Digitisers use Boxcar function to bring clarity to signals

May 16, 2017 // By Jean-Pierre Joosting
Spectrum GmbH has added a Boxcar averaging function to its extensive range of modularly designed digitizers that smooths out high frequency noise to give a clear signal.

The Boxcar function is most useful in situations where signals have been oversampled and it can be used to increase vertical resolution, lower noise and improve dynamic characteristics such as signal-to-noise ratio (SNR) and spurious free dynamic range (SFDR).  It uses a mathematical signal processing function to effectively recalculate the vertical value of each acquired data point by averaging it with adjacent sample points.

In the image above, the top trace (blue) shows a sine wave with a significant level of noise. The signal displays the raw data that has been acquired by an M4i.4450 digitizer with a resolution of 14 Bits and a sampling rate of 500 MS/s. The middle trace (orange) and lower trace (red) show the effect of applying Boxcar averaging functions to the raw data using 8 and 64 adjacent points respectively. Both of the averaged traces display significantly lower noise levels.

To allow maximum flexibility, the Boxcar function allows users to select the number of adjacent points to be averaged from 2 to 256. The averaged data is subsequently stored with higher resolution by an amount that is proportional to the number of points selected. For example, selecting two adjacent points would increase the resolution of a 16 Bit digitizer to that equivalent to 17 Bits, selecting four adjacent points equates to 18 Bits and selecting the maximum 256 points would produce a theoretical 24 Bits.

“Boxcar averaging is an excellent tool for improving the measurement performance of high-speed high-resolution digitizers,” said Spectrum’s Director of Technology, Oliver Rovini. “By taking advantage of situations where signals have been oversampled, the function delivers a number of clear benefits. Not only does Boxcar averaging reduce the effect of high frequency noise but it also improves results in low noise level situations. For example, on high purity signals our own tests have shown that a digitizer’s performance, as measured by its effective number of bits (ENOB), can be improved by at least 2 Bits when using Boxcar averaging. SFDR and SNR measurements are also better by more than 12 dB.”