Test: Types of Oscillator - 2 - UPSC MCQ

RC phase shift oscillator:

• RC phase shift oscillator as shown in the figure is used to invert the input for a 180° phase difference. Hence, the gain of the RC phase shift oscillator is negative. (Statement 1 is correct and 3 is incorrect)
• The single R-C network gives the 60° phase shift. Hence the total phase shift provided by the feedback network is 180⁰. (Statement 2 is incorrect and 4 is correct)
• In the RC phase shift oscillator, the inverting amplifier produces a 180° phase shift.
• The RC Phase shift Oscillator: has a fixed frequency and is used at lower frequencies 

Concept:

Oscillators:

1. An oscillator is a circuit that produces a continuous, repeated, alternating waveform without any input.

2. An oscillator is an amplifier with +ive feedback.

3. The source of oscillation in an oscillator is the DC switching transient noise.

4. switch debouncing → Switch debouncing in an electronic design ensures that the device that is sampling the switching waveform does not misinterpret a single button press as many. 

5. an oscillator is not used as a switch debouncing. (option 3 is incorrect statement)

Crystal Oscillator:

• Crystal oscillators are fixed frequency oscillators with a high Q-factor.
• It operates on the principle of the inverse piezoelectric effect in which alternating voltage applied across the crystal surfaces causes it to vibrate at its natural frequency.
• It is these vibrations that eventually get converted into oscillations.
• These oscillators are made of Quartz crystal Rochelle salt and Tourmaline.
• Quartz is inexpensive, naturally available, and mechanically strong when compared to others.

In a crystal oscillator, the crystal is suitably cut and mounted between two metallic plates as shown in the figure.

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The R-C phase shift oscillator and Wein bridge oscillator are the commonly used circuits for generating a sinusoidal waveform of required frequency.

RC Phase shift oscillator:

The circuit diagram of the RC phase shift oscillator is shown below:

• The phase shift oscillator is a linear electronic circuit that produces a sine wave output.
• It consists of an inverting amplifier element such as a transistor or op-amp with its output feedback to its input through a phase shift network consisting of resistors and capacitors in a ladder network.
• The feedback network shifts the phase of the amplifier output by 1800 at the oscillation frequency to give positive feedback. 

Wein bridge oscillator:

The circuit diagram of the Wein bridge oscillator is shown below:

• The Wein bridge oscillator uses two RC networks connected together to produce a sinusoidal oscillator.
• The Wein bridge oscillator uses a feedback circuit consisting of a series RC circuit connected with a parallel RC of the same component values producing a phase delay or phase advance depending upon the circuit frequency
• At the resonant frequency, the phase shift is 00.

Hartley Oscillator:

In a Hartley oscillator the positive output freed-back is inductively coupled by tank circuit consisting inductor coil with center tap.

The tank circuit of Hartley Oscillator is shown:

Applications

• The Hartley oscillator is to produce a sine wave with the desired frequency
• Hartley oscillators are mainly used as radio receivers
• The Hartley oscillator is Suitable for oscillations in RF (Radio-Frequency) range, up to 30MHZ

 A Wien Bridge oscillator is a two-stage RC coupled amplifier circuit that has good stability at the resonant frequency. It has low distortion and is easy to tune. It is called so because the circuit is based on a frequency-selective form of the Wheatstone bridge.

Voltage series feedback is used in Colpitts oscillator. In voltage series feedback a part of output voltage is fed back to input.

The crystal oscillator is highly stable with time, highly stable with temperature, highly selective. The frequency of crystal is a constant regardless of output circuits. The frequency is dependent on the crystal shape which does not differ much at all.

Clapp oscillator is a colpitts oscillator which has an extra capacitor series with an inductor. Hence it is an LC oscillator, with 3 capacitors and one single inductor to set its frequency, and the clap circuit is often used as a variable frequency oscillator, by making one capacitor variable.

f = 1/2πRC
R = 1/2πfC = 63Ω.

RFC choke present in Colpitts oscillator is to provide high reactance to AC oscillation and low reactance to DC supply to improve performance. If resistor is used there will be some power loss in DC biasing which is unnecessary.

The oscillator has two resonant frequencies, parallel and series. Parallel resonance occurs when the reactance of series leg becomes equal to that of the parallel capacitor, and during this time, impedance offered is very high, almost infinite.

The stray third capacitor in the clap oscillator doesn’t have a negative effect and the frequency obtained is instead more stable and accurate than colpitt oscillator, hence the clap oscillator is preferred for a variable frequency oscillator.

A Wien Bridge oscillator provides a stable low distortion output over a wide range of frequency. However, the number of components required is high and the Wheatstone bridge applied cannot be used for high resistance values.

Crystal Oscillator:

A crystal oscillator is the most stable frequency oscillator.

Advantages:

• The crystal oscillator is possible to obtain a very high precise and stable frequency of oscillators
• It has very-low-frequency drift due to change in temperature and other parameters
• The Q is very high
• It has automatic amplitude control

Disadvantages:

• These are suitable for high-frequency application
• Crystals of low fundamental frequencies are not easily available

important point

• Hartley and Colpitts's oscillators are LC oscillators.
• LC oscillators are unstable oscillators.
• Phase shift oscillator is suitable for oscillations in AF range up to 1 kHz
• Crystals like quartz have high-quality factors, Q (range: 10⁴ - 10⁶). The high-quality factor will result in high-frequency stability.