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Phase-Locked Oscillators

Key Takeaways

  • Phase-locked oscillators are oscillators in which the phase of the output signal is locked to a reference signal.

  • The advantages of using indirect synthesis in phase-locked oscillators are that they are less complex than direct synthesis and offer low noise and spurious signals.

  • The implementation of a phase-locked oscillator depends on the output frequency, reference frequency, phase noise, spurious signals, and harmonic limits. 

Phase Locked Oscillator

      The frequency of the oscillator signal output is dependent on the phase difference between the reference and output

In radar and communication applications, frequency generation is necessary. Phase-locked oscillators are employed in such applications as stable frequency sources. The phase-locked oscillator offers features such as low phase noise and spurious signals and is the preferred choice for frequency synthesizing applications. Let’s read more about phase-locked oscillators in this article.

Phase-Locked Oscillator

Phase-locked oscillators utilize phase-locked loops to generate oscillations. The phase of the output signal is locked to a reference signal, which can be either internal or external. The operating principle of a phase-locked oscillator is that the phase difference between the oscillator output and input reference is detected, and the frequency of the oscillator is controlled with respect to this phase difference.

Phase-Locked Oscillators Function as Frequency Synthesizers

Phase-locked oscillators can be considered fixed-frequency synthesizers. The phase-locked oscillator generates stable output frequency, irrespective of vibrations, aging, temperature variations, etc.

When employed as frequency synthesizers, phase-locked oscillators can be implemented using three topologies:

  1. Direct synthesis
  2. Direct digital synthesis
  3. Indirect synthesis

Generally, phase-locked oscillators are implemented using indirect synthesis, where the output frequency is a rational multiple of the reference frequency.

Advantages of using indirect synthesis in phase-locked oscillators:

  • Less complex than direct synthesis
  • Low noise and spurious signals

The next section explains the indirect synthesis topology of the phase-locked oscillator along with a block diagram representation.

The Basic Configuration of Phase-Locked Oscillators

Block diagram of Phase-Locked Oscillator

The block diagram given above shows the basic configuration of phase-locked oscillators using the indirect synthesis topology. In the block diagram, the reference signal is generated by a quartz crystal oscillator. The phase of the crystal oscillator-generated signal is compared with the feedback signal.

The output frequency signal is divided into the frequency range of the reference signal and then fed back. The feedback signal frequency is a part of the output frequency. A phase comparison is made between the feedback signal and the reference signal. The voltage-controlled oscillator in the block diagram is tuned with respect to the phase difference, and the output signal is generated.

Components of Phase-Locked Oscillators



Quartz Crystal Oscillator

The standard reference frequency for the PLL oscillator is developed by the crystal oscillator. Usually, quartz crystal oscillators are preferred when the frequency range is between 10 to 100 MHz.

Phase Comparator

Phase comparators are made up of logical gates and flip-flops. Whenever there is a phase difference between the reference and feedback signal, pulses are generated for that time duration.

Low Pass Filter

Low pass filters consisting of capacitors are used to smooth the tuning voltage to VCO. They also reduce the reference of current peaks.


To obtain the feedback signal, which is in the frequency range of the reference signal, the output frequency is divided using a pre-divider.


Counter circuits allow the VCO to oscillate at frequencies higher than the reference frequency.

Features of Phase-Locked Oscillators

Some of the features of phase-locked oscillators are:

  • Precise frequency control
  • High-frequency stability
  • High accuracy
  • Superior flexibility 
  • Low phase noise
  • Excellent short-term stability
  • Less interference of extraneous and spurious signals
  • Output signal strength maintained throughout
  • Immunity to environmental conditions such as humidity, temperature, etc.
  • Minimized EMI radiations, preventing the antenna operation of the system 

Phase-Locked Oscillator Implementation

When implementing the phase-locked oscillator, there are a few requirements to be considered. The implementation of a phase-locked oscillator depends on:

  1. Output frequency
  2. Internal or external reference frequency
  3. Spectral quality parameters such as phase noise, spurious signals, and harmonic limits

Applications of Phased-Locked Oscillators

In various applications, phase-locked oscillators are employed. A few of them are discussed below.



Power electronic converter control

To accurately control the timed firing pulses to the static power electronic converters interfaced with the grid, phase-locked oscillators are commonly used. The use of phase-locked oscillators in the pulse timing control system offers exceptionally low harmonic generation.

Satellite communication

Ku and Ka-band frequencies are widely utilized for satellite communication. Phase-locked oscillators can be implemented in Ka and Ku band applications.


There are instruments, such as spectrum analyzers, network analyzers, test equipment, etc., that depend on phase-locked oscillators for operation.

5G systems

The proliferation of 5G infrastructure is increasing, and 28 GHz is taken as the standard carrier frequency of 5G systems. Phase-locked oscillators are employed in 5G applications for various operational purposes.

Phase-locked oscillator applications are not limited to telecommunication, medical devices, industrial automation, aerospace, and defense systems. Cadence AWR design platform supports the design and simulation of phase-locked oscillators.

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