Using Bode Plots in APx

Created on 2013-12-16 23:59:00

Bode Plots in APx500

Q: Can the APx500 software make Bode plots?

A: A Bode plot is a graph of the transfer function of a system showing the magnitude and phase plotted on a logarithmic frequency axis. In a Bode plot, usually the phase is plotted on a separate graph above the magnitude graph, but sometimes magnitude is plotted above phase, or they can also be shown on one graph with dual vertical axes.

In APx, the transfer function of a system (a Device Under Test (DUT) or a circuit) can be measured using one of the frequency response measurements (Frequency Response, Stepped Frequency Sweep, Continuous Sweep, Acoustic Response or Multitone Analyzer). Of these, the easiest and most convenient to use is Continuous Sweep. Although it has 16 primary results, the only two of interest for a Bode plot are Gain (which is the transfer function magnitude) and Phase. Note that below the Phase graph there is a Mode control which can be used to select “relative to Ch1”, etc. For a Bode phase plot, this control should be set to “Input-to-output, wrapped”, as shown below:

APx Phase Mode Setting

As a simple example, in the attached project file, a Continuous Sweep measurement has been used to measure the frequency response (or transfer function) of an analog A-weighting filter. Note that all the primary results except Gain and Phase have been deleted, and the Gain result has been renamed to “Gain (Transfer Function Magnitude)”. These results are the components of a Bode plot.

For those users who need to see them plotted one above the other, the project file contains a report using the APx custom reporting feature to plot the phase graph directly above the magnitude graph.