D-4
D Functional Description of the Controller
Down-mixing
of the
frequency
Down-mixing of the frequency in the input section is required to convert the
carrier frequency of the FM signal from 40 MHz originally to lower
intermediate frequencies. With these intermediate frequencies, the velocity
decoder can work optimally in the individual measurement ranges. Down-
mixing is carried out by a mixing process which does not affect the modulation
content of the FM signal. The variable frequency f
LO
is produced by the so-
called local oscillator in a fixed relationship to the drive frequency of the
Bragg cell.
D.3 Oscillator Section
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.
Summary of Contents for OFV-3001
Page 28: ...3 16 3 First Steps...
Page 54: ...5 12 5 Operating the Vibrometer...
Page 94: ...D 10 D Functional Description of the Controller...
