Have you ever wondered what is going on inside your Zurich Instruments lock-in amplifier? Lock-in amplifiers enable precise measurements of small signals buried in noise. While analog instruments have been used for decades, the development of analog-to-digital converters with high speed, resolution, and linearity has more recently enabled the realization of digital instruments where all signal processing is carried out numerically. This development helped to push the frequency range, input noise, and dynamic reserve to new limits. In addition, digital signal processing is much less prone to errors introduced by a mismatch of signal pathways, to cross-talk, and to drifts caused by temperature changes, for instance. The availability of field-programmable gate arrays (FPGA) with high computing power, abundant memory, and high speed has enabled to build instruments that contain several lock-in amplifier units, PID controllers, boxcar averagers, and arbitrary waveform generators - all in one box. In this blog post, we provide an overview of how this is done in a Zurich Instruments lock-in amplifier and illustrate how this implementation enables to perform many measurements in parallel.
Instrument Architecture
The heart of a Zurich Instruments lock-in amplifier is the FPGA where the entire signal processing is implemented. The FPGA is connected to an analog and digital interface as shown in Figure 1. This interface include the signal inputs, signal outputs, auxiliary and trigger channels and a digital DIO port. At the input, the signal is digitized using an analog-to-digital-converter (ADC) and then routed to the FPGA where it can be split into several paths. In the digital domain, a duplication of the signal is possible without any signal loss, which constitutes a great advantage over analog instruments where the analog signal has to be divided. Duplication of the digital signal in combination with the high processing power of the FPGA enables the operation of many signal-analysis tools in parallel.
Zurich Instruments' lock-in amplifiers are entirely computer-controlled through the LabOne® software. In addition to providing a variety of possibilities to display and analyze the results, this functionality is also useful when working remotely or discussing measurements with colleagues. The instrument is connected to a host computer where the dedicated LabOne Data Server is in charge of all communication to and from the instrument. Data from the FPGA is continuously streamed via USB or Ethernet to the LabOne Data Server, which distributes it to all the clients that subscribe to it. A client can be the web browser-based LabOne user interface or the application programming interfaces (APIs). It is important to note that the data server runs locally on the host computer, so no internet access is needed to operate the instrument and the data never goes out of the user's control. The LabOne software contains a set of versatile time- and frequency-domain analysis tools such as an oscilloscope, data plotter, parametric sweeper, and FFT spectrum analyzer. Additional functionality can be obtained on the instrument with upgrade options that increase the number of demodulators or add PID/PLL controllers, boxcar averagers or an arbitrary waveform generator. The measurement results from different analysis tools can be saved as numerical files; they can also be provided as analog signals on one of the Auxiliary Output channels and can be routed to another signal processing unit on the FPGA.
