Bandwidth and Passband Flatness

The bandwidth of a signal source is defined as the frequency at which the amplitude of the frequency response is 3 dB lower than the amplitude of the frequency response at DC or a low frequency. The bandwidth of a 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 capability of the source to create signals with specific frequency content.

Note  On some NI signal generators you can enable or disable the filters.

Passband flatness is a measure of the amplitude accuracy of the frequency response with respect to frequency. Passband flatness is usually specified in ±dB, and it is usually referenced to the amplitude of the frequency response at a designated frequency.

For example, a specification might be listed as ±1 dB with respect to the amplitude of the frequency response at 50 kHz. This method is used because two different metrology instruments, a digital multimeter (DMM) and a power meter, are used to measure passband flatness. The power meter is an excellent metrology instrument for measuring passband flatness, but its performance can be improved by calibrating its frequency response at a low frequency, such as 50 kHz, with a DMM. In other words, the DMM measures the amplitude of a 50 kHz tone, and the power meter measures the amplitude of all other frequencies with respect to the amplitude of the 50 kHz tone measured by the DMM.

Passband flatness is important in many applications. For example, if the sensitivity of a receiver is being tested, it is important to know the variation of the amplitude of the test tone as it is swept across the frequency band of interest. Some NI signal generators have a Direct Output analog path that has been optimized for passband flatness. Others allow you to select a frequency at which the calibrated amplitudes can be finely adjusted to achieve the best amplitude accuracy near the selected frequency.