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The oscillator section generates all drive frequencies for operating the other
subassemblies in the vibrometer. The drive frequency for the Bragg cell is of
central importance as it directly determines the optical frequency offset in the
interferometer and thus the center frequency of the carrier signal. The phase
reference signal
ϕ
ref
for the displacement decoder is generated with a fixed
relationship to the carrier frequency f
B
.
The variable mixing frequency f
LO
is automatically set by the system control
dependent on the selected velocity measurement range.
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Technically it is just about impossible to attain all desirable and realizable
characteristics of the velocity decoder in a single universal subassembly. The
modular concept of the controller thus permits the use of two different velocity
decoders which can be installed simultaneously. The selection of the decod-
ers depends on the application. The models available are the OVD-01 and
OVD-02 which feature the following characteristics.
29'
The OVD-01 is a velocity decoder for applications in the acoustic frequency
range up to approximately 20kHz. In this range, it has excellent linearity and
accuracy characteristics. With five measurement ranges from 1 to
1,000 it covers the full dynamic range of the vibrometer with high resolu-
tion. The upper four measurement ranges can be used up to frequencies of
50kHz and higher while retaining their good characteristics, however ampli-
tude accuracy and linearity decrease with higher frequencies. The measure-
ment ranges 125 and 1,000 can be used from the frequency 0Hz
(full DC capability).
29'
The OVD-02 as a broad band decoder is suitable universally for almost all
applications in the frequency range up to 1.5MHz. Four measurement ranges
from 5 to 1,000 cover most technical applications with sufficient
amplitude resolution. Characteristic is the excellent amplitude and phase fre-
quency response with extremely good amplitude flatness up to the highest
frequencies. In the measurement ranges 125 and 1,000 the
OVD-02 can detect DC velocity components. This is useful e.g. when measur-
ing on rotating discs. When setting up the sensor head, large DC components
which might otherwise overload the measurement range can then be mini-
mized.
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