BASIC MEASUREMENT SYSTEM
SLOS578 – MAY 2008
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This section focuses on methods that use the basic equipment listed below:
•
Audio analyzer or spectrum analyzer
•
Digital multi meter (DMM)
•
Oscilloscope
•
Twisted-pair wires
•
Signal generator
•
Power resistor(s)
•
Linear regulated power supply
•
Filter components
•
EVM or other complete audio circuit
shows the block diagrams of basic measurement systems for class-AB and class-D amplifiers. A sine
wave is normally used as the input signal because it consists of the fundamental frequency only (no other
harmonics are present). An analyzer is then connected to the audio power amplifier (APA) output to measure the
voltage output. The analyzer must be capable of measuring the entire audio bandwidth. A regulated dc power
supply is used to reduce the noise and distortion injected into the APA through the power pins. A System Two™
audio measurement system (AP-II) by Audio Precision™ includes the signal generator and analyzer in one
package.
The generator output and amplifier input must be ac-coupled. However, the EVMs already have the ac-coupling
capacitors C
IN
, so no additional coupling is required. The generator output impedance should be low to avoid
attenuating the test signal, and is important because the input resistance of APAs is not high. Conversely, the
analyzer input impedance should be high. The output resistance, R
OUT
, of the APA is normally in the hundreds of
milliohms and can be ignored for all but the power-related calculations.
(a) shows a class-AB amplifier system. It takes an analog signal input and produces an analog signal
output. This amplifier circuit can be directly connected to the AP-II or other analyzer input.
This is not true of the class-D amplifier system shown in
(b), which requires low-pass filters in most
cases in order to measure the audio output waveforms. This is because it takes an analog input signal and
converts it into a pulse-width modulated (PWM) output signal that is not accurately processed by some
analyzers.
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Copyright © 2008, Texas Instruments Incorporated
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