Text preview for : Creating Custom Multitone with Keysight U8903A Audio Analyzer - Application Note 5991-2396EN c201407 part of Agilent Creating Custom Multitone with Keysight U8903A Audio Analyzer - Application Note 5991-2396EN c201407 Agilent Creating Custom Multitone with Keysight U8903A Audio Analyzer - Application Note 5991-2396EN c20140729 [8].pdf
Back to : Creating Custom Multitone | Home
Keysight Technologies
Creating Custom Multitone with
Keysight U8903A Audio Analyzer
Application Note
Introduction
A multitone waveform is an audio signal comprised of multiple tones. Figure 1 shows an
example of a multitone signal consisting of 20 tones from 1 kHz to 20 kHz. Multitone wave-
forms are widely used in modern audio measurements to test consumer and professional
audio devices, broadcast devices, and internet audio. A multitone is an ideal test signal for
an open-loop audio test where the analyzer does not need to coordinate with the genera-
tor. This feature allows an audio signal to be tested over a long distance, such as audio via
internet, or a playback device such as a mobile phone.
Figure 1. An example of a multitone signal consisting of 20 tones
Standard multitone signals have evenly-spaced tones with equal amplitudes as shown in
Figure 1. However standard multitone signal do not represent real-world test conditions,
making it necessary to create custom multitones.
For example, when testing a filter, additional tones can provide detailed characterization
of specific frequency ranges, such as the rising and falling edges of the filter curve. When
testing a pre-amplifier, it may be desirable to lower individual tone amplitudes to avoid
overloading the device under test (DUT). In other cases, it may be necessary to create
custom multitones defined by industry standards such as Dolby or DTS.
Creating a custom multitone waveform with the Keysight Technologies, Inc. audio analyzer
is quick and convenient. This application note shows the Keysight U8903A audio analyzer
can be used to create a standard multitone and then used to modify the parameters of the
standard multitone to create a custom multitone waveform. Throughout the example, bold
text is used to indicate keys on the instrument. Text in [ ] refers to softkeys presented on
the instrument's screen.
Process Overview There are four basic steps to creating custom multitone signals:
1. Create a standard (equally-spaced) multitone waveform.
2. Modify the frequency, amplitude, and phase values.
3. Verify the custom multitone waveform.
4. Save and recall the multitone waveform as an arbitrary file.
To illustrate this process, the following sections demonstrate how to create a custom, six-
tone multitone waveform as specified in Table 1.
Table 1. The parameters for the six-tone custom multitone waveform to be created.
Tone number 1 2 3 4 5 6
Frequency 2 kHz 3.5 kHz 4.8 kHz 5 kHz 5.2 kHz 5.5 kHz
Relative amplitude 10 mVrms 10 mVrms 20 mVrms 20 mVrms 20 mVrms 20 mVrms
Phase Random phase
Total amplitude 1 Vrms
Crest factor < 3.2
DC offset 0 Vrms
1. Notes:
2. The relative amplitude shown in Table 1 is a relative value. The actual amplitude of the
multitone is determined by the total amplitude setting. The sum of the individual tone
amplitudes is not the multitone's amplitude. Most multitone testing requires only the
amplitude of the whole multitone. In this example, the total amplitude is 1 Vrms, not 100
mVrms (= 10 Vrms + 10 Vrms + 20 Vrms + 20 Vrms + 20 Vrms + 20 Vrms).
3. Determining each tone's amplitude can be calculated with the formula:
Vrms = V12 rms + V 2rms+ ... + Vn2 rms. Vrms is the multitone amplitude and Vnrms is the
amplitude of number n tone. Knowing the relative amplitude of each tone, each individual
tone's amplitude can be calculated using this formula. For example the amplitude of 3.5
kHz tone is 235.7 mVrms.
03
Creating a Standard The initial step is to create a standard, equally-spaced multitone.
Multitone Waveform
1. Press Generator until AG1 appears in the top frame of the Analog Generator
window.
2. In the Analog Generator channel 1 (AG1) frame, press [Waveform] then select
Multitone from the drop-down menu and press Enter.
3. Select [Waveform Config], and then enter the following values:
[Start Freq] (start frequency): 2 kHz
[Stop Freq] (stop frequency): 5.5 kHz
[Tones Count]: 6
[Amplitude]: 1 Vrms
Use the default values for the other parameters. Figure 2 shows the AG1 frame on the
Analog Generator screen after selecting these parameters.
Figure 2. A standard multitone with 6 tones
Modifying Frequency, Using the standard multitone, in this section the frequency, amplitude, and phase will be
Amplitude, and Phase modified to create a custom multitone.
1. From the Analog Generator Waveform Config screen, press [More (1/2)].
2. Set both [Record Length] and [Waveform Length] to 8192 (points). Note: the
record length should be equal to or greater than the waveform length.
3. Press [Create Custom]. The screen will display a list of tones which will be modified.
04
Modifying Frequency, 4. Using the arrow key and Enter, select and set the frequency and relative
Amplitude, and Phase amplitude of each tone per the specifications listed in Table 1. The Phase values should
(continued) remain at the default values.
Note that the resulting tone frequency shown on the display will not be identical to
the value entered. For example, if 2 kHz is entered the listed value will be 1.9836 kHz.
This is due to the limited number of sampling points used in creating the waverform.
The frequency value is rounded to an integer number of points that have a complete
cycle within the multitone waveform. The greater the number of sampling points set
in Waveform Length, the closer the resulting value will be to the entered value. The
U8903A supports up to 32,768 points. However, the greater the number of sampling
points chosen, the longer it will take to build the waveform.
5. To modify the crest factor, press [Randomize Phase] to apply a random phase to
each tone. Most multitone testing requires the design of low-crest-factor test signals.
To lower the crest factor, press [Randomize Phase]. Alternatively, the phase of each
tone may be entered individually to generate a preferred crest factor.
6. Press [Apply Settings] and [Yes] to confirm all of the settings.
The display should look similar to that shown in Figure 3, and the values for Phase will be
ramdomized. In the list, the amplitude of each tone is the relative value.
Figure 3. Six tone multitone signal with adjusted parameters
05
Verifying the Custom 1. The multitone waveform can be verified using the U8903A's Frequency Domain
Multitone Waveform function.
2. Connect the analog generator of channel 1 (AG1) to the analyzer channel 1 (AA1) using
a BNC cable as shown in Figure 4.
Figure 4. Connecting AG1 to AA1
3. Press Frequency Domain. The U8903A will display the spectrum of a
custom multitone.
4. Press [Monitor Setting] and set [Points] to 8192.
Remember the larger the number of points, the better resolution of the spectrum, but it
will lengthen the time needed to refresh the spectrum graph.
5. Press [Return] to go back to main menu.
6. Press [Axis Settings] and then [Grid Settings].
7. Set the [Left] to 1700 Hz, [Right] to 6000kHz, and [Bottom] to