1GA16_1L.DOC
ROHDE & SCHWARZ
9
4
4 Measurement Modes
Measurement Modes
4
4.1
.1 Modes of Frequency Response Analysis
Modes of Frequency Response Analysis
Measuring the frequency response of various parameters is particularly important in development and
production testing. Such parameters are for instance:
•
electrical transfer function of crossover units,
•
acoustic transfer function of loudspeakers,
•
impedance characteristic,
•
sound pressure characteristic,
•
phase characteristic and group delay,
•
distortion.
These parameters can be measured by the frequency sweep of a sinewave signal and displayed in a
tabulated or graphical form. Each curve may contain of up to 1024 single measurements with linear or
logarithmic scaling as required.
Frequency sweeps are very time-consuming because of the generator setting time, DUT settling time,
measurement time and, for acoustic measurements, the signal travel time involved. Particularly at low
frequencies, settling time and measurement time may be too long when it comes to alignments or
production testing.
All measurements based on voltage measurements may also be carried out with a noise generator and an
FFT analyzer. The (quasi)-noise generator of the UPD/UPL uses discrete sinusoidal lines and may be
synchronized to the FFT, ie the sinusoidal audio signals produced by the generator are of the same
frequency spacing as the FFT (called FFT noise in the text below). Consequently the measurement signal
at the display edges of the FFT can be continued without any transitions that no window function is
required for suppressing the FFT continuation error and an optimum frequency and level accuracy of the
FFT is obtained.
Measurement time and frequency resolution depend on the selected FFT size or on the frequency
stepwidth of the noise generator.
The smallest step of the 25-kHz generator is 5.86 Hz. This is therefore the maximum permissible
resolution for the FFT analyzer although a much better resolution could be obtained in the analyzer (with
the aid of ZOOM FFT). It is advisable to use 8 k FFT where the measurement speed still allows several
measurements per second to be made. The measurement speed can be further increased by reducing
either the frequency resolution or the FFT size.
At the generator side, generation and phase optimization of the noise signal involves much more
computation work than the generation of a simple sinewave signal. If all available sinusoidal lines are
used, the noise signal is available after about 1 minute. However, this time is only required once and any
number of highly accurate measurements can be carried out thereafter. By reducing the FFT size
(analyzer) or the frequency range (generator) the number of discrete sine tones can be reduced and the
signal generation time drastically shortened.
Note
: The highest resolution (2.9296875 Hz at 8 k zoom FFT) makes only sense in the UPL, as the
UPD is not able to track a resolution of < 5.859375 Hz because of its higher internal generator
clock rate.
The frequency sweep and the FFT noise mode each have their pros and cons so that both modes are
employed (as far as possible) for the measurements described in the Application Note.
Содержание UPD
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