RF Oscillators are an essential component to most RF signal chains, as they are used as local oscillators (LOs) with frequency conversion circuits, as a reference frequency, and in frequency synthesis/generation circuits. The basic purpose of RF Oscillators is to generate a periodic and continuous RF signal, typically sine waves, with input from DC power supplies. All RF sources have some form of oscillator to generate the initial RF signal. This includes use as clock generators for digital circuits and converters, such as Analog-to-Digital Converters (ADCs) and Digital-to-Analog Converters (DACs).
There are two main types of RF oscillators, crystal-based oscillators (XOs) and negative-resistance (NR) solid-state (SS) oscillators. Crystal oscillators are very common and have been around for over 100 years. NR-SS oscillators are more recent and are comprised of dc biased SS devices, such as Gunn diodes, impact ionization avalanche transit time (IMPATT) diodes, resonant tunneling diodes (RTDs), and there are transistor types as well. There are basic oscillator types that generate a single frequency high fidelity signal, these are generally called Reference Oscillators. There are also oscillator types that have additional circuits to enhance their performance. This includes Phase Locked Oscillators and Voltage Controlled Oscillators. There are also crystal oscillators that include temperature compensation to better ensure the accuracy of the oscillator signal; these are temperature controlled crystal oscillators (TCXO).
A Phase Locked Oscillator is typically a dielectric resonator oscillator (DRO) with additional circuitry (phase-locked loop) that combines the close to carrier performance of a reference oscillator with the good far from carrier noise floor of the DRO. This type of circuit is designed to improve the phase noise of the oscillator output for microwave applications.
A Voltage Controlled Oscillator (VCO) is an oscillator where the output frequency can be controlled using an input voltage, typically a DC voltage. There are a variety of methods used to realize voltage control of an oscillator, including using voltage-controlled capacitors. The following is a collection of RF oscillator key performance parameters:
RF Oscillators Key Parameters
- Center/Output Frequency (Hz)
- Tuning Frequency Range (Hz) *if applicable
- Reference Frequency (Hz) *if applicable
- Reference Impedance (Ohms) *if applicable
- Output Power (dB/dBm)
- Frequency Stability (MHz/deg C or ppm)
- Power Stability (dB/deg C)
- Phase Noise (dBc/Hz)
- Harmonics (dBc) and 2nd Harmonics (dBc)
- Spurious (dBc)
- Bias/Supply Voltage (Vdc)
- Bias/Supply Current (Idc or mA)
- Tuning Voltage Range (Vdc) *if applicable
- Impedance Output/Input (Ohms)
- Operating Temperature
- Aging per year (ppm) *if applicable