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.
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.
- 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.