for this to happen positive feedback is used around an amplifying device such as a transistor or op-amp.
If negative feedback is applied to an amplifier the gain of the amplifier is decreased but the stability is increased. With positive feedback however, the gain is increased but the stability is decreased. This increase in gain produces a situation where an alternating sinusoidal output is obtained without a signal input. The amplifier has now become an oscillator giving an alternating output with the energy required to maintain this oscillation is obtained from the d.c. supply.
A tank-circuit consisting of a parallel-tuned LC circuit or RC circuit is used as the frequency determining unit which is “tuned” to give oscillations around its resonant frequency, hence the name tuned oscillator. The output from this device is feedback to its own input in such a way that the feedback signal aids the change in input signal. No input signal is required because the frequency determining unit provides its own signal via the feedback network in such a way that the circuit is self-exciting. Then this type of circuit is known generally as a Feedback Oscillator (positive feedback) and oscillators which use this technique are:
LC Oscillators: As their name implies, LC oscillators consist of a parallel tuned inductor-capacitor tank circuit as their frequency determining unit. The capacitor is constantly charging and discharging through the inductor coil at its selected resonant frequency but due to the heavy losses in the resistive element of the coil, the dielectric of the capacitor, and in radiation from the circuit. So in a practicle LC circuit the amplitude of the oscillatory voltage decreases at each half cycle and these oscillations would eventually die away to zero. If sufficient energy is applied at the appropriate time from a d.c. power supply in the cycle to overcome these losses then oscillations will continue at a constant frequency and amplitude indefinitely. Resonant frequency occurs when the coils inductive reactance (XL) equals that of the capacitive reactance (XC). Oscillations are controlled by varying the value of the capacitor (varactor).
Tuned oscillator circuits which use an LC (Inductor/Capacitor) tank circuit include:
- Hartley Oscillator
- Clapp Oscillator
- Colpitts Oscillator
- Tuned Collector Oscillator
- Pierce Oscillator
- Miller Oscillator
RC Oscillators: RC oscillators are also known as “phase shift oscillators” because their oscillating elements are made up of resistor-capacitor circuits which produce a phase-shifting circuit which corresponds to positive feedback. RC networks are not naturally oscillating circuits but become oscillating elements when connected around transistor or operational amplifiers.
RC oscillators do not use inductors but instead produces oscillations at a frequency at which the RC network produces a 180 deg. phase shift. A single stage amplifier will produce 180 deg. phase shift between its input and output and which can be used as a stage to produce the required positive feedback. The output from the amplifier is fed back via the RC network to its input. The input is shifted 180 deg. through the amplifier and 180 deg. through the RC network and 180 deg. + 180 deg. = 360 deg. or zero phase shift.
One useful property of the RC oscillator is that output frequency is inversely proportional to capacitance which means that a change in capacitance produces a much higher frequency compared to the LC oscillator. However, the disadvantages are that the output power of the RC oscillator is low due to dissipation in the resistive elements and for positive feedback to occur the amplifier gain must be greater than 29.
Tuned oscillator circuits which use a RC (Resistor/Capacitor) phase shifting circuit include:
- Ladder Phase Shift Oscillator
- Relaxation Oscillator
- Quadrature Oscillator
- Wein Bridge Oscillator
- Switched Capacitor Oscillator
- Digitally Switched Oscillators
- Phase Advance Oscillator (current transfer)
- Phase Retard Oscillator (voltage transfer)
Crystal Oscillators: Quartz and some other crystalline substances exhibit the “piezo-electric” effect. When a mechanical stress or physical deformation is applied to two surfaces of a suitably cut crystal it will produce a voltage between the surfaces. Likewise, when a voltage is applied to the crystal it causes a small physical deformation to the actual shape of the crystal.
Then if the voltage produced by mechanical deformation is fed back in some way, it will itself produce mechanical distortion in the crystal which will produce a voltage, which will…continue forever. This forms the basis of a number of crystal oscillators, because this feedback occurs only at the natural frequency of vibration of the crystal with this natural frequency value being determined by the “cut” of the crystal. Then the crystal in fact behaves as a resonant circuit with a very narrow bandwidth.
There is a limit to the stability and frequency that can be obtained from normal LC or RC tuned oscillators. Quartz crystal oscillators operate at very high frequencies up to 10Mhz when operating in the parallel mode. They also have very high stability and a resonant frequency with a very high Q factor making them ideal for use in CPU, microcontroller and video applications.
“Untuned Oscillators”
Unlike the tuned oscillators above, an untuned oscillator has no LC tank-circuit, frequency-selective RC or crystal circuit in its feedback path. Instead, an untuned oscillator uses nonlinear feedback and generally, the output waveform from an untuned oscillator is non-sinusoidal such as square-wave, triangular-wave or pulse being characterised by a sudden transition from one condition of stability or state to the next. Untuned oscillators are more commonly known as relaxation oscillators. Types of untuned oscillators include:
Ring Oscillator: Ring oscillators consist of an “odd” number of logic gates or amplifiers connected together in a series chain so that the output of the last is connected to the input of the first producing a ring type circuit. The frequency of oscillation depends upon the proporgation delay of the components used and the number of odd “stages” that are within the ring. Oscillation frequency is very high as to is the power consumption. Ring oscillators are more of a novelty as their high frequency and use of components make them impractical as an oscillator.
Relaxation Oscillators: Relaxation oscillators are more commonly known as multivibrators. They are a class of oscillator in which the active devices (usually a transistor) in the circuit are driven well beyond their cut-off and saturation regions for a period of time. Relaxation oscillators are cheap and easy to build with the three main types of multivibrator being.
- Astable Multivibrator: – has no stable state.
- Monostable Multivibrator: – has one stable state.
- Bistable Multivibrator: – has two stable states.
555 and Timer Chips: As well as our old favourite the NE555 timer and its variations, there are a whole host of different chips available in both TTL and CMOS that can be used to generate a variety of different waveforms and signals with some of the most popular being the: 74LS121, 74LS123, 74LS221 and their variants.
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