Oscillator
- 1. Oscillator Oscillators convert direct current (DC) from a power supply to an alternating current (AC) signal. Electronic oscillator is an electronic circuit that produces a periodic, oscillating electronic signal. Principle An oscillator always employs a sensitive amplifier whose output is fed back to the input in phase. Thus, the signal regenerates and sustains itself. Types of Oscillators basedon frequency Low-frequency oscillator (LFO) Audio-frequency oscillator Radio-Frequency oscillator Ultrahigh frequency oscillator (UFO) Microwave oscillator Oscillatorstypes in general LC Oscillators : Meissener oscillator, clapp oscillator, hartley oscillator, colpitts oscillator, lampkin oscillator, seiler oscillator, Meacham bridge Oscillator. RC Oscillators : phase shift oscillator, twin – T oscillator, Wein bridge Oscillator. Quartz Oscillators: butler oscillator, pierce oscillator, tri-tet Oscillator. RelaxationOscillators : blockingoscillator , multivibrator , ring oscillator,Pearson- anson oscillator. Other oscillators: cavity oscillator, deley line oscillator, opto electronic oscillator, robinson oscillator , gunn oscillator Hartley oscillator Hartley oscillator is an electronic oscillator circuit in which the oscillation frequency is determined by a tuned circuit consisting of capacitors and inductors (two series- connected coils i.e a tapped coil in parallel with a capacitor ), that is, an LC oscillator.
- 3. Working An npntransistor connected in a common emitter configuration works as the active device in amplifier stage. R1 and R2 are biasing resistors and RFC is the radio frequency choke, which provides the isolation between AC and DC operation . Athighfrequencies,the reactance value of thischoke isveryhigh,hence itcanbe treatedas an open circuit.The reactance iszerofor DC condition,hence causesnoproblemforDCcapacitors.The CE is the emitterbypass capacitorandRE isalso be a biasingresistor. When the DC supply (Vcc) is given to the circuit, the collector current starts raising and begins with the charging of the capacitor C. Once capacitor C is fully charged, it starts discharging through L1 and L2 and again starts charging. This back-and-fourth voltage waveform is a sine wave which is a small and leads with its negative alteration. It will eventually die out unless it is amplified. Now the transistor comes into the picture. The sine wave generated by the tank circuit is coupled to the base of the transistor through the capacitor CC1. the transistor is configured as common-emitter, it takes the input from tank circuit and inverts it to a standard sine wave with a leading positive alteration. Frequency of oscillation : If the capacitance of the tank capacitor is C and the total inductance of the tapped coil is L Then L = L1 + L2 + M where m is the mutual inductance of coils . Advantages The output amplitude remains constant over the working frequency range. frequency is simply varied by the net value of C in the tank circuit. Very few components are needed . Instead of two separate coils a single coil of bare wire can be used it grounded at any point . Disadvantages It cannot be a low frequency oscillator. Harmonic distortions are present.
- 4. Applications Hartley oscillators are mainly used as radio receivers. It is Suitable for oscillations in RF (Radio-Frequency) range, up to 30MHZ . Desired frequency of sine wave is produced by this oscillator . Colpitts oscillator Colpitts circuit consists of a gain device with its output connected to its input in a feedback loop containing a parallel LC circuit (tuned circuit), which functions as a bandpass filter to set the frequency of oscillation . Circuit diagram Stimulation circuit
- 5. Output signal Working When power supply is switched ON, capacitors C1 and C2 starts charging. When they are fully charged they starts discharging through the inductor L1. When the capacitors are fully discharged, the electrostatic energy stored in the capacitors gets transferred to the inductor as magnetic flux. The the inductor starts discharging and capacitors gets charged again. This transfer of energy back and forth between capacitors and inductor is the basis of oscillation. Voltage across C2 is phase opposite to that of the voltage across the C1 and it is the voltage across C2 that is fed back to the transistor. The feedback signal at the base base of transistor appears in the amplified form across the collector and emitter of the transistor.The energy lost in the tank circuit is compensated by the transistor and the oscillations are sustained. The tank circuit produces 180° phase shift and the transistor itself produces another 180° phase shift. That means the input and output are in phase and it is a necessary condition of positive feedback for maintaining sustained oscillations. Frequency of oscilation is given by ƒ =1/(2П√(L*C)) Where L and c are inductance and capacitance of tank circuit C =(C1*C2)/((C1+C2))
- 6. Advantages Good wave purity High performance at high frequency Good stability at high frequency It also has wide operating range of 10kHz – 300MHz Disadvantages Because of inductor circuit becomes bulk and cost of circuit high Difficult to adjust feedback as value of capacitor changed Poor isolation Hard to design Applications It is used for development of radio and mobile communications . This oscillator preferred in situation where it is to be intented to withstand high and low temperature frequently . It is used in generation of sinusoidal output signal with high frequncices It used in SAW devices which can be used as different types of sensors i.e temperture sensor . Franklin oscillator It is an electrical oscillator which uses two transistors and a LC circuit.The two transistors are connected as a two stage amplifier, both stages are inverting. the output of the second is coupled to the input of the first by a tuned circuit. Provided the amplifiers have enough gain, the circuit will oscillate, because the two amplifiers each give 180 degree phase shift, so the output is in phase with the input, so the oscillations build, until they are limited in amplitude by gain compression.
- 7. Circuit diagram Applications The simple, two diode balanced modulator is still one of the best ways of generating a DSB/SSB signal. The local oscillator used here is Franklin oscillator. You can use Silicon switching diodes, 1N914, 1N4148, Germanium point contact diodes, 1N34, or Schottky (Hot Carrier) diodes. Decide what band you want to use, work out the local oscillator frequency, allowing for the IF offset, then build a suitable oscillator / PLL / DDS. An SSB transceiver requires the 80 Metre band. The IF frequency here is 7.8MHz, the local oscillator frequency range required is, 1 1.3 MHz to 11.6 MHz to cover from 3.5 to 3.8 Mhz. This local Franklin oscillator can easily be modified to work on other frequencies, if you are using a different IF. Wein bridge oscillator The Wien bridge oscillator is an electronic oscillator and produces the sine waves. It is a two stage RC circuit amplifier circuit and it has high quality of resonant frequency, low distortion, and also in the tuning. Consider the very simple sine wave oscillator used by the RC circuit and place in the conventional LC circuit, construct the output of sinusoidal waveform is called as an Wien bridge oscillator. The Wien bridge oscillator is also called as a Wheatstone bridge circuit.
- 8. Circuit diagram Working The circuit is in the oscillation mode and the base current of the first transistor is changed randomly because it is due to the difference in voltage of DC supply. The base current is applied to the collector terminal of the first transistor and the phase shift is about the 180°. The output of the first transistor is given to the base terminal of the second transistor Q2 with the help of the capacitor C4. Further, this process is amplified and from the second transistor of collector terminal the phase reversed signal is collected. The output signal is connected to the phase with the help of the first transistor to the base terminal. The input point of the bridge circuit is from the point A to point C the feedback of this circuit is the output signal at the second transistor. The feedback signal is given to the resistor R4 which gives the negative feedback. In this same way the feedback signal is given to the base bias resistor R4 and it produces the positive feedback signal. By using the two capacitors C1 and C2 in this oscillator, it can behave continuous frequency variation. These capacitors are the air gang capacitors and we can also change the values of the frequency range of the oscillator. Applications It is used to measure the audio frequency. Wien bridge oscillator designs the long range of frequencies Advantages Distortion testing of power amplifier.
- 9. It supplies the signals for testing filters. Excitation for AC Bridge. To fabricate pure tune. Long distance can be spanned by the resting beams. Disadvantages The Wheatstone bridge is not used for the high resistance. The circuit needs the high no. of other components. The limited output frequency is obtained because the amplitude and the phase shift characters of the amplifier.