Fragen und Antworten zur Signalerzeugung mit dem Signalgenerator

Why do you need a signal generator?

A signal generator (also known as a function generator) is used in the characterization, testing and fault analysis of electronic designs and components. The signal generator is required to test whether a module or circuit (= device under test, e.g. an amplifier or filter) generates the specified output signal from an input signal. The signal generator generates the defined input signal for the device under test (DUT).

The output signal from the DUT is in turn recorded and evaluated using a measuring device. This allows conclusions to be drawn about the functionality of the module. A signal generator can be used in a variety of ways, for example to replicate sensor signals or to generate HF and serial high-speed signals - including in development and verification, for testing applications for wireless communication, GNSS and radar technology. The signal generated is either analog or digital.

What is the difference between a signal generator and a function generator?

"Signal generator" is the generic term for all signal-generating devices in measurement technology. As a rule, they are divided into subgroups, for example (arbitrary) function generators, vector signal generators, pulse generators (also pulse generators) and logic generators. The function generator is therefore a signal generator. A function generator produces periodic electrical signals with different waveforms, typically with sine, square, triangular or sawtooth oscillations. The amplitude, frequency and offset of these waveforms can be user-defined.

A digital function generator works with direct digital synthesis (DDS). The accuracy depends primarily on the internal resolution of the DDS. A quartz oscillator is usually used as the clocking source for the DDS. Any number of programmable waveforms can also be generated and output periodically using changeable memory maps. These function generators are referred to as arbitrary generators (also arbitrary function generators).

What is an arbitrary generator?

An arbitrary generator (also known as an arbitrary function generator) can generate a user-defined sequence of amplitudes. If the end of the specified list is reached, the signal waveform is repeated. A direct digital synthesis (DDS) technique is usually used for this. However, it is also possible to load (real) signals recorded by measurement into the generator and retrieve them. The complexity of the signals is limited by the device-specific sampling rate and memory depth.

Arbitrary generators are used as flexible signal sources in research and development as well as in the service sector for troubleshooting, circuit development and optimization, and also for testing purposes. More complex signals can be created on an external PC using special software programs and loaded into the generator via an interface. For example, it is possible to superimpose defined interference or noise onto a test signal. Digital signals, for example for testing serial data transmission, can also be generated with the arbitrary generator.

Example: A person's electrocardiogram (ECG) has a specific functional sequence that cannot be reproduced with the classic function generator. Such an ECG curve can be recorded with an oscilloscope, for example, and transferred to an arbitrary waveform generator and retrieved. To generate anomalies, the ECG waveform can now be modified easily by changing some data within the data set. A circuit designed to detect these anomalies can be tested with the ECG curve stored in the arbitrary generator. There are arbitrary waveform generators that are specially designed for applications in medical technology and equipped with standardized waveforms for human medicine.

What is a logic generator?

A logic generator (also known as a logic pattern generator or digital pattern generator) is a special signal generator designed to generate digital signals. It can usually be programmed arbitrarily. Depending on the application, logic generators offer different signal widths (8-, 16-, 32-bit) and can be set to the typical voltage levels in the logic sector (e.g. 5 V). Applications with the logic generator include testing and troubleshooting in digital electronics and embedded systems, the stimulation of hardware for digital signal processing and the stimulation of digital-to-analog converters.

What are the advantages of a segmented device memory?

Particularly complex communication signals, for example in the field of robotics and the automotive industry, are usually not repeated for several seconds. Function generators with a large memory depth make it possible to generate individual long signal waveforms or divide them into smaller segments. In robotics, the same signal is repeated thousands of times, but with varying rising and falling edges. This is where segmentation offers advantages: It is easier to set up and adapt flexibly and requires less storage capacity.

The arbitrary function can also be used to insert transients or signal dropouts, for example to test the reaction of a circuit to rare signal events. Segmentation makes it easier to play one-off cycles or to vary the ratio between good and bad cycles. The sequencing of signal waveforms can significantly improve test efficiency. It enables a variety of signal combinations by changing the sequence and number of cycles of each segment.

What is an additive noise signal (sum signal)?

Additive noise signals (sum signals) are suitable for testing the immunity of an electronic system. A sum signal is the superposition of several sine waves or signal forms, each of which has its own amplitude, phase and frequency. Such a test signal is specified in many test specifications because it allows circuits or components to be stimulated quickly and efficiently over the entire bandwidth. You can also determine the frequency response of a device under test for a desired selection of frequencies or carry out measurements such as intermodulation distortion. Two methods can be used to obtain an additive noise signal.

Method 1: Two (or more) function generators are connected in parallel and their amplitudes are added to form a single waveform. The clocking source and phase control of the function generators must match in order to be able to synchronize the signals.
Method 2: The waveform is created mathematically and the result is loaded into the arbitrary function generator. Depending on the test duration, individual arbitrary waveforms or a repeatable signal segment without anomalies can be generated.

How can I generate user-defined pulses with the function generator?

A function generator can also be used to generate trigger signals, clocking signals and logical control signals. A special pulse generator is not required for this. When designing a control circuit, the function generator can be used as an external source for trigger delays. Function generators generally have three functions for generating pulses: the square wave function, the arbitrary function and the pulse mode.

Rectangular function: Generation of pulses by varying the duty cycle of a square wave signal, usually between 20 and 80%. The burst mode can be used for a higher or lower duty cycle. This can be used to output single-cycle pulses of a square wave signal as well as to set the waiting time (dead time) before the next pulse sequence is sent.

Arbitrary function: If a variety of pulse sequences and patterns are required, you can use the arbitrary function to define user-defined shapes and parameters. For optimum time resolution, as many data points as possible are required to describe the pulse. With the appropriate software support, a function generator can be controlled and automated via the PC, which considerably simplifies the creation of arbitrary signals.

Pulse mode: Some function generators have an integrated pulse function. By entering the main parameters such as period, pulse width, rise time and fall time, pulses can be created particularly easily and flexibly. The burst mode can also be used to generate complex pulse sequences. In combination with a trigger delay, this enables high-precision trigger signals if an external gating signal is available.

How can I test the purity of a function generator?

Circuits that amplify, mix or modulate signals can be tested efficiently with a pure sinusoidal signal that has just a low harmonic distortion. The cleaner a function generator outputs the sine function, the more precisely errors can be detected and rectified. All periodic signals can be decomposed in accordance with Fourier as the sum of their harmonics with the respective partial amplitudes of the fundamental wave. Signal components that do not belong to the ideal signal form can be identified easily with a spectrum analyzer (distortions caused by non-linearities, interference, etc.).

The noise level of a function generator can be checked with a spectrum analyzer by testing whether the specified bandwidth corresponds to the 3-dB point and whether the generator can output sine waves or square wave functions up to the specified bandwidth. In the case of a pure sine wave, the ratio of the sum of all harmonics to the fundamental frequency (fundamental component) is the harmonic distortion of the generator (total harmonic distortion, THD).

The cabling of the measurement setup can have a considerable influence on the noise. Interference can be minimized by using high-quality test conductors that are as short as possible and suitable connectors that only transmit the desired signal to the circuit. A double-shielded coaxial cable prevents unwanted signals from coupling into the source signal. Impedance matching of the signal source, cable and circuitry eliminates reflections and maximizes the transmitted power.

What are the selection criteria for a function generator?

Whether a function generator is suitable for your application depends on technical parameters as well as the integrated and optional functionalities, among other things:

Frequency range: highest and lowest required signal frequency (decisive for bandwidth and sampling rate)
Output voltage (amplitude)
Number of output channels (analog and digital)
Accuracy: vertical resolution, spectral purity, frequency and time accuracy, jitter (results from the required signal quality)
Type and complexity of the output signals: decisive for memory depth, sequencer, modulation functions
Integrated signal functions (e.g. pulse mode and arbitrary function)
Modulation types (typical modulation types: Amplitude Modulation AM, Frequency Modulation FM, Pulse Width Modulation PWM, Amplitude Shift Keying ASK, Frequency Shift Keying FSK, Phase Shift Keying PSK, Binary Phase Shift Keying BPSK, Quadrature Amplitude Modulation QAM)
Offset range: affects the usable signal amplitude if a DC voltage component must also be superimposed
Signal outputs (ground-related, differential)
Trigger inputs
Synchronization connections: for time synchronization of the output signals with other measuring devices
Software options (operability via PC, automation, remote control, etc.)
Hardware interfaces (connection options, data transfer, etc.)

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