D-3
D Functional Description of the Controller
Drop-out
reduction via
the tracking
filter
Drop-out reduction via the tracking filter plays a very important role in optical
signal processing. The light scattered back from the object has a speckled
nature i.e. at any instant the detector sees a light or a dark speckle. The low
signal amplitude of the dark speckle can lead to loss of signal, so-called drop-
outs. When decoding the velocity, this interruption of the input signal causes
short but high noise signals, so-called spikes which make it very difficult to
analyze the output signal. These drop-outs are effectively reduced by a so-
called tracking filter integrated in the input section of the controller. This is
done by an electronic circuit to regenerate high frequency signals based on
the principle of the phase locked loop (PLL).
The principle of signal regeneration by the tracking filter is based on replacing
the input signal with a distorted amplitude by a stable signal from a voltage
controlled oscillator which is synchronized with the frequency and the phase
of the input signal. Suitable circuit design can make it possible to maintain the
synchronized condition approximately, even if the input signal is temporarily
lost. The mechanical analog for this design is a flywheel which may lose a
small portion of its energy if the driving force is briefly interrupted but
continues to run at almost the same number of revolutions per minute and can
drive a subsequent mechanism without disruption. It is easy to see that this
effect gets better, the higher the inertia of the wheel is. At the same time the
flywheel however loses the ability to follow rapid changes in the revolutions
per minute i.e. the dynamic response of the drive system gets worse. The
same correlation also applies to the electronic tracking filter which thus
always represents a compromise between the regeneration effect and the
dynamic tracking behavior of the input signal. Basically, good drop-out
elimination or noise suppression is always involved with limited dynamic
response. If the maximum acceleration is exceeded, the synchronization
between the input signal and the oscillator is lost (the tracking filter loses lock)
which leads to drastic signal distortions at the signal output. Practical advice
for setting the tracking filter can be found in
The internal structure of the tracking filter circuit is shown in
. The
function of the voltage controlled oscillator (VCO) has already been
mentioned. The control signal which synchronizes the oscillator is generated
in the phase detector which monitors the phase difference between the input
signal and the oscillator signal. The dynamic characteristics of the
configuration are mainly determined by the internal low pass filter. The
maximum acceleration which the tracking filter can still follow depends on the
filter bandwidth. The low pass time constant is switched between SLOW and
FAST via the system control and thus adapts the dynamic characteristics to
the application. The tracking filter can be turned off via a bypass if the
accelerations are too high or in the case of good optical signals.
Figure D.2: Block diagram of the tracking filter circuit
A m p l i f i e r
I n p u t
P h a s e
d e t e c t o r
L o w p a s s
f i l t e r
V
f
V C O
o f f
o n
O u t p u t
C o n t r o l B u s
