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Fakultet elektrotehnike i računarstva
Zavod za elektroakustiku
EXERCISES FROM AUDIOTECHNICS
EXERCISE 1
Measurements of linear and nonlinear distortion on a power amplifier
In general, distortion can be defined as unwanted changes of audio signal during its
transmission.
We define two cases, when:
-
The output signal from an amplifier is distorted, without generating new spectral
components;
-
The output signal from an amplifier is distorted, with generating new spectral
components, which cannot be found in the input signal.
In the first case, we talk about linear, and in the second case about nonlinear signal
distortion. Along with this basic differentiation of distortion during audio signal transmission, there
are also transient distortion, which due their specific features are placed in the third group.
1.
Linear distortions
The linear distortions do not generate new spectral components, but effectively change
amplitude and time relationships among the existing frequency components in the signal, which
obtains new and different shape. Linear distortions include changes in amplitude and phase.
Amplitude distortions include changes in amplification at various frequencies. In audio
amplifiers and loudspeakers, these distortions are in general unavoidable, and occur in cases of
changes in frequency response of an audio component. Amplitude distortions are shown with
frequency response of an audio device. This frequency response shows how amplification depends
on frequency.
Phase distortions occur when phase response of an audio device changes with frequency.
These distortions occur due to changes in phase relations among signal’s frequency components.
Phase distortions usually occur when a reactive component is installed in an electronic circuit, in
order to limit or shape the frequency response. Therefore, the largest changes of phase shift will
occur around limiting frequencies, while intensity of these changes will depend on device’s quality.
Phase distortions are also called time distortions or time delay, because phase difference in
frequency domain corresponds to time shift in time domain.
2.
Nonlinear distortions
If there is no direct linear connection between input and output values we talk about
nonlinear systems, where nonlinear distortions occur, which could be harmonic and non-harmonic.
A basic characteristics of nonlinear distortions is generation of new output spectral components in
comparison to the input signal’s spectrum.
Autor: prof.dr.sc. Ivan Đurek
Sveučilište u Zagrebu
Fakultet elektrotehnike i računarstva
Zavod za elektroakustiku
EXERCISES FROM AUDIOTECHNICS
2.1.
Harmonic distortions
These nonlinear distortions occur when a nonlinear element is driven with single sine signal.
If we for the input signal take a voltage signal in the form of U ul  U sin  t , it can be seen
that in a nonlinear system output signal will include the basic harmonic, and harmonic components
with frequencies 2t, 3t, 4t, etc. The component with double frequency is called the second
harmonic, component with triple frequency the third harmonic, etc.
Measurements are made with a sine signal with fixed frequency, with rated voltage on a
rated load (rated output power), using two methods:
-
Measurements of total harmonic distortion factor (THD) with a notch filter, which
removes the basic harmonic, including level measurements of the remaining harmonics
and noise;
-
Measurements of RMS level of each harmonic, so called harmonic analysis, with an
instrument called spectrum analyzer.
If it is possible to measure harmonic components’ level, we can determine how nonlinear an
audio component is.
kuk 
U 2f 2  U 2f 3  U 2f 4    
U 2f 1  U 2f 2  U 2f 3  U 2f 4    
 100 %
Harmonic distortion factor given in percentage is called total harmonic distortions factor
(THD), and represents a value of harmonic distortions in an audio device.
2.2.
Non-harmonic distortions
These nonlinear distortions occur, when a nonlinear element is driven with a signal with two
or more spectral components. If we take a driving signal in a form of
U ul  U1 sin 1 t   U 2 sin  2 t  , output signal will together with original spectral components
include the following spectral components, 21, 22, (1±2), (1±22), (21±2), 2(1±2), etc.
Non-harmonic components in the output signal show there is an amplitude and frequency
modulation of higher frequency components with low frequency components. This means means
there are also spectral components which are result of mixing or intermodulation of two signals, and
therefore these distortions are also called intermodulation distortions.
Selective voltmeter is used to measure levels of higher frequency harmonic (Uf2) and mixing
products (Uf1±f2, etc. – measurements ti the fourth order will be sufficient). Using these measured
values it is possible to calculate intermodulation distortion factor:
m
U
 U f 2  f 1   U f 2  2 f 1  U f 2  2 f 1   U f 2  3 f 1  U f 2 3 f 1     
2
f 2 f 1
Autor: prof.dr.sc. Ivan Đurek
2
Uf2
2
 100 %
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Fakultet elektrotehnike i računarstva
Zavod za elektroakustiku
EXERCISES FROM AUDIOTECHNICS
3. Measurements
Measurements on a power amplifier will be made
using the HP 35665A spectral analyzer. This
device includes signal generator and spectral
analyzer.
3.1 Measurements of linear distortions
On the spectral analyzer load a measurement state
"THD.STA". Set the input signal, i.e. amplifier
gain, in order to have a RMS output signal, whose
level corresponds to power of 1 W on rated load
(Rload= 8 ohm).
Load measurement state "FREK.STA". Press
START button. Measure frequency and phase
response, and copy the diagrams. Normalize
signal gain to the gain on signal with frequency of
1 kHz (at this frequency, the gain will be 0 dB).
Schematics of measurements
Determine upper and lower frequency limit: fLL = ________
Autor: prof.dr.sc. Ivan Đurek
fUL = __________
Sveučilište u Zagrebu
Fakultet elektrotehnike i računarstva
Zavod za elektroakustiku
EXERCISES FROM AUDIOTECHNICS
3.2 Measurements of nonlinear distortions
This part of the exercise includes measurement of THD factor with output power at three
frequencies.
On the spectral analyzer load measuring state "THD.STA".
Set the spectral analyzer generator’s frequency according to the table below. Set the signal’s
amplitude in order to get output power according to the table. Pause the reading, and using the
cursor read level of harmonics and calculate total harmonic distortion factor. Repeat the procedure
for other powers and frequencies from the table.
Snaga
10 mW
100 mW
1W
5W
10W
THD (%) 100 Hz
THD (%) 1 kHz
THD (%) 10 kHz
Draw a diagram which shows how THD factor depends on output power for all three frequencies.
Autor: prof.dr.sc. Ivan Đurek