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A2 Audio Test & Measurement System
User Manual
User Manual
A2 Audio Test & Measurement System
Noise Function
The Noise function of the A2 measures noise according to the CCIR Recommendation 468/3
and 468/4, in either absolute or relative terms. The signal path in the Noise function includes
either the Audio bandpass filter or the weighting filter defined in the above standard. The internal rectifier of the A2/A2-D always measures Quasi-peak in function Noise.
The Quasi-peak rectifier is a RC-network that has a very short attack time but quite a long
decay time. This type of rectifier especially weights the spikes in the noise signal, so that the
result corresponds closely to the audibility of the signal.
Nevertheless, some applications (e.g. signal -to-noise ratio require to measure noise RMS
rectified, maybe even with a specific weighting filter. The A2/A2-D can also perform these
measurements in the LEVEL function with the corresponding filter (e.g. A-weighting).
To perform a Noise measurement, apply the signal to be measured to the active input connector and press the <NOISE> key.
Fig. 21 Noise Weighting Characteristics
Each further press to the <NOISE> button will toggle between the absolute and relative
Noise measurement function, denoted by NOISE and N.REL in the display. Select NOISE for
absolute measurements.
The METER display shows the result together with the weighting mode on top, along with the
generator / input level and frequency.
If a signal consisting of random noise is applied to the A2/A2-D, the frequency reading will
not be stable, since no periodic signal is found.
Units
Applications
Noise measurements are helpful if the noise content of the signal is dominant, e.g. to
measure the minimum output level of an amplifier. Please notice, that it is essential to terminate the inputs of the DUT properly, in order to avoid the introduction of external noise.
NOTE Do not use the Noise function to evaluate the Signal-to-Noise ratio. The
reason is, that the Noise mode of the A2/A2-D measures according to the
CCIR standard (peak-weighted), while for the S/N ratio, a RMS-weighted
measurement has to be executed (see p. 48).
The available units in function Noise are dBq and Vq, where the "q" stands for Quasi-peak
rectified. The reference voltage (0dBq) for the Quasi-peak rectifier is 0.775V.
Filters
Noise measurements can be done either weighted or unweighted, whereof in both modes a
specific weighting filter is active. This is necessary according to the standards, since unlimited bandwidth would lead to an unlimited high noise signal.
• Press the <WTD UWTD> key to enter the weighted mode, where the CCIR-468/3 filter is
active (LED WTD lights up).
• If the unweighted mode is selected, the 22Hz-22kHz Audio Bandpass is activated
automatically (LEDs UWTD and 22 -22k are lit).
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