RF oscillator is a component that produces a periodic, oscillating electronic signal in the radio frequency range, using DC voltage.
Different types of commonly available RF oscillators are XO - Crystal Oscillators, VCXO - Voltage Controlled Crystal Oscillators, OCXO - Oven Controlled Crystal Oscillators, and TCXO - Temperature Compensated Crystal Oscillators.
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In this RF and microwave oscillator article, we will cover,
An RF Oscillators convert direct current (DC) into periodic, continuous, alternating RF signal waveform, which is widely used in many RF and microwave electronic devices as a source of the signal. RF oscillators are primarily characterized by the output frequency and the phase noise of the generated signal.
The basic working principle of oscillators can be explained by analyzing the LC tank circuit behaviour, which employs a completely pre-charged capacitor: C and an uncharged inductor: L, as its components. At first, the capacitor starts to discharge via the Inductor, which causes the generation of an electromagnetic field across the inductor and it gets charged until the capacitor discharges completely. Through this process stored direct electrical signal is converted into an electromagnetic signal.
Once the Inductor is fully charged, it starts discharged across the capacitor and the capacitor gets charged still inductor is fully discharged. Through this process, the electromagnetic signal is back converted into an electrical signal. This process will continue until the electromagnetic field collapses. The signal output is taken across the capacitor. The charging and discharging of the capacitor cause the generation of the alternating current in the circuit.
This cycle will repeat and generate a periodic signal as long as the LC circuit is biased using a DC supply to compensate for the loss during the charging and the discharging of the capacitors.
Oscillators are basically classified into 3 types on the base of the frequency range.
low-frequency oscillator: A low-frequency oscillator will generate a frequency signal below 20 Hz.
Audio oscillator: An audio oscillator generate a frequencies signal in the range of 20 Hz to 20 kHz I.e in the audio range.
RF oscillator: An RF oscillator generates the frequency signals in the range of 100 kHz to 100 GHz I.e. RF and Microwave frequency range.
There are basically two types of oscillator are available non-linear or relaxation oscillator and the linear or harmonic oscillator.
The harmonic oscillator or linear oscillator produces a sinusoidal continuous output. Harmonics oscillators are of two types.
Feedback amplifiers are designed using transistor or operational amplifiers and connecting the positive feedback loop through frequency-selective filters. When the power supply is switched on to the amplifier, the internal electronic noise in the circuit provides an oscillation. Then these noises get amplified and filtered until reach a required sine wave single frequency. Feedback oscillator circuits are classified on the base of the frequency-selective feedback loop used.
The active devices such as magnetron tubes, IMPATT diodes, tunnel diodes, and Gunn diodes are used to design a Negative-resistance oscillator. These oscillators are high frequencies oscillators that can be used in the microwave range and above. The Resonant circuit, such as an LC circuit, crystal etc is connected with negative differential resistance in the negative-resistance oscillators.
Relaxation or a nonlinear oscillator generates a non-sinusoidal output, such as a sawtooth, square, or triangle wave. A nonlinear oscillator consists of a capacitor or inductor like an energy-storing element and a nonlinear switching device like a latch, Schmitt trigger, connected in a feedback loop. The switching device in the loop periodically charges and discharges the energy storage element in the circuit and it causes the generation of the output waveform. The frequency stability of the non-linear oscillators is poorer than linear oscillators.
Square-wave relaxation oscillators are most commonly used as a clock signal for digital circuits. The relaxation oscillators are used for applications at lower frequencies.
In voltage controlled oscillator, the oscillation frequency can be tuned over some range by an input control voltage or current. These oscillators are used with phase-locked loops oscillators. The RF VCOs are usually designed by adding a varactor diode to the tuned circuit for changing the capacitance of the capacitor in the LC circuit by DC voltage, which in turn changes the resonant frequency of the tuned circuit.
RF oscillators are used in a wide range of RF and Microwave applications ranging from simple clock generators for digital instruments and baseband systems to RF carrier generators for RF modulators. The most common application of RF oscillators includes signal generation for the synthesizers ( Local Oscillators) for the communication systems like Radio Transceiver. A few of the industrial applications are listed below.
While selecting an RF Oscillator for the RF and Microwave design application, frequency stability and phase noise are the prime specifications that need to consider. In addition, it is ideal to check all the below-listed parameters for better characteristics of the system.
The frequency of the oscillator indicates the output frequency from an oscillator ( Repetition rate of the signal output from the oscillator). The frequency of the oscillators are measured in hertz (Hz). For the fixed oscillators the output frequency will be a fixed value. For the variable oscillators, the output frequency can be tuned to any specific value in the operating frequency range by a control voltage.
The frequency stability of an oscillator is expressed in parts per million (ppm) or parts per billion (ppb) and is represents the deviation of output frequency from its designed ideal frequency value. These oscillator output frequency variations will be based on external conditions like temperature, control voltage stability etc. The smaller stability number ( Higher Frequency stability range) means better performance.
RF Oscillators are available with different output signal formats like sine, square etc. RF designers need to select an oscillator with a suitable signal output.
The Phase noise (Time-domain counterpart: Jitter), have a direct impact on system performance, such as bit-error-ratio (BER) in serial data communication systems. As the clock is the base of any digital communication system, phase noise in the clock will lead to degrading the performance of the entire system. Phase noise of the oscillator indicates the clock noise in the frequency domain. Lower the phase noise figure of the oscillator better for the performance. For the general applications phase noise of -105 dBc at 1KHz and -150 dBc at 1MHz away from the centre, frequency is an acceptable figure.
The supply voltage of the oscillators is specified in volts (V). It is the power input required for the operation of the oscillator. The supply voltage is powered through the VDD pin of the oscillators and the standard voltages range for oscillators are 1.8 to 3.3 V.
The supply current of an oscillator is measured in microamps (µA) or milliamps (mA) and it is the maximum operating current an oscillator is required to generate a required output signal.
The output power level of an oscillator indicates the maximum output power level of the desired signal from the oscillators. It is mentioned in dBm. Designers need to select an oscillator with desired output power to derive the required circuits.
Harmonics of the oscillator indicate the power level of the signal at the harmonic frequency. Lower the harmonic power level, better for the applications.
The operating temperature range of the oscillator indicates the ambient temperature over which the oscillator is expected to operate by meeting the specifications. It is expressed in Degree centigrade in a range. For the commercial, Automotive Grade oscillators are of operating temperature range of 4 to +70°C, Commercial grade oscillators are of −20 to +70°C and for Military applications, it will be in the range of −55 to 125°C.
RF Oscillators generally come in an SMD or are housed in metal, plastic or ceramic industrial standard packages. The connector modules are coming with standard RF and Microwave connectors for the output RF interface and Vdd pins for the DC supply.
As RF oscillators are used to generate the basic clock signals, it is one of the prime components in any communication system. An oscillator is used to generate the required recurring frequency from the DC signal and Crystal Oscillator uses the resonance property of a quartz crystal to generate the required frequency.
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