Lock-in amplifier with optimal noise suppression

Lock-in amplifiers are firmly anchored in everyday research. They offer excellent noise suppression for repeating signals. The degree of suppression, however, depends critically on the low-pass filter at the end of the signal processing chain. Until now, simple narrow-band filters are generally used which have an infinite settling time and a suboptimal capability of supressing a white noise background (Fig. 1, showing 6dB, 12dB, 18dB, and 24dB resistor capacitator (RC)-filters). A German university strives to overcome these problems. They are developing soft rectangle filters that offer a finite settling time and probably the best root mean square (RMS) spectral width for a desired suppression of a white noise background (Fig. 2). Reaching the same noise levels, these filters considerably cut down measurement times. Fig. 3 compares output of a lock-in amplifier using the new soft rectangle filter with a standard 24 dB RC filter after a switching event. The soft rectangle filter shows a 1.52 improvement in settling time. Even a factor of up to 2.3 compared with a first-order RC-filter and a factor of up to 3.4 compared with a fourth-order Butterworth-filter can be obtained (for a typical 99%-settling time). Moreover, the finite settling time completely eliminates contaminations of the measured data point by a previous data point. The dramatic speedup of measurements will result in a more economic use of expensive lab equipment and personnel and allow for the measurement of even weaker signals on a noise background. Lock-in amplifiers are widespread in general laboratory environments, the semiconductor industry, and healthcare as a growing market for their use . A lock-in amplifier device that provides the soft-rectangle window for cutting down measurement times will certainly have a clear advantage over the standard RC-filters. Thus there is good market potential a lock-in amplifier with an improved signal-to-noise ratio. The technology is at concept stage. Partners from healthcare or semiconductor industry with an interest in integrating the solution are sought for licensing and technical cooperation for the further development of the solution.