Low-cost method for measuring the frequency content of a signal above the acquisition rate

Harvard researchers have developed a new method for determining the PSD of a signal that dramatically extends the effective detection bandwidth of current instruments. The invention circumvents previous limitations of acquisition rate and detector frequency bandwidth by modulating the signal before detection or sampling. The bandwidth of this new system is governed by the maximum modulation frequency, enabling the power spectrum of a signal to be measured above the Nyquist limit (half of the acquisition rate). This dramatic increase in bandwidth can be used to push the envelope of high frequency measurement and to realize significant cost savings in instrumentation. As an example, the method was used to transform a standard video camera into a spectrum analyzer with a frequency range of up to ~300 kHz, simply by modulating an LED light source.


There is a substantial need in science and engineering to accurately characterize various processes involving fluctuations (e.g. Brownian motion, mechanical vibrations, electronic noise). However, measurements of these fluctuations via the ubiquitous power spectral density (PSD) are restricted in frequency by instrument limitations stemming from the acquisition rate and detector frequency response. While common to all instruments, these limitations can be especially detrimental to video applications (e.g. video microscopy, single molecule measurements, microrheology), where camera frame rates are often intrinsically slow or are necessarily slowed to accommodate long exposure times (e.g. fluorescence imaging). These pervasive limitations are manifested in the inability to make certain high-frequency measurements, or in the significant expense required to upgrade to faster measurement equipment.

Intellectual Property Status: Patent(s) Pending