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Chapter 6
Advanced Concepts
NI-DSA Software User Manual
Auto-ranging extends the dynamic range and auto-level increases the signal
to noise ratio. If the system's transfer function has large variations within
the measurement span, a swept-sine analysis can often give you greater
dynamic range than an FFT analysis. Because the FFT measures all the
frequency components at the same time, the source must contain energy
at all of the measured frequencies. Generally each individual component's
amplitude is about 30 dB less than the overall source amplitude in the time
domain. So each frequency is actually measured at –30 dB relative to full
scale. This effectively reduces the dynamic range of the measurement. A
swept-sine measurement, on the other hand, can eliminate the 30 dB loss
because it measures one frequency at a time, and this frequency can be
applied full-scale.
With auto-ranging, swept-sine measurements can be further optimized for
each frequency according to the output level of the device under test, so the
signal level is as close to full scale as possible. Optimizing the input range
at each frequency rather than applying the frequency-rich time-domain data
can extend the dynamic range of the measurement to beyond 140 dB. For
example, if the transfer function has both gain and attenuation, the
auto-level control will adjust the source level to compensate for the output
of the system. If the system under test has gain at one certain frequency, the
amplitude of the sweep source can be reduced automatically to prevent the
output of the system from overloading the DSA input channel. At the points
with significant attenuation, the source level can be increased to boost the
system output level to provide a better signal to noise ratio. Point-by-point
optimization gives swept-sine analysis a great advantage over FFT
measurement.
Using auto-resolution mode can greatly speed up your swept-sine
measurements. Often, you want to measure a frequency response with a
large span and you want to preserve the frequency resolution. With the FFT
method, the only option is to increase the size of the FFT. This consumes
memory and increases measurement time. In auto-resolution mode, even
though a large number of points is still needed to increase the frequency
resolution, the analyzer can skip points in the range where the frequency
response is flat, and proceed step-by-step where a sharp transition occurs.
This results in fewer measurement points without a loss of frequency
resolution.
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