Chapter 3
Theory of Operation
P/N 79000035_SM, Version 1.5 3 - 11 Focus 30/35 Service Manual
Functional description
The purpose of the Distance meter unit is to measure the distance to a target with the use of an red
Laser beam. This is achieved in the following steps:
• Transmitting of an red Laser signal.
• Receiving of the reflected red Laser signal.
• Determine the phase difference between the transmitted and the received signals.
• Upon the phase difference, calculate the distance to the target.
The calculation of the distance is made from the known formula:
Description of the transmitting signal from Transmitter unit to Receiver unit
The transmitting signal from the transmitter laser diode for the internal path is decoupled from a
reference mirror in front of the transmitter laser diode.
In reflector mode the transmitting signal is sent through a filter to reduce output power. This filter is
controlled by a filter motor and in DR mode this filter is removed from the transmitting signal path.
Next the signal will reflect through a transmitter prism and the front lens on the way to the target
(reflector or DR).
The returned target signal will reflect back through the front lens and via two reflectors and then into
the receiver fibre.
The amplitude of the received target signal is controlled by a greywedge. The greywedge is driven
by a step motor. A reflex coupler on the transceiver board indicates the open greywedge position.
The measurement is divided in an internal and an external part to compensate thermic drift. The
internal path comes from the transmitting signal decoupled from a reference mirror. The switching
between internal and external path works with a rhomboid prism on the switch motor.
The receiver diode receives the target signal and calculates a distance in the transceiver board. The
distance information is sent to the MCB board.
The phase measurement needs multiple modulation frequencies to reach a non ambiguous results
because the phase measurement can only measure the phase of the wave rest piece.
Distance = speed x time
• Speed, the speed of light is known  300 000 km/s.
• Time, the time is measured by the phase difference of reference signal and received signal.
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