Boxcar Averager Option

The VHF-BOX option adds one 50/200 MHz boxcar averager unit to the VHFLI Lock-in Amplifier, increasing the measurement speed and signal-to-noise ratio (SNR) for applications that use non-sinusoidal waveforms. In contrast to using a digitizer or an oscilloscope, the measurement results from a boxcar averager are streamed continuously in the digital domain and as analog signals with user-defined offset and scaling factors.

A boxcar measurement is easily set up thanks to the LabOne^® user interface: the boxcar gate start and width can be set by visualizing the input signal directly on the Scope or Sweeper. A second baseline window can be set to reduce baseline fluctuations and enable differential measurements. To achieve the best trade-off between measurement speed and SNR, the result can be averaged over consecutive periods between 1 and 1 million times.

Lock-in vs boxcar

While lock-in amplifiers are ideal for analyzing sinusoidal signals and extracting specific components of a frequency spectrum, boxcar averagers are the tool of choice for low-duty-cycle signals (see figure). A typical input signal from a pulsed experiment has a pulse width T_p, repetition period of the pulses T_rep, and duty cycle d. In boxcar averaging, the input signal is multiplied with a boxcar function, also know as rectangular pulse train, with pulse width T_Box. In this way, noise contributions outside of the boxcar window are rejected (see panel c). Inside the boxcar gate, signal integration results in a single value per period later averaged over N periods.

Data extraction from low-duty-cycle signals requires information from the base frequency and many higher harmonics simultaneously. To capture this additional information, boxcar averagers need a signal input bandwidth that is many times larger than the base frequency, so that setting up a boxcar measurement requires careful control of more parameters compared to a lock-in measurement. The payoff usually comes in the form of shorter measurement times and higher SNR, which can make a big difference in areas such as imaging. Therefore, lock-in measurements and boxcar measurements are complementary tools for analyzing periodic signals.