The Basics of Frequency Synthesizers and Oscillators

Modern transmitters and receivers leverage oscillators and frequency synthesizers to generate carrier signals, upconversion/downconversion local oscillator frequencies, aid in frequency tuning, and more. There are many different types of materials and technologies used in precision frequency generation. This post will provide a brief explanation of the different types of oscillators and frequency synthesizers common in RF and microwave technology.

Basics of Oscillators

Oscillators are materials and structures, that when stimulated, produce a repeatable and predictable frequency response. These devices are a critical component in many analog, digital, and RF circuits. Some oscillator types are comprised of electrical elements interacting to produce oscillation and some have material properties that produce resonant signals when stimulated. To enhance an oscillator’s electrical behavior and consistency over time, temperature, material and element degradation, and other factors, some oscillator types also leverage digital or analog control circuitry.

LC Circuit Oscillators

Some of the original oscillators leveraged inductor and capacitor circuits to generate resonance and oscillation signals. Early radio systems used knobs and switches to adjust inductor and capacitor values to change the resonant behavior. These systems were typically large compared to other oscillator technologies and required consistent tuning to maintain the desired oscillation performance. LC Circuit oscillators have been largely superseded by crystal oscillators and other electrical circuit type oscillators found in integrated circuits.

Coaxial Resonator Oscillators

Coaxial elements can also be used to create inductance and capacitance behaviors at specific frequencies coinciding with the element and cavity geometries. These devices are limited in miniaturization by the desired frequency of operation, but typically are very stable and provide excellent phase noise performance from several hundred megahertz to a few gigahertz.

Crystal Oscillators

Crystal oscillators are comprised of crystal structures that generate resonance behavior when electrically stimulated. They are typically very stable, exhibit excellent phase noise, and can be made relatively compact. For more demanding applications, these oscillators are often incorporated into more complex analog or digital circuits with one or more oscillators and environmental control systems to provide more reliable behavior that limits frequency and phase drift over time. The frequency of oscillation is proportional to the size of the crystal, and crystal oscillators can be fabricated to resonant from several hertz to over a gigahertz.

Dielectric Resonator Oscillators (DROs)

DROs are microwave and millimeter-wave frequency oscillators that leverage dielectric resonators to produce very stable and high Q resonant behavior with very low microphonics. Typically, a puck of dielectric with a high dielectric constant and low dissipation factor is used as the resonant element of the DRO, and lick other resonant structures, the frequency of operation is related to the physical dimensions of the puck. DROs are similar to coaxial resonator oscillators/cavity resonators, but do not suffer from the increased resistive losses at higher frequencies that metal cavities do.

Phase Locked Oscillators (PLOs)

PLOs are compound devices that are comprised of an oscillator and additional control circuitry, either analog or digital, that helps to augment an oscillator’s stability. PLOs rely on adjustments to the resonant behavior or oscillator stimulus with a feedback network that corrects for, and “locks” in, the desired frequency behavior. The requirements of PLO circuitry depend on the type of oscillator, frequency requirements, spectral quality requirements, power output, and other environmental factors.

Voltage Controlled Oscillators (VCO)

A VCO is an oscillator that can be controlled with an input voltage signal, typically a DC signal. There are several types of VCOs, including voltage-controlled crystal oscillators (VCXO) with variable capacitor (varactor) diode devices, and yttrium-iron-garnet (YIG) tuned oscillators.

Basics of Frequency Synthesizers

A frequency synthesizer is a circuit which can generate multiple frequencies from a single frequency reference. Typically, frequency synthesizers use various techniques to produce or modify the signals from an oscillator, including frequency multiplication/division, frequency mixing, or phase-locked loops. A more recent method, known as direct digital synthesis, using digitally programed methods, such as a lookup table, to create the desired output frequencies.

Phase Locked Loop (PLL) Frequency Synthesizers

PLLS are similar to PLOs, with the exception that the feedback control circuitry is additionally capable of predictably adjusting the output frequency. Like the PLO, the goal of the feedback circuitry is to lock the output phase to the input phase of the circuitry, but in a PLL, the resultant control signals are also used to stimulate a VCO. Some PLL synthesizer architectures are indirect digital synthesizers using Integer-N or Fractional-N designs.

Integer-N

This type of PLL synthesizer uses frequency multiplication with negative feedback to produce an output frequency as an integer of the frequency reference. Typically, this control function is performed with digital circuitry producing and analog voltage output controlling a VCO. An unavoidable consequence of Integer-N PLLs, is the increase in the signal’s phase noise as a function of the frequency multiplication.

Fractional-N

This type of PLL synthesizer improves on the Integer-N architecture by enabling the output frequency to be ratio of the fractional modulus and fractional channel of operation figure. Hence producing frequency resolution that is a fraction of the phase detector frequency, effectively reducing the phase noise compared to Integer-N typologies. This type of PLL, however, must be precisely designed and is of greater complexity than other PLL architectures.

Direct Digital Synthesis (DDS) Frequency Synthesizers

Recent advances in RF signal generation from digital synthesis has enabled the direct generation of RF signals from a digital reference. Typically, this would involve the creation of a digital waveform, which would then be converted into an RF signal with a digital-to-analog converter (D/A). DDS devices are often packaged as ICs, and only require a clock that allows enough headroom and accuracy to produce the desired signals.