Unveiling Supercon 2024: Delving Deeper into Signals with Oscilloscopes, Beyond the Noise Barrier
Discovering elusive signals hidden beneath the noise floor can be a challenging endeavor, but with the right tools and techniques, it's possible to extract these faint signals with precision. James Rowley and Mark Omo revealed the secrets of lock-in amplifiers at the 2024 Superconference, demonstrating how to utilize this powerful technique to unearth signals that would otherwise be impossible to detect.
Noise is an unavoidable obstacle when trying to detect weak signals. However, the magic of lock-in amplifiers allows you to reject all noise, focusing on the signal you're interested in. This technique, explained James, can be executed using an analog-to-digital converter and digital signal processing (DSP) that you may already have at your disposal, such as the oscilloscope in your workshop. Essentially, a lock-in amplifier acts as an ultra-narrow bandpass filter, enabling the measurement of signals even when they have a negative signal-to-noise ratio.
In his talk, James provided a simple analogy to explain lock-in amplification's principles. Imagine a speaker and a microphone—in an ideal world, the microphone would pick up only the sound from the speaker, but real-world noise sources can often swamp the desired signal. With a lock-in amplifier, you would be able to isolate just the sound from the speaker, effectively rejecting all other noise sources. This powerful technique is applicable to a wide range of applications, from measuring extremely sensitive signals from load-cells to detecting heart catheter locations during complex medical procedures.
Mark then delved into the digital signal processing tricks required to find signals hidden beneath the noise floor. By heavily filtering out noise outside the area of interest, it's possible to effectively increase the signal-to-noise ratio and identify the desired signal, even if it's quite faint. The challenge is finding a filter narrow enough to exclude noise while still passing the relevant signal information. To achieve this, the measured signal is first shifted down to zero hertz and averaged over time. This approach, though somewhat unconventional, can be used for signals of any given bandwidth with the help of a reference signal to maintain phase coherence.
Building a lock-in amplifier with hardware is possible, but it tends to be expensive and fussy. As an alternative, you can use an analog-to-digital converter, like the one found in a common oscilloscope. The better the ADC in your oscilloscope, the better its performance. Something like the Rigol DS1054Z has enough memory depth to achieve a 1700x reduction in noise, making it a viable option for those looking to get started with lock-in amplification. For the curious, the code for this technique is available on GitHub.
The talk concluded with a practical demonstration of lock-in amplification in action. A microphone and speaker were set up with a known phase shift of 90 degrees between the input signal and the output. Despite being in a loud room with a full crowd, Mark and James were able to accurately measure the phase shift of the desired signal with their lock-in amplifier setup.
If you frequently struggle to measure weak signals that you know are present in your environment, you may find these techniques highly valuable. This talk serves as an excellent starting point for leveraging the power of digital signal processing to uncover signals hidden beneath the noise floor.
Equipment in science and technology, such as an analog-to-digital converter and digital signal processing (DSP), can help in rejecting noise and unearthing weak signals with precision, just like a lock-in amplifier. These tools can be utilized in various applications, even when dealing with signals that have a negative signal-to-noise ratio, making them highly valuable, especially for detecting faint signals.
