Rohde & Schwarz UPD, Application Note

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1GA16_1L.DOC ROHDE & SCHWARZ 12
4 4.4 .4 Swept Burst Measurement Swept Burst Measurement
In this mode, not a continuous but a burst sinewave signal is swept, ie varied in frequency. A whole
number of periods is measured, the burst duration is one period longer than the measurement time. This
mode is used for measuring the transmission range as with this measurement reflections in the room can
be eliminated by simple means.
Between the echo signal delay and the signal delay only the direct measurement signal is present at the
microphone. This period defines the lower frequency limit for an echofree operation: all frequencies with a
shorter signal period can be measured without any echoes. The procedure is described in detail in section
5.2.6, Frequency Response of Sound-Pressure Level and Transmission range.
4.4.1 Advantages and Main Fields of Application
•
Echofree measurements above a certain lower frequency limit also outside anechoic chambers
•
the measurement is not impaired by the echo of the preceding sweep point
•
short burst pulses are easier to support than continuous sinewave signals.
Because of the longer measurement time compared with those of other modes and the susceptibility to
interference, this mode can only be used in development laboratories.
4.4.2 Program Settings
Since the burst sweep cannot be directly selected as a mode in the device software, each sweep point
has to be set by the application software:
•
Select SINE BURST as a generator signal.
•
Set analyzer to a special RMS mode (TRIGGERED) where the measurement is automatically triggered
on detection of the signal level.
•
Same as with the sinewave sweep, vary the frequency depending on the values set for start, stop,
points and spacing.
•
The measurement time is a multiple of one period, but at least one period. Towards the upper end the
number of periods is limited such that the measurement is terminated before the first echo arrives.
•
The burst duration is one period longer than the measurement time and also an integer (to avoid DC
components).
•
The burst-on delay is constant and should compensate the internal measurement delay.
•
The input level range is fixed and maintained for all sweep points. An automatic routine defines the
level range so that the reference frequency (SNS FREQ) is optimally driven.
•
The trigger threshold at which the RMS measurement is started can be selected (TRGLEVEL softkey).
The threshold should be selected so that it is exceeded by all sweep points but sufficiently away from
ambient noise. Values around 1% are recommended.
4 4.5 .5 GO/NOGO Tests GO/NOGO Tests
For applications in production, the impedance characteristic and transmission range of a reference
loudspeaker can be recorded, tolerance values defined and DUTs checked for compliance with set
tolerances (GO/NOGO tests).
The measured characteristic of this "golden unit" is stored as a trace and loaded into the display panel as
a reference trace for the DUTs. The tolerance curves are then displayed as horizontal lines at a specified
distance above and below the 0-dB line. The advantage of this method is that deviations from the ideal
trace (0-dB line) can be directly read in dB. An ideal DUT would produce a horizontal line of exactly 0 dB.