9.16.5 Why Are Frequency Synthesizers Used Instead of Crystal Oscillators?
- Why Is Frequency Stability So Important?
- What Is a Crystal Oscillator?
- Why Can't One Crystal Generate Every Frequency?
- What Is a Frequency Synthesizer?
- How Does a Frequency Synthesizer Work?
- What Is a Phase-Locked Loop?
- What Are the Main Parts of a PLL?
- What Is a Voltage-Controlled Oscillator?
- What Is Direct Digital Synthesis?
- How Does DDS Compare with PLL?
- What Is Phase Noise?
- What Is Frequency Accuracy?
- What Is Frequency Stability?
- What Is a TCXO?
- What Is an OCXO?
- What Is a GPS-Disciplined Oscillator?
- Where Are Frequency Synthesizers Used?
- Why Are Fast Frequency Changes Important?
- How Have Synthesizers Changed Radio Design?
- Why Are Frequency Synthesizers Important?
Description
Understand how modern radios generate thousands of highly accurate frequencies from a single reference oscillator. Compare crystal oscillators, PLL synthesizers, DDS, and GPS-disciplined oscillators, and learn why frequency stability is essential.
Introduction
Every radio transmitter and receiver depends upon one essential requirement: an extremely accurate and stable operating frequency. Whether a satellite communicates with an earth station, an aircraft contacts air traffic control, or a smartphone connects to a cellular base station, both ends of the communication link must operate at precisely the intended frequency. Even a small frequency error can degrade reception, increase interference, or prevent communication altogether.
In the earliest radio systems, generating an accurate frequency was relatively straightforward because transmitters often operated on only one channel. A quartz crystal oscillator provided an exceptionally stable frequency reference, and the equipment rarely required retuning. As communication systems became more sophisticated, however, radios needed to operate on hundreds, thousands, or even millions of different frequencies. Equipping a radio with a separate crystal for every channel quickly became impractical.
The solution was the frequency synthesizer. Rather than relying on numerous fixed-frequency oscillators, a synthesizer generates a wide range of accurately controlled frequencies from a single highly stable reference. Modern synthesizers allow radios to change frequency almost instantaneously while maintaining excellent stability and spectral purity.
Frequency synthesizers have become fundamental to almost every modern communication system. They are used in mobile telephones, Wi-Fi equipment, satellite terminals, navigation systems, broadcasting transmitters, radar systems, software-defined radios, and countless other electronic devices.
Why Is Frequency Stability So Important?
Every communication system operates within an allocated frequency band.
If a transmitter or receiver drifts away from its assigned frequency:
- communication quality deteriorates;
- interference with neighbouring channels may occur;
- synchronization may be lost;
- receiver performance decreases; and
- regulatory limits may be exceeded.
Maintaining precise frequency stability is therefore essential.
What Is a Crystal Oscillator?
A crystal oscillator generates an electrical signal using the mechanical resonance of a quartz crystal.
Quartz possesses highly predictable resonant properties. When placed within an electronic oscillator circuit, the crystal determines the operating frequency with remarkable accuracy.
Crystal oscillators became the standard frequency reference for radio equipment because they combine:
Why Can't One Crystal Generate Every Frequency?
A quartz crystal naturally resonates at only one specific frequency.
Although small adjustments are possible, the tuning range is extremely limited. Consequently:
- every operating frequency would require a different crystal;
- radios with many channels would become bulky;
- manufacturing costs would increase; and
- changing frequency would require replacing crystals.
As communication systems expanded, a more flexible solution became necessary.
What Is a Frequency Synthesizer?
A frequency synthesizer is an electronic circuit that generates many accurately controlled output frequencies from a single stable reference oscillator.
Instead of using hundreds of crystals, one reference oscillator supplies the required accuracy.
The synthesizer then produces the desired operating frequency electronically.
How Does a Frequency Synthesizer Work?
Although several different designs exist, most synthesizers begin with:
- a highly stable reference oscillator;
- frequency multiplication or division;
- electronic control circuits; and
- filtering.
The selected frequency is then supplied to the transmitter or receiver as required.
Changing channels simply requires changing the synthesizer settings rather than replacing hardware.
What Is a Phase-Locked Loop?
The most common synthesizer architecture is the phase-locked loop (PLL).
A PLL continually compares:
- the reference frequency; and
- the synthesized output.
If any difference exists, the circuit automatically adjusts the oscillator until both signals maintain a constant phase relationship.
This continuous feedback provides excellent frequency accuracy and stability.
What Are the Main Parts of a PLL?
A typical PLL synthesizer contains:
- a stable reference oscillator;
- a phase detector;
- a loop filter;
- a voltage-controlled oscillator (VCO); and
- programmable frequency dividers.
Together these components generate a wide range of accurately controlled frequencies.
What Is a Voltage-Controlled Oscillator?
A voltage-controlled oscillator (VCO) produces an output frequency that varies according to an applied control voltage.
Within a PLL, the control voltage is generated automatically by the feedback loop.
The VCO therefore locks precisely onto the desired operating frequency.
What Is Direct Digital Synthesis?
Direct Digital Synthesis (DDS) generates frequencies digitally rather than by continuously tuning an analogue oscillator.
Instead of varying circuit components, DDS calculates digital samples of the desired waveform. These samples are converted into an analogue signal by a digital-to-analogue converter. DDS offers several advantages:
- extremely fine frequency resolution;
- rapid frequency switching;
- precise frequency control;
- programmable operation; and
- straightforward digital integration.
How Does DDS Compare with PLL?
Both techniques have strengths.
PLL synthesizers generally provide:
- excellent spectral purity;
- wide frequency coverage;
- low phase noise; and
- high output power.
DDS offers:
- extremely fast tuning;
- exceptional frequency resolution;
- direct digital control; and
- flexible waveform generation.
Many modern communication systems combine both techniques to exploit their respective advantages.
What Is Phase Noise?
No oscillator produces a perfectly pure sinusoidal waveform.
Small random fluctuations occur in its phase. These fluctuations are known as phase noise. Excessive phase noise can:
- degrade receiver sensitivity;
- increase adjacent-channel interference;
- reduce modulation accuracy; and
- limit communication performance.
Minimizing phase noise is therefore an important design objective.
What Is Frequency Accuracy?
Frequency accuracy describes how closely the generated frequency matches its intended value.
It is often specified in:
- parts per million (ppm);
- parts per billion (ppb); or
- parts per trillion (ppt).
Modern communication systems often require accuracies of only a few parts per billion.
What Is Frequency Stability?
Accuracy describes the initial frequency error.
Stability describes how well that frequency remains constant over time despite changes in:
- temperature;
- supply voltage;
- component ageing; and
- mechanical vibration.
A highly stable oscillator changes frequency very little under varying operating conditions.
What Is a TCXO?
A Temperature-Compensated Crystal Oscillator (TCXO) automatically compensates for the small frequency changes caused by temperature variations.
Compared with ordinary crystal oscillators, TCXOs provide:
- improved stability;
- better accuracy;
- reliable operation over wide temperature ranges; and
- suitability for portable equipment.
Many smartphones and GPS receivers employ TCXOs.
What Is an OCXO?
An Oven-Controlled Crystal Oscillator (OCXO) encloses the crystal within a miniature temperature-controlled chamber.
The crystal operates at a constant elevated temperature regardless of the surrounding environment. This provides exceptionally stable performance. OCXOs are commonly used in:
- laboratory equipment;
- broadcasting systems;
- satellite earth stations; and
- telecommunications infrastructure.
What Is a GPS-Disciplined Oscillator?
A GPS-disciplined oscillator (GPSDO) combines a highly stable local oscillator with timing information received from Global Navigation Satellite Systems.
The satellite signals continually correct the local oscillator. The result is extremely high long-term frequency accuracy.
GPSDOs are widely used where precise synchronization is required.
Where Are Frequency Synthesizers Used?
Frequency synthesizers appear in almost every modern radio system.
Applications include:
- mobile telephones;
- Wi-Fi equipment;
- Bluetooth devices;
- satellite communications;
- radar systems;
- television broadcasting;
- aircraft radios;
- marine communications;
- software-defined radios; and
- test and measurement equipment.
Their versatility explains their universal adoption.
Why Are Fast Frequency Changes Important?
Many communication systems frequently change operating frequency.
Examples include:
- frequency-hopping radios;
- cellular handovers;
- software-defined radios;
- radar systems; and
- spectrum-monitoring equipment.
Fast synthesizers allow these changes to occur almost instantaneously while maintaining excellent frequency accuracy.
How Have Synthesizers Changed Radio Design?
Before synthesizers became common, multi-channel radios required large numbers of crystals and complicated switching arrangements.
Modern synthesizers allow one compact circuit to generate thousands—or even millions—of frequencies under software control. This has greatly reduced:
- equipment size;
- manufacturing cost;
- maintenance requirements;
- power consumption; and
- system complexity.
At the same time, it has dramatically increased flexibility.
Why Are Frequency Synthesizers Important?
Frequency synthesizers transformed radio engineering by allowing highly accurate and stable frequencies to be generated electronically from a single reference oscillator. They eliminated the need for large banks of crystals, simplified radio design, enabled rapid frequency changes, and made programmable communication systems practical.
Modern wireless communications—including cellular networks, satellite systems, software-defined radios, and radar—would be almost impossible without the flexibility provided by frequency synthesizers.
Summary
Frequency synthesizers generate large numbers of accurate and stable operating frequencies from a single reference oscillator. Technologies such as phase-locked loops, direct digital synthesis, temperature-compensated crystal oscillators, oven-controlled crystal oscillators, and GPS-disciplined oscillators provide the accuracy required by modern communication systems.
By replacing banks of fixed-frequency crystal oscillators with programmable electronic circuits, frequency synthesizers have made today's radios smaller, more flexible, more reliable, and capable of supporting the enormous variety of communication standards used throughout the world.
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