Periodic Waveform Analyzer

Zurich Instruments introduces a new class of instruments consisting of a unique combination of new and existing capabilities.

A New Class of Instruments

The Periodic Waveform Analyzer (PWA) is a type of synchronous detector which is useful for the analysis of periodic signals. Synchronous detection comes in a variety of different flavors and is a useful method to optimize signal-to-noise ratio when extracting signals from a noisy background. Different synchronous detection schemes all make use of the fact that the signal of interest is periodic and the associated frequency is a known reference. Lock-in detection and boxcar averaging are two practical examples of synchronous detection schemes, each tailored to a specific signal type - sinusoidal signals for lock-in detection and low duty cycle signals for boxcar averaging.

The PWA fits into this landscape by taking a continuous stream of samples and associating them synchronously to the actual phase of the reference oscillator. Additionally, the samples are averaged with the previous samples with the same phase. Doing so over many periods of the reference frequency, leads to a dense set of phase and sample pairs, where the phase is a value between 0 and 2π. This is a kind of digital signal averager for periodic signals. The plot of the averaged values with the phase on the X axis can be called phase domain representation.

The PWA also overcomes the limit imposed by the physical sampling rate of the ADC by several orders of magnitude, similarly to a sampling oscilloscope.

Zoom into any Periodic Waveform

The UHF-BOX Boxcar Averager categorizes the phase values into 1024 bins, leading to a waveform with sub-degree (i.e., 2π/1024 or 0.4°) resolution, providing a phase domain representation. If this resolution is not sufficient, one can reference the input signal to a higher harmonic of the reference frequency which allows zooming into the region of interest and hence increasing the temporal resolution down to millidegree (m°). That gives a precise analysis for pulsed signals with low duty cycles or any other periodically repeating transient.

Boxcar or Lock-in? Ask the Harmonic Analyzer

The PWA makes it easy to select the most efficient signal recovery method. The duty cycle and the signal energy that is available in the first harmonic scale almost linearly. For instance in a rectangular signal pulse with 50% duty cycle only 1/3 of the signal is in the first harmonic, and if the duty cycle is divided by a factor of two, also the signal energy in the first harmonic is halved.

The FFT performed on the PWA dataset yields the spectral distribution of the signal amplitude over the higher harmonics of the reference frequency. In case of the UHF-BOX PWA, this is the energy distribution at DC plus the first 511 harmonics of the reference frequency. Hence the user can easily determine if lock-in amplification or boxcar averaging is the more appropriate measurement method. For instance, if one sees the signal spread out over many harmonic components without any prominent peak, a boxcar detection scheme might be the wiser choice to achieve best possible signal to noise ratio.

Averaging of Periodic Signals (Signal Averager)

The reference frequency for the PWA is either provided by the instrument itself or, when externally generated, carefully tracked with a phase-locked loop of adjustable bandwidth. Compared to an analysis performed with a standard oscilloscope this offers the significant advantage of averaging without trigger jitter. Even more important is the possibility of dead-time free detection, whereas many real-time oscilloscopes have a considerable re-arm delay after each triggering event. In comparison, the UHF-BOX PWA can capture and average a data stream of 10 seconds or more, corresponding to 800 million periods for an 80 MHz laser without missing a single sample.

Low Duty-cycle Signal Analysis

The scope-like functionality of the PWA is also helpful to select a window of interest for boxcar measurements. In the phase domain representation one can easily pick the region that contains the valuable signal and, in case the resolution of 512 bins is insufficient, one can reference the input signal to a higher harmonic of the reference frequency which allows zooming into the region of interest and increasing the temporal resolution as much as is needed.

 

Summarizing, the periodic waveform analyzer is a versatile and powerful measurement instrument, that provides a lot of value when combined with instruments like a lock-in amplifier, a signal averager or a boxcar averager. The PWA replaces a sampling oscilloscope in many setups and could be part of any set of standard measurement instrumentation.

call +41 44 515 04 10
or leave us a message.