Rohde & Schwarz UPD - Page 22

1GA16_1L.DOC ROHDE & SCHWARZ21
5.2.1.1 Near-Field Measurements
For a near-field measurement the microphone is positioned as closely as possible to the loudspeaker.
Thus an extremely high signal level is obtained and reflections are negligible in most cases.
Microphone and loudspeaker should be placed in the center of an imaginary sphere, the surface of which
touches the nearest wall.
Near-field measurements can also be performed in rooms that are not completely sound-absorbing.
However, the results obtained are only valid at low frequencies. The equation
2
⋅ ⋅ <π λ
r
,
λ =
c f/
yields a frequency limit of
f
c
r
max
=
⋅ ⋅2 π
with r being the radius of the loudspeaker.
5.2.1.2 Far-Field Measurement
For far-field measurements, the distance between microphone and loudspeaker should be at least 3-times
the loudspeaker size. In practice, this distance is typically 1 m.
Microphone and loudspeaker should be placed at the focuses of an ellipsoid, the surface of which touches
the nearest wall.
Since the far-field measurement is very sensitive to echoes it should be performed either in the open field
or in an anechoic room. A time-selective measurement with burst signals is also recommended.
5.2.2 Adjusting the Output Power
The output power is adjusted by means of the POWERAMP softkey (F11, level 3) which opens a softkey
menu where the user can select manual or automatic control and make necessary entries.
In both cases the nominal loudspeaker impedance has to be entered (REF IMPE, F6). The selection
between the manual and automatic mode is made by pressing softkey MANU (F11) or AUTO (F12) which
starts the measurement or control. After the start a new softkey level is displayed which offers only the
STOP softkey (F10) for terminating the measurement.
Manual mode: After starting the power measurement the user has to adjust the amplifier exactly to the
desired power. In this case the UPD generator outputs the constant voltage selected under VOLTAGE
(F6, level 3). The amplifier output power is constantly measured by the UPD analyzer and displayed in a
bargraph (trace B) and as a numerical value (function window of channel 1).
Automatic mode: The desired output voltage is adjusted through control of the generator output voltage.
The user has to enter the desired power (POWER softkey, F8) and the maximum permissible amplifier
input voltage (MAX VOLT, F9, for protecting the power amplifier). After the start (AUTO, F12) the
generator output voltage is regulated (starting at 1 mV) using the specified values until the STOP softkey
is pressed. The current output power (measured value) can be read in the display or seen from the
bargraph. The power amplifier can be set before the start of the measurement routine and/or during the
execution.
Recommended: First set the power amplifier with MANU to roughly the required power (preferably to
a slightly higher value) and then trigger the fine adjustment of the generator voltage with AUTO.