Joe Begin
Chief Test Engineer, Audio Precision
#audioanalyzer #signalgenerator #apx500

Signal Generator Regulation in APx500 Audio Analyzers – Part 2: Regula


In Part 1 of this blog post on signal generator regulation, we discussed the Auto-Set Gen Level feature which is available both in Sequence Mode and Bench Mode in the APx500 software. In this second part, we’ll cover additional generator regulation measurements in both APx500 operating modes.

Power Measurements

First a note about units for power measurements. When an analog input is selected in the APx500 software, measurement results can be displayed in any unit which is a member of the rms voltage family of units, using the units pulldown control above the result graph (Figure 1). This family includes units of watts (W), which is convenient to use when conducting measurements like power ratings.

Figure 1. Using the Units control above a result graph to view results in units of watts (W).

When watts (W) is selected as the unit, APx converts the measured rms voltage (V) to power (P) in watts using the equation , where R is the Input Reference for W(watts) as set in the Reference sub-panel of Signal Path Setup (Figure 2). This control has a default value of 8 ohms. When displaying measurement results in units of watts, it’s important to set the Input Watts reference to match the impedance magnitude of the load(s) used in the test. Otherwise, the results displayed in watts will be incorrect.

Figure 2. The watts Input Reference.

Sequence Mode Measurements

In Sequence Mode, there are three additional measurements which regulate the signal generator level:

  • -Maximum Output
  • -Maximum Output (CEA 2006)
  • -Regulated Frequency Sweep

The Generator/Analyzer configuration panels for these three measurements is shown in Figure 2.

Figure 3. Measurement configuration panels for (a) Maximum Output, (b) Maximum Output (CEA-2006) and (c) Regulated Frequency Sweep.

Maximum Output

This measurement is intended to address the classic use case of finding the rated output of a power amplifier at one frequency. As mentioned in Part 1, the maximum output power of an amplifier is typically specified as the measured output level due to a mid-frequency sine stimulus when the THD+N Ratio just reaches one percent. As shown in Figure 1a, the configuration panel for the Maximum Output panel is almost identical to the Automatically Set Generator Level dialog. The main difference is that in Maximum Output, the regulation target is fixed at THD+N. Having Maximum Output as an explicit measurement in sequence mode adds benefits such as the ability to put limits on results, add sequence steps to the measurement and have the results flow into the sequence report.

Maximum Output (CEA 2006)

This measurement specifically addresses the Dynamic Power Rating requirement in standard ANSI/CEA-2006-B - Testing and Measurement Methods for In-Vehicle Audio Amplifiers. For full-bandwidth amplifiers, the standard specifies a 1.0 kHz sine-burst signal with 20 on cycles (signal at high level) followed by 480 rest cycles (signal 20 dB lower than the on level). For bandwidth-limited amplifiers (e.g., a subwoofer amplifier) the standard specifies a 50 Hz sine-burst signal with 10 on cycles followed by 20 rest cycles. The Dynamic Power Rating is the output power delivered during the on portion of the burst signal while maintaining no more than 1.0% THD+N.

The configuration panel for Maximum Output (CEA 2006) is almost identical to that of Maximum Output (Figure 1b versus Figure 1a). This measurement uses the same regulation process, except the signal is one of the specified sine-bursts instead of a continuous sine signal. The signal processing in this case is tricky, especially for the 1.0 kHz sine-burst. To determine the THD+N at each level step, the algorithm has to first extract the on portion of the signal, then compute the THD+N based on FFT analysis. At 1.0 kHz, the 20-cycle on portion of the signal is only 20 milliseconds long, which leaves almost no time for signal transients to be eliminated. Therefore, for this measurement we recommend using DC-coupled inputs (i.e., setting the High-Pass control to DC in the Input Filters section of Signal Path Setup). Other filter settings – even the default setting of AC (< 10 Hz) can induce filter transients which take time to settle, and can interfere with the regulation process.

Regulated Frequency Sweep

As the name implies, the Regulated Frequency Sweep measurement is a stepped frequency sweep in which generator regulation is applied at every frequency step. As shown in Figure 1c, its measurement configuration panel is a blend of Auto-Set Gen Level and Stepped Frequency Sweep. It provides for regulating to a target of either RMS Level or THD+N ratio, and the regulation is performed at each frequency point. This measurement addresses the requirements of some specifications which stipulate that output power be measured over a range of frequencies. For example, ANSI/CTA 2006-C specifies that an amplifier be driven at full power (the level resulting in 1.0% THD+N) with a sine signal swept across its rated frequency range. The rated output power is taken as the minimum power measured over this entire frequency range.

Bench Mode Measurements

In addition to the Auto-Set Gen Level feature described in Part 1 of this post, Bench Mode’s very flexible Sweep measurement offers a variety of regulation capabilities. When, the Regulate each point checkbox in the sweep settings panel is checked (Figure 4a), the gear-shaped button to the right of it becomes enabled. Clicking this settings button invokes the Regulation Settings dialog (Figure 4b). At first glance, this dialog appears identical to the regulation settings dialog in other regulation measurements, such as Auto-Set Gen Level. However, there are two major differences, as shown in Figure 4b:

  1. 1. The Regulate pulldown menu includes a choice of all meters available in the current measurement context – everything from Bandpass Level to THD+N Ratio.
  2. 2. The Adjust pulldown menu also has many choices, including Generator Level or Frequency, Jitter Frequency, Jitter Level and DCX DC voltage level.

This variety of Regulate and Adjust parameter choices gives the Sweep measurement tremendous flexibility in terms of regulation. But this flexibility does come at some cost in that some combinations of parameters may not make sense. This is an advanced feature and as such, user discretion is advised.

Figure 4. (a) The Regulate each point checkbox in the Sweep settings panel and (b) the Regulation Settings dialog.


This concludes our discussion of signal generator regulation features built in to APx500 audio analyzers. The most common regulation problems can be addressed with the Auto-Set Gen Level feature available in Signal Path Setup of both Bench Mode and Sequence Mode. Sequence Mode also has a way to automate this feature, as well as three specific regulation measurements. And Bench Mode has a very flexible Sweep engine with even more regulation capabilities.