Top 10 Tips for Your Boxcar Measurement

Experiments involving the measurement and characterization of pulsed, low-duty cycle signals are becoming increasingly common, especially in -- but not limited to -- the optics and photonics application realm. Boxcar averaging turns out to be the best measurement approach to analyzing low-duty cycle signals, thanks to the ability to capture the information present at the fundamental frequency and higher harmonics.

In this blog, similar to what we presented in this blog for lock-in measurements, we share a compact compilation of ten tips to boost your boxcar averager measurement with the Zurich Instruments 500 kHz/5 MHz MFLI Lock-in Amplifier equipped with the MF-BOX option or the UHFLI 600 MHz Lock-in Amplifier equipped with the UHF-BOX Boxcar Averager option. 

1. Use about 50% of the input range

Sacrifice 1 bit of your ADC for protection against clipping. It's better to feel safe than to worry about signal clipping all the time. Also, the input amplifiers work best in this range, where they show the smallest distortion. The "Auto Range" function of the MFLI and UHFLI lock-in amplifiers will automatically set a 50% input range.

2. Remove the DC offset from the input signal

A large DC component at the input can deplete most of the instrument’s dynamic range before the signal is analyzed by the boxcar, resulting in reduced measurement performance and limited resolution. Setting the input to AC coupling adds a high-pass filter on the input path that removes the DC offset. The cut-off frequency of the high-pass filter on the UHFLI 600 MHz lock-in amplifier with the UHF-BOX Boxcar Averager option is 320 kHz at 50 Ohm input impedance and 80Hz at high input impedance.

3. Optimize your boxcar gate width and position for maximum Signal-to-noise ratio (SNR)

Start with a large boxcar gate centered around the peak of your signal, and then reduce its width until the SNR peaks. 
It is often the case that the SNR is maximized for a boxcar gate width smaller compared to the full width of the input signal pulse. Thanks to the LabOne Periodic Waveform Analyzer (PWA) and plotter tool, you can easily adjust and optimize the width and position of the boxcar gate while simultaneously monitoring the SNR in real-time. The position of the gate window can be chosen such that the entire pulse is captured or only a part of the pulse that carries the relevant information.

4. Always use the baseline suppression window, even when no reference pulse is present

For fully differential measurements, e.g., with two pulses in the same period, the boxcar gate should be placed around the pulse of interest and the reference window around the reference pulse. With the reference window enabled, the output of the boxcar unit will then subtract the integrated signal during the baseline window from the integrated signal during the boxcar gate. 
For the case when no reference pulse is present, setting the reference window somewhere in the period (sufficiently far away from the boxcar gate) removes additional DC and/or slowly varying offset. This leads to a general increase in the SNR and the rejection of disturbing input signals that are phase-shifted with respect to the signal of interest at the fundamental frequency

5. Use the appropriate number of averaging periods according to the dynamics of your signal

If the dynamic of your signal is slow, suppress as much noise as possible by increasing the number of averaging periods until the SNR reaches an adequate value. If you’re interested in performing fast measurements (e.g., in imaging applications) or pulse-to-pulse variations instead, set the averaging periods to 1 --  i.e., no averaging is performed, so that the integrated value of each individual pulse is output from the boxcar unit.

6. Reference the boxcar averager to a subharmonic of the repetition rate if you are only interested in one of many pulses or want to perform differential measurements of consecutive pulses

In some instances of pulsed experiments, one might be interested only in the information of one pulse among many. A very common scenario is when every second pulse carries the relevant information about the parameter of interest (e.g., optical pump-probe with modulation at ½ the repetition rate of the laser). In these cases, choosing a submultiple of the reference frequency isolates a specific number of pulses, enabling setting up the boxcar gate only around the relevant one.

7. Use the Arithmetic Unit to perform operations between different boxcar units

The UHF-BOX Boxcar Averager offers two independent boxcar units, each with baseline suppression, and a built-in Arithmetic Unit. The Arithmetic Unit can provide real-time results based on a combination of two parallel boxcar measurements with arbitrary scaling factors, for example, in a pump-probe experiment to capture the relative pump-induced intensity variations on the probe pulses on a shot-to-shot basis by normalizing the result with a second boxcar measurement.

8. If you are not sure whether lock-in amplification or boxcar averaging leads to the best measurement result, compare the SNR of the two methods firsthand

Owing to its digital nature, the UHFLI and MFLI lock-in amplifiers offer both functionalities in the device. This allows you to compare the performance of lock-in amplification and boxcar averaging consistently and in parallel using the same instrument, making the comparison straightforward and enabling you to make an informed choice for your experiment. For more information on this topic, you can have a look at this blog post or this video.

9. Directly stream the boxcar output to the PC, when possible

Direct data stream to the host PC removes the need for an additional signal acquisition system, reduces your complexity, and makes your setup more robust. The UHF-BOX on the UHFLI 600 MHz lock-in amplifiers allows for sustained data rates, up to 3.5 MSa/s. In case of a measurement bandwidth higher than the maximum data rate of the instrument, one can route the boxcar result to the Auxiliary Output. 

10. If your pulsed signal is modulated, perform a subsequent lock-in measurement directly on the boxcar output

The measurement result of the boxcar averager units can be easily internally rerouted to one of the demodulators of the lock-in unit. This allows the user to extract the amplitude of an additional modulation on the pulsed input signal. Importantly, the averaging bandwidth, inversely proportional to the number of averaging periods, must be set so as to avoid the suppression of the modulation. An overview of this procedure is described in this blog post.