Usually, signal generators produce electrical waves that repeat in a controllable manner. Each full repetition of a wave is known as ‘period.’ A waveform is actually a graphic representation of the wave’s variation over time.
Amplitude. Amplitude is a measure of the voltage strength of the waveform. Amplitude of an AC waveform periodically changes with time. Signal generators allow you to set a voltage range over which the signal continuously fluctuates with a frequency selected by you. The various amplitudes of interest in a waveform are peak amplitude, peak-to-peak amplitude and RMS amplitude (peak amplitude/√2), as indicated in Fig. 2.
The amplitude specification for a common signal generator is 10-20V (open-circuit) peak-to-peak with 50 ohms impedance. 10V peak-to-peak sine wave has an RMS voltage of 10/√2, which implies that the generator will be able to deliver only 10/50 √2 amperes of RMS current at peak output. If you need more current or voltage than what is specified above, you will have to use a suitable amplifier or go for specialised function generators. So carefully check whether this specification suits your requirement.
For frequency response testing, amplitude flatness is also important. Amplitude flatness means that the fixed amplitude does not vary with the change in frequency.
Frequency. This is the rate at which full waveform cycles occur. Frequency is measured in Hertz (Hz), also known as cycles per second. It is inversely related to the wavelength (shown in Fig. 3 as point 4) of the waveform, which is a measure of the distance between two similar peaks on adjacent waves. The higher the frequency of the waveform, the shorter the time period.
The frequency specification of a signal generator is the highest sine wave frequency that the generator is capable of delivering. Many signal generators will have a lower maximum frequency specification for other waveforms. Here the minimum frequency that the generator can deliver is also important. For some applications, you will need very low frequencies. Signal generators with 1-micro-hertz resolution are readily available.
Phase. Phase in sinusoidal functions or in periodic waves has two different but closely related meanings. One is the initial angle of a sinusoidal function at its origin, and is sometimes called phase offset or phase difference. Another usage is the fraction of the wave cycle that has elapsed relative to the origin. Refer Fig. 3, where the phase difference between the red and blue waves is ‘θ.’
Phase shifts are common in electronics. The amplitude, frequency and phase characteristics of a waveform are the building blocks that a signal generator uses to optimise waveforms for almost any application. These are implemented as controlled variables in all the signal generators.
Rise and fall times. Rise and fall times, also known as edge transition times, are characteristics usually described for pulses and square waves. These are the measure of the time it takes for a signal edge to make a transition from one state to another. With the advancement in technology, these values have typically gone down to nanoseconds range.
Both rise and fall times are measured between 10% and 90% points of the static voltage levels before and after the transition as shown in Fig. 4. You will need a functionality in the signal generator where you can vary the rise time and fall time independently for a waveform while testing applications such as an amplifier with asymmetrical slew rates. So look for a signal generator that supports this function.
Pulse width. Pulse width is the time elapsed between the leading and trailing edges of a pulse. Pulse width measurement is expressed as the time between the 50 per cent amplitude points of the respective edges. This is an important variable function for a signal generator, which you will need while experimenting with servo motor, LED intensity, etc. Fig. 5 shows a signal with different duty cycles.
DC offset. Many signals have their amplitude variations centred above or below 0V reference. ‘Offset’ voltage is the voltage between circuit ground and the centre of the signal’s amplitude Variation. In effect, the offset voltage expresses the DC component of a signal containing both AC and DC values. This DC component needs to be handled carefully in most of the designs that require a stimulus system like signal generator in the design phase, which can vary this component in a controlled manner to test the implementation.