Signal Generator Terminology and Specifications

The bandwidth of a signal generator is the frequency at which the output signal is attenuated 3 dB relative to the amplitude of a DC or low frequency signal. The bandwidth of a signal source is limited by the output amplifier design or by filters in the analog output circuit. Bandwidth is one of the factors that determines the signal generator’s maximum output frequency.

For example, Figure 1 shows the frequency response of the NI PXI-5422 arbitrary waveform generator. As shown, the bandwidth of the PXI-5422 is 80 MHz, meaning the generator can reliably output signals with a frequency range of 0-80 MHz with less than 3dB attenuation. The frequency components above 80 MHz of any output signal will be attenuated greater than 3 dB.

Sample rate is the rate at which digital data is transferred from the memory to the digital-to-analog converter (DAC). A digital waveform must be updated at least twice as fast as the highest frequency of the desired signal to be accurately generated. Ideally, though, a sample rate many times greater than the signal’s highest frequency produces accurate waveforms. A higher sample rate also captures more waveform details.

For example, Figure 2 illustrates a 1 MHz sine wave generated by an ideal 20 MS/s DAC. The black line represents the actual sine wave, while the red line signifies the actual signal output from the DAC. Notice that the DAC holds its output voltage until updated. In general, the higher sample rate more accurately defines the waveform shape.