Essential tools for Accelerated Sensor Testing
In our recent sensors webinar, Kivanç and Jim explored how Zurich Instruments lock-in amplifiers and LabOne software have revolutionized sensor development processes. This blog post summarises the webinar, focusing on the importance of lock-in detection, the integration of essential tools to complement the lock-in amplifier, and our commitment to providing unparalleled support.
The webinar outlines the typical characterization workflow for sensor testing, such as the importance of analyzing your signal before demodulation, and it then goes on to explain how lock-in amplifiers can not just provide precise and repeatable measurements of small signals buried in noise, but also how they can be used to set up feedback loops to control and stabilize your sensor. All this can be done with one instrument, and at the pre-ASIC stage to speed up your development.
Why are lock-in amplifiers pivotal in sensing applications?
Lock-in amplifiers allow us to capture the tiny signals coming from from sensors amidst a large amount of noise and disturbances, much like finding a needle in a haystack. However, this critical step is just the starting point for effective sensor development.
Lock-in amplifiers provide an essential toolset for analyzing signals with efficient workflows. By isolating the desired signal from background noise, these devices boost the signal-to-noise ratio, ensuring that every subtle measurement is precise. This capability is crucial when dealing with signals that are easily overwhelmed by ambient noise, such as those found in low-frequency environments or high-sensitivity applications like quantum computing.
A lock-in amplifier works by modulating the sensor signal and detecting it at a specific frequency, filtering out noise and disturbances that do not match the modulation frequency. This process not only enhances the clarity of the measurement but also significantly improves the precision and reliability of the data collected. The result is a clean, precise signal that can be analyzed and used to inform further development steps.
Integrating tools with LabOne software
To fully harness the power of lock-in detection, Zurich Instruments offers the LabOne control software, a comprehensive platform that integrates all necessary tools for sensor characterization and control. LabOne provides a robust user interface, designed to streamline the entire measurement process.
LabOne facilitates on-the-fly analysis through various features like fitting tools and trend analysis. All tools within LabOne can operate in parallel on a single screen, or across multiple screens. Once downloaded, it does not need any internet connection to ensure privacy and flexibility. It harnesses the power of your browser, giving platform independence to ensure that LabOne is not only convenient but also adaptable to any working environment, whether you're in the lab or out in the field.
Moreover, LabOne seamlessly integrates with five programming libraries (Python, C, .NET, LabVIEW and MATLAB), allowing for efficient integration with your experimental setup and facilitating automation. This means you can easily customize and automate your workflows to fit specific project requirements, saving valuable time and reducing the potential for human error.
Additionally, LabOne includes an advanced toolset, including and oscilloscope and spectrum analyzer, which are integral for characterizing the frequency response of sensors. These tools allow you to observe the behavior of the sensor in both the time and frequency domains, offering a comprehensive understanding of its performance.
Phase-Locked Loops and Feedback Control
Ensuring stable and precise sensor operation often requires sophisticated feedback control systems, like Phase-Locked Loops (PLLs). PLLs play a crucial role in maintaining the reliability of sensor measurements by locking the oscillator to the resonator's frequency. This stability is essential for achieving accurate and reproducible results.
To define a feedback loop and show how to set one up on a digital lock-in amplifier, consider a Phase-Locked Loop (PLL) as an example.
In a PLL, we start with an oscillator signal that we feed to a resonator. The lock-in amplifier measures the phase of the signal returning from the resonator, and this phase varies through the resonance. This allows us to set our position relative to the resonance accurately.
We choose a set point, which is the phase we’d like the PLL to maintain. Then, we take the measured phase and subtract the set point to create an error signal. This error signal is fed into a PID (Proportional-Integral-Derivative) controller, which adjusts the oscillator frequency to sit at the desired point relative to the resonance. When this is achieved, we say that the oscillator is locked to the resonator.
Here is the workflow for setting up a PLL with a digital lock-in amplifier:
- Using the sweeper tool, we measure the transfer function of the device under test.
- Use the math tools in the sweeper to calculate the resonance frequency and linewidth.
- Open the PID tool in the LabOne software and specify the type of loop, its input, and output. Input the resonance parameters are obtained from the sweeper tool in step 1.
- Use the PID advisor to generate PID coefficients, and copy these coefficients into the PID controller.
- Activate the PID controller to lock the oscillator to the resonator.
Once the loop is set up, you want to ensure it is locked and operating correctly. The LabOne software includes a lock indicator and if more detail is needed, you can plot the resonance frequency over time in the Plotter tool to monitor performance. You can also test the step response by adjusting the set point abruptly. This allows you to fine-tune the PID coefficients, optimizing the step response for a quick reaction without overshoot or ringing.
Practical Outcome of Feedback Control
Consider a concrete example of how a PLL and automatic gain control improve sensor measurements. Initially, without control, the signal fluctuates significantly and lacks stability. By enabling the PLL, we achieve a stable response. With the addition of automatic gain control, the amplitude becomes even more stable. These two feedback loops work in tandem, enhancing the sensor’s sensitivity and response time, which is crucial for applications requiring high precision and stability.
Practical Applications
To illustrate the practical applications of these technologies, let’s explore three real-world examples:
- Optical Balance for Aerosols: particle in an optical trap, schematic as shown in figure 2. This setup involves an optical trap using counterpropagating Gaussian beams to hold a particle at the center. We modulate the balance between the lasers and demodulate the signal using a lock-in amplifier. The phase response of this system, modeled as an overdamped harmonic oscillator, provides insights into the particle’s mass. By fitting the phase response, we can determine the damping coefficient, resonance frequency, and hence, calculate the mass of the particle. This method allows us to measure the mass of saturated particles that are difficult to assess in a beaker.
- Nanomechanical cantilever-based mass sensing: this example involves a cantilever with multiple modes as shown in the schematic in figure 3. We use frequency response analysis to characterize each mode's frequency, quality factor, and phase response. Using multiple PLLs running in parallel, we can track phase shifts for each mode when a particle lands on the cantilever. These shifts are different for each mode due to the particle’s position. By analyzing this data, we can determine the added mass and its distribution with high resolution. This approach allows us to visualize individual particles absorbed on the cantilever surface.
3. Capillary suspended resonator: in this setup shown in figure 4, particles pass through a suspended capillary tube that acts as a double-clamped resonator. The actuation is monitored using reflected laser light. The amplitude (blue curve) and phase (red curve) responses are used to measure frequency shifts. Due to the quality factor (Q), frequency shifts directly relate to phase changes, unaffected by the Q factor. This setup measures the buoyant mass of particles passing through. To overcome the challenge of buoyant mass sensitivity, we employ optical scattering techniques. Different particles scatter light differently, allowing us to deconvolute the mass spectrum obtained from frequency measurements using the scattered light signal, revealing distributions of different particle types.
These examples demonstrate how various techniques can be applied to similar systems to accurately characterize and control sensors. The combined use of open-loop and closed-loop systems enhances the precision and efficiency of sensor measurements, making these techniques invaluable for advanced sensor development.
Advanced Sensing Techniques
Advanced sensing techniques like Pound-Drever-Hall (PDH) sensing, offer precise measurements of linewidth and quality factor (Q) of a resonator. PDH sensing employs feedback control to lock an oscillator to the resonance frequency of a resonator, providing a robust method to measure Q accurately.
There are several methods to measure linewidth, with the frequency response analyzer being one common approach. In this method, the resonator is expected to stay at the same frequency while the analyzer sweeps across it. However, if the resonator frequency varies during the measurement, it can lead to a false estimate where the linewidth appears too broad and the Q factor too low. Variations are most likely when you have a long measurement time at low power, where the signal-to-noise ratio is low, necessitating longer integration times.
PDH sensing offers a second-order measure of Q by detecting the second harmonic. This approach provides a direct measure of Q, with the signal peaking at the center of the resonance. This peak goes through a maximum, and to first order, there is no variation of the signal when the resonance shifts in frequency. To keep the oscillator centered on that resonance, we use a feedback loop.
By implementing this technique, we achieve a more accurate measure of Q. Employing PDH sensing with feedback control allows us to better lock the oscillator to the resonator and obtain a precise measurement of the quality factor.
Traditionally, amplitude modulation is detected using a power detector diode or a nonlinear device, which measures the power. However, our approach leverages digital lock-in amplifiers, which offer distinct advantages for PDH sensing:
- Signal generation: digital lock-in amplifiers generate frequency-modulated signals over a wide dynamic range, which is especially useful for low-power measurements needed to study two-level systems (TLS) responsible for noise.
- Direct measurement: instead of using a nonlinear detector, digital lock-in amplifiers directly measure these carriers, providing a clearer and more accurate analysis.
Our Commitment to Support
Finally, our greatest asset at Zurich Instruments is our team. We have PhD application scientists with backgrounds in many fields relevant to sensor development, ready to offer premium support to our customers. We are passionate about sharing our experience to help you accelerate your sensor development projects.
Our team’s expertise enables us to provide tailored advice and solutions, ensuring that you can navigate even the most complex sensor development challenges. From initial material selection and characterization to advanced feedback control and final implementation, we are here to support you every step of the way.
By leveraging Zurich Instruments’ cutting-edge technology and expert support, you can significantly streamline and enhance your sensor development process. We invite you to challenge us with your project details and look forward to collaborating with you to achieve your goals. Please get in touch to set up a demo or to learn more about how Zurich Instruments can hemp you accelerate your sensor development.