Fast-Track Your Sensor Research: Essential Tools for Accelerated Testing - Q&A
I teamed up with Jim Phillips in this webinar to share essential workflows to help you advance your sensor research and meet project deadlines. We covered various topics, including the key measurement steps required to identify a sensor's optimal operating conditions and several control strategies, such as Phase-locked Loops (PLL) and Pound-Drever-Hall (PDH). We also discussed how efficient workflows and the correct instruments can enable sensing yoctograms and attonewtons.
The event was recorded, and the video is available here. This blog post answers the many and much-appreciated questions asked by the audience during the webinar.
We also provide a practical guide for understanding the working principles of lock-in amplifiers and feedback controllers:
Lock-in detection is crucial in extracting sensor signals from background noise. It can be visualized as discerning the sound of an analog watch ticking amidst the roar of a jet engine. For a detailed discussion and rigorous mathematical treatment of the lock-in amplifier, check out our Principles of lock-in detection. Another crucial element in sensing is controlling the sensor's state to ensure the optimal operation conditions using feedback loops. Our Principles of phase-locked loops and Principles of PID controllers provide a practical guide.
Questions and Answers
What is the primary use of a lock-in amplifier?
Lock-in amplifiers are used where a periodic signal must be separated from noise that is either random or at other frequencies.
They have been used for many decades and have taken the data for 1000s of papers and many Nobel Prizes. They are primarily used where the signal-to-noise ratio is low, but also where it is higher and where high accuracy is required. A modern digital lock-in amplifier can serve many other functions in developing the experiment: oscilloscope, parameter sweeper, mapping software, PID and PLL feedback loops, and even an arbitrary waveform generator.
What does demodulation mean, and what role does it play in lock-in detection?
Suppose that our signal is at 1 kHz. Demodulation is the process of converting this signal to DC. In the process, the lock-in amplifier determines the amplitude and phase of the signal's 1 kHz component. Demodulation is at the heart of lock-in detection.
How do you select the low pass filter bandwidth of the lock-in amplifier?
The setting of the low-pass filter's -3 dB frequency is a balance. Setting it to a low value improves noise suppression, while setting it to a high value increases the measurement speed. You can monitor both aspects using LabOne's data analysis tools and tune the low-pass filter to your needs. For example, a faster measurement is required when mapping rapidly, as in a scanning probe microscope, while keeping a satisfactory signal-to-noise ratio.
Does the reference signal for the lock-in detection need to be a sine?
There are two modes of operation with lock-in amplifiers when it comes to the reference signal. The internal reference mode is when the lock-in amplifier itself generates the reference signal. This signal is used both in demodulation and in generating the drive Signal. Zurich Instruments' lock-in amplifiers use precise numerical oscillators for pure sine waveforms to eliminate any error in the demodulation process. This ensures precise measurement of the amplitude and phase at the frequency of interest. You can also supply the reference signal externally, called the external reference mode. In this mode, the lock-in amplifier can take on various waveforms as long as its main periodicity corresponds to the correct frequency. However, such a signal consists of various frequency components and would lead to imprecise results if used directly for demodulation. In their external reference mode, Zurich Instruments' lock-in amplifiers automatically lock to the main frequency component of the external waveform and use their pure sine generators for the demodulation to achieve the best performance.
Does LabOne provide a way to do a 2D scan?
LabOne's Sweeper allows a parameter sweep while measuring with the demodulators. It handles various aspects, from configuring the sweep grid to averaging for accurate results. Scripting 2D sweeps using the APIs is possible with a for loop to step one parameter while calling a sweeper module to sweep the other parameter at each step.
Can we measure neuromorphic sensors?
It is possible; however, understanding application requirements is crucial for the correct measurement solution. Often, neuromorphic sensors rely on pulse sequences to bring the sensor to the proper state. Our lock-in amplifiers and arbitrary waveform generators can achieve this.
How are the disturbances and errors analyzed in the sensors? Are they ignored as the optimum drive signals are determined?
Various approaches depend on the sensing scheme and the device's properties. For example, Allan variance measurements are helpful when working with frequency shift sensing to assess phase and frequency noise. Don't hesitate to contact us to find the right approach for your sensor; we will be happy to help.
Can we use your instruments for educational purposes?
Absolutely. We are happy to collaborate in teaching activities. We can provide material to support teaching or organize workshop events with our experienced application scientists.
How can car sensors detect if the driver is close to hitting something, especially when it is dark outside?
Collision avoidance systems are highly regulated, covering all perceivable scenarios and environmental conditions. They use various sensing instruments, including, but not limited to, cameras with image recognition, radar, and lidar. Radar is unaffected by darkness, and optical approaches can employ a spectrum beyond visible light to operate at night.
Where could we get the LabVIEW program?
The LabOne software also provides LabVIEW API libraries with various example codes and tutorial scripts. The API allows you to control LabOne functions from your LabView program. The installation guide is in our user manual and can be found via this link.
Can we use lock-in detection for analog pressure sensors?
Pressure sensors work in static or dynamic mode and are sensitive to different variables. In dynamic mode, especially in the presence of an AC signal, lock-in amplifiers are crucial in filtering the signal of interest.
Can you tell us more about the PDH measuring three frequencies? How does it improve the measurement?
The common method for frequency shift sensing is using phase-locked loops and PLLs. A PLL measures at a single frequency and relies on the phase shift due to the device under test, DUT. However, this is prone to errors resulting from spurious phase shifts such as those due to stray capacitance and phase noise in low-temperature microwave amplifiers. Pound-Drever-Hall sensing, PDH, on the other hand, measures at three closely-spaced frequencies. The spurious phase shift is largely common to the three signals used. The PDH signal takes the differences of the three signals' phases, so the errors cancel. Also, PDH provides a means for the continuous and real-time measurement of linewidth, or Q, which a PLL does not.
Does the latest APS paper detail your method of determining Q by demodulation at twice the modulation frequency?
The APS paper discusses this, and you can listen to the recorded lecture. For a written treatment, see this blog. Contact us for more details.
Are there situations where the feedback may bias the measurement results?
It is possible. For example, we must rely on a phase setpoint when using a PLL to track a resonator. This is a powerful yet simple technique in most situations. However, if the DUT's phase delay at its resonance drifts due to external factors, the results based on a constant setpoint become inaccurate. It is essential to have a measurement toolset that allows monitoring and identifying such issues. We provide these with the LabOne software and feedback control-capable Lock-in amplifiers. A flexible toolset to enable other feedback schemes is also beneficial when such an issue occurs. For instance, PDH lock, as we described during the webinar, is significantly less sensitive to such spurious phase shifts. We are happy to help you find the best approach.
When studying mode-coupling of a NEMS resonator, is it possible to simultaneously measure the amplitude response at f,2f, 3f,… etc., while sweeping the driving frequency (f) around its fundamental frequency (in sweeper mode)?
Yes. Zurich Instruments' lock-in amplifiers incorporate multiple oscillators and demodulators, allowing harmonic analysis and parametric drive schemes. For the first, a single numerical oscillator can simultaneously generate reference signals to demodulate fundamentals and harmonics. This ensures perfect synchronization out of the box in such simultaneous measurements. It is also possible to use LabOne's sweeper tool to realize such sweeps on the fundamental tone while measuring at harmonics. Depending on the instruments, this might require a software upgrade. Please get in touch to discuss possibilities.
Do you have a blog post/tutorial for resonance tracking of MEMS/NEMS resonators using PLL?
Yes. Please look at the Principles of phase-locked loops with the associated video as a practical guide to using PLLs. The principles explained can be applied in many applications, including MEMS/NEMS resonator tracking. Beyond frequency tracking with PLLs, additional PID controllers are used to control amplitude. This scheme is called automatic gain control, and the Principles of PID controllers also provide insights on this topic.
Thanks for conducting the webinar!
A number of you wrote to thank us for conducting the webinar. We, in turn, thank you for those notes. We greatly enjoy this work and love sharing our knowledge with our community. It feels great to hear that you appreciated the presentation.