36
H. Vallen
voltage UF. The transmitting sensitivity is obtained by using the laser vibrometer
setup described hereafter.
3.5 Basics of the LVM Setup
LVM stands for Laser Vibrometer. The LVM-setup is used for determining the
transmitting sensitivity spectrum of a transmitter. The setup is similar to the F2F
setup. Instead of a SUT and a transient recorder the LVM measures the motion pulse
generated by the transmitter. Figure 3.2 shows the block diagram of the LVM setup.
Since the LVM is less sensitive than a SUT the transmitter (TM) needs a stronger
excitation pulse in order to cause a sufficient signal-to-noise ratio of the LVM output.
A high voltage amplifier (x50) has been used which delivered the transmitter voltage
UL, a pulse amplitude of almost 100 V (peak). Since this is too high a voltage for
the reference channel of the LVM, a high impedance voltage divider (39:1) has been
used to feed UL/39 into the reference channel. The synch output of the AFG triggers
the LVM to measure both, the motion pulse VL and the reference channel. 200
measurements per position have been averaged for an optimal signal-to-noise ratio.
Since the SUT converts the average motion over its sensitive face, the transmitter’s
motion has been measured at 145 positions arranged in 9 concentric rings of 16
positions each and the central position. In this way 9 velocity signals, averaged
over 9 different diameters have been derived, so that for each SUT a velocity signal
averaged over a diameter near to that of the SUT’s sensitive face can be selected for
sensitivity determination.
The motion pulse is measured at the free face of the transmitter. The sensor’s
receiving sensitivity shall be scaled to the motion at the free surface of the test object
(according to E 1106). The acoustic impedance of the test object has influence on
the sensitivity and must be considered. Annex B of the initial draft deals with the
consideration of the acoustic impedance of the material the sensor shall be mounted
to.
The reference channel output UR and the velocitv output VF are processed by
FFT with a resolution of 1.22 kHz.
Fig. 3.2 Block diagram of
the LVM-setup
H. Vallen
voltage UF. The transmitting sensitivity is obtained by using the laser vibrometer
setup described hereafter.
3.5 Basics of the LVM Setup
LVM stands for Laser Vibrometer. The LVM-setup is used for determining the
transmitting sensitivity spectrum of a transmitter. The setup is similar to the F2F
setup. Instead of a SUT and a transient recorder the LVM measures the motion pulse
generated by the transmitter. Figure 3.2 shows the block diagram of the LVM setup.
Since the LVM is less sensitive than a SUT the transmitter (TM) needs a stronger
excitation pulse in order to cause a sufficient signal-to-noise ratio of the LVM output.
A high voltage amplifier (x50) has been used which delivered the transmitter voltage
UL, a pulse amplitude of almost 100 V (peak). Since this is too high a voltage for
the reference channel of the LVM, a high impedance voltage divider (39:1) has been
used to feed UL/39 into the reference channel. The synch output of the AFG triggers
the LVM to measure both, the motion pulse VL and the reference channel. 200
measurements per position have been averaged for an optimal signal-to-noise ratio.
Since the SUT converts the average motion over its sensitive face, the transmitter’s
motion has been measured at 145 positions arranged in 9 concentric rings of 16
positions each and the central position. In this way 9 velocity signals, averaged
over 9 different diameters have been derived, so that for each SUT a velocity signal
averaged over a diameter near to that of the SUT’s sensitive face can be selected for
sensitivity determination.
The motion pulse is measured at the free face of the transmitter. The sensor’s
receiving sensitivity shall be scaled to the motion at the free surface of the test object
(according to E 1106). The acoustic impedance of the test object has influence on
the sensitivity and must be considered. Annex B of the initial draft deals with the
consideration of the acoustic impedance of the material the sensor shall be mounted
to.
The reference channel output UR and the velocitv output VF are processed by
FFT with a resolution of 1.22 kHz.
Fig. 3.2 Block diagram of
the LVM-setup