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National Instruments Corporation
5-1
NI PXIe-5450 User Manual
5
Signal Generation
Fundamentals
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.
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.
