Rohde & Schwarz UPD, Примечание к применению

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1GA16_1L.DOC ROHDE & SCHWARZ 25
Since the burst sweep is not directly available as a measurement function in the UPD or UPL, its
function will be briefly described below:
Burst sinewave signals with a length of integral periods of the sweep frequency are used as
measurement signals (to avoid DC components). The number of burst periods is selected such that
no more periods follow after the end of the measurement. The measurement is performed with a
special RMS mode (TRIGGERED). After the start of the measurement, the mode waits for the first
burst signal to arrive (ie a selected trigger threshold to be exceeded) and then starts the RMS
measurement.
The measurement time has to be selected so that
•
integral multiples of a period (at least one) are measured to avoid errors caused by interruptions,
•
a maximum number of periods is measured to obtain a high accuracy,
•
measurements are only performed before the first echo arrives.
Echofree measurements can only be performed in the short time between the echo delay and the
signal delay. The frequency of exactly one measurement period during this time is the theoretical
lower frequency limit.
f
T T
m s
L L
e a e a
min
/
=
−
=
−
1 330
where L
a
= microphone distance, L
e
= shortest echo path.
This means that below this frequency limit also parts of the echo signal are measured. In spite of
this, these measurements still have an advantage as against the sinewave sweep. Since the signal
is switched off after the measurement and a new measurement is only started after the elapse of
the reverberation (REVERBER, F10 under MIC&ROOM), the measurements cannot be affected by
echoes from a previous measurement. In practice, the frequency limit is slightly higher because of
the delayed start of the measurement.
FIG 11: Measurement of transmission range using a burst (far field) and a sinewave sweep (near field).