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Phase-Locked Loops

Phase-locked loops (PLLs) are closed-loop negative-feedback control systems that maintain the phases of two periodic signals in a well-defined phase relation. Consequently, PLLs are versatile tools for measuring and tracking a signal's frequency, for extracting a given frequency component of the original signal while eliminating noise and spurious components, or for synthesizing new signals based on the input signal.

In addition, PLLs can provide feedback to an external system to drive it at certain points of its transfer function, for example at resonance, or to synchronize two external oscillators by tracking their beat note as often done in optical PLLs. This versatility makes PLLs great tools for physics and engineering applications such as scanning probe microscopy, MEMS, NEMS and resonators, electronic engineering, optics and photonics.

PLLs for Lock-in Amplifiers

All Zurich Instruments' PLLs are implemented by means of digital signal processing on an FPGA with multiple numerically controlled oscillators available as signal sources. The phase detector is realized as the dual-phase demodulator of a lock-in amplifier with a low-pass filter that rejects many of the unwanted spectral components.

Providing a clean signal to the PID controller increases the stability of the PLL. Zurich Instruments' PLLs can realize basic PLL configurations as well as more complex measurement and control schemes with a single instrument, because the PLLs are upgrade options for our lock-in amplifiers and can run in parallel with other built-in functionality such as feedback controllers, demodulators, and data capture and analysis tools. This white paper provides a more detailed discussion of lock-in amplifiers and phase detection.

PLL functionality based on digital signal processing on an FPGA

4 PLLs for 500 kHz / 5 MHz Lock-in Amplifier

2 PLLs for 50 MHz Lock-in Amplifier

4 PLLs for 600 MHz Lock-in Amplifier

  • 50 kHz max. closed-loop bandwidth
  • 4 PID controllers
  • Phase unwrap, Auto Tune
  • 50 kHz max. closed-loop bandwidth
  • 2 PID controllers
  • Additional controllers as a separate option
  • 300 kHz max. closed-loop bandwidth
  • 4 PID controllers
  • Phase unwrap, Auto Tune

LabOne instrument control software

All Zurich Instruments' lock-in amplifiers are equipped with the LabOne® toolset that allows users to fully characterize their system with a parametric sweeper, an oscilloscope, and many other data acquisition tools. For instance, it is possible to visualize the PID error as a histogram to spot deviations from a normal distribution, which may indicate that something in the setup does not work as expected. Further, the PLL's bandwidth can be measured under real experimental conditions using a frequency modulation method, as shown in the figure to the right.

The PLL advisor in the LabOne software

User Benefits

  • An all-digital PLL integrated in a lock-in amplifier provides a straightforward implementation of phase detection, closed-loop control, and signal generation within a single instrument, thus reducing the overall complexity of the experimental setup.
  • The PID Advisor makes it possible to model the setup and calculate sensible starting parameters.
  • The LabOne toolset consisting of Scope, Spectrum Analyzer, Sweeper and Plotter facilitates an integrated analysis and monitoring of the locking quality.
  • The phase unwrap functionality over the range ±1024π expands the 'capture range' of the PLL from the typical ±π and ensures robust operation.


PID/PLL upgrade options

PID/PLL Upgrade options

Sensor characterization and control

Zurich Instruments Webinar - Sensor Characterization and Control

Boost your SPM applications

Boost your SPM Applications

Application Notes

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Optomechanical mass sensing

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Control of MEMS Coriolis Vibratory Gyroscopes

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Parametric feedback cooling

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Interferometer Stabilization with Linear Phase Control Made Easy

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Stabilizing the piezoresponse for accurate and crosstalk-free ferroelectric domain characterization via dual frequency resonance tracking


Chien, M., Brameshuber, M., Rossboth, B., Schütz, G. & Schmid, S.

Single-molecule optical absorption imaging by nanomechanical photothermal sensing

PNAS 115, 11150-11155 (2018)

Sarrafan, A., Azimi, S., Golnaraghi, F. & Bahreyni, B.

A nonlinear rate microsensor utilising internal resonance

Sci. Rep. 9, 8648 (2019)

Martin-Jimenez, D. et al.

Bond-level imaging of organic molecules using Q-controlled amplitude modulation atomic force microscopy

Appl. Phys. Lett. 117, 131601 (2020)

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