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Sum Function Capability Makes Testing Easy

2020-03-04  |  9 min read 

Sum Function Capability Makes Testing Easy

If you want to test a device, product, or system properly, you need a signal that consists of the sum of two waveforms. Here are some examples:

  • Telephones produce dual-tone sine wave signals when receiving and sending information. You can test the receivers in these telephony systems by subjecting them to various dual-tone input signals.
  • Audio amplifiers cause small amounts of distortion of their input waveforms. You can evaluate the distortion introduced by the amplifier using an input signal consisting of the sum of a square wave and a sine wave.
  • Clock signals with noise can cause timing errors. You can test immunity to noise on a clock signal by creating a signal that adds noise to a square wave.

The testing required in each of these examples uses a signal consisting of the sum of two waveforms. This blog post covers these examples and provides a simple method for creating summed waveforms using the Keysight Trueform Series waveform generator.

Testing Telephony DTMF Decoders

We are all familiar with the typical tones telephone systems use. Each time we press a phone key, we hear a tone through the earpiece. The system typically uses these tones to route a call from one location to another. Other telephony applications, such as entering passwords and commands into an answering machine, interacting with an automated banking system, or traversing the branches of a business phone tree, use these tones. If you listen carefully, you will notice that each tone is actually two frequencies, each a pure sine wave, playing simultaneously. These are dual-tone multi-frequency (DTMF) tones.

The tone you hear is a combination of one low-frequency tone and one high-frequency tone. It is determined by the intersection of the row and column selected by pressing a phone key. See Figure 1. The A, B, C, and D keys depicted in the figure are not on a standard phone, but they are part of the DTMF push-button phone definition.

Figure 1. DTMF keypad frequencies — the sum of one low-frequency and one high-frequency sine wave represents each of the 16 keys (four rows by four columns)

In the telephony world, standards exist for these tones so that all the systems that use them work together gracefully. To ensure that the DTMF decoding electronics in these systems work properly, you must test the decoders by subjecting them to all forms of DTMF generation tones.

The Trueform Series waveform generator has a sum function that allows you to add an internally or externally generated signal to the primary signal on a single channel. If the primary signal is a sine wave and the added sum signal is also a sine wave, the combined waveform is exactly what you need to generate and control a DTMF tone for testing decoders. You can easily adjust the amplitude, frequency, and duration of each of the dual tones to the limits specified in the regulatory standard.

Testing Audio Amplifier Intermodulation Distortion

When applying one sine wave signal to a nonlinear system, the output of the system contains harmonics related to the frequency of the input signal. When applying multiple sine wave signals to a nonlinear system, the output of the system contains harmonics of the input signals and intermodulation products related to the sums and differences of the input frequencies. This is known as intermodulation distortion. This intermodulation distortion can create problems on the output of the system. For example, if the nonlinear system is an audio amplifier and the intermodulation distortion is high, the sound coming from the speakers driven by the audio amplifier will be harsh. Dynamic intermodulation distortion is particularly disruptive. You should evaluate it on systems such as audio amplifiers.

One method for evaluating dynamic intermodulation distortion is to provide a square-sine wave input to the nonlinear system and examine the response of the system on the output using a spectrum analyzer. Using the sum function in the Trueform Series waveform generator, you can create the required square-sine wave signal. Figure 2 shows an example. With the summed signal applied to the input of the amplifier, the spectrum analyzer shows the amplitudes of the intermodulation of the output waveform. You can use that to calculate the total intermodulation distortion.


Figure 2. Square-sine waveform showing a 3.18 kHz square wave summed with a 15 kHz sine wave. The summed waveform serves as the input signal to test audio amplifier transient intermodulation distortion.

Testing Clock Signal Noise Immunity

Many electronics include digital circuitry or a microprocessor. A clock signal is responsible for pacing and coordinating the actions of the circuits. Any noise that appears on the clock signal can cause problems with the circuits it is controlling. Problems include performance degradation, loss of transmitted data, undesired sounds in audio, and unwanted image effects in a video.

The sources of noise are thermal, shot, and flicker. Interference noise includes cross talk, electromagnetic interference, and switching devices. Ensuring that the circuits connected to your clock signal are immune to possible noise sources is imperative to guarantee the proper operation of your circuit. Therefore, it is important to test your circuits with noise intentionally added to the clock signal.

Consider a square-wave clock subjected to a source of white noise (additive white Gaussian noise), corrupting the otherwise clean signal. The circuits driven by this clock could be adversely affected, depending on the magnitude of the noise relative to the clock signal magnitude. Once again, using the sum function of the Trueform Series function generator, you can easily produce a noisy clock signal, as shown in Figure 3:

  • Figure 3a shows the clean square-wave clock produced by the Trueform Series waveform generator.
  • Figure 3b shows the same clock signal with a small amount of white noise added using the sum function.
  • Figure 3c shows the same clock with a large amount of white noise added.

                            3a                                               3b                                                     3c

Figure 3. A clock signal showing no added noise (3a), a small amount of added noise (3b), and a large amount of added noise (3c)

Trueform Series Sum Function Simplifies Test

Many test applications require a signal consisting of two waveforms added together. DTMF testing requires two sine waves summed together. You can use the sum of a sine wave and a square wave to evaluate audio amplifier intermodulation distortion. You can evaluate immunity to noisy clock signals by summing noise with a square wave. In each of these tests, use of the sum function in the Trueform Series waveform generator greatly simplifies the test by providing a method for creating a signal consisting of the sum of two waveforms. Making adjustments to the frequency and amplitude of each waveform is easy. You can test your device against all the extremes of these variable parameters.

To learn more about Keysight’s Series Trueform waveform generator, please go to