TDA2030A

Transient intermodulation distortion (TIM)

Doc ID 1459 Rev 2

15/23

The "inverting-sawtooth" method of measurement is based on the response of an amplifier

to a 20 kHz sawtooth waveform. The amplifier has no difficulty following the slow ramp, but it

cannot follow the fast edge. The output will follow the upper line in Figure 26 cutting of the

shaded area and thus increasing the mean level. If this output signal is filtered to remove the

sawtooth, direct voltage remains which indicates the amount of TIM distortion, although it is

difficult to measure because it is indistinguishable from the DC offset of the amplifier. This

problem is neatly avoided in the IS-TIM method by periodically inverting the sawtooth

waveform at a low audio frequency as shown in Figure 27.

Figure 26.

20 kHz sawtooth waveform

Figure 27.

Inverting sawtooth waveform

In the case of the sawtooth in Figure 27 the mean level was increased by the TIM distortion,

for a sawtooth in the other direction, the opposite is true. The result is an AC signal at the

output whose peak-to-peak value is the TIM voltage, which can be measured easily with an

oscilloscope. If the peak-to-peak value of the signal and the peak-to-peak of the inverting

sawtooth are measured, the TIM can be found very simply from:

In Figure 28 the experimental results are shown for the 30 W amplifier using the TDA2030A

as a driver and a low-cost complementary pair. A simple RC filter on the input of the

amplifier to limit the maximum signal slope (SS) is an effective way to reduce TIM.

TIM

V

OUT

V

sawtooth

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Obsolete

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