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Model 5501A
Theory of Operation
3-20. System operating power is distributed to the laser head and other transducer units via theconneaor
board. Safety switch S 2 opens when the laser head cover is removed. This disconnects -15 Vdc from the
piezoelectric transducer (PZT) power supply and the high voltage power supply. As a result, these power
supplies become inoperative. The +I5 Vdc input is applied to a regulator on the A7 Control Board. This
regulator provides +5 volts for use within the laser head.
NOTE
High or low indicates voltage deviation of approximatley 1 volt, or
more. For example, consider the +I5 volt input going more
positive by 1 volt. The inverted input to the comparator switch
goes high (i.e., more positive than ground). This results in the
cathode of LED DS1 switch going toground, activating the UNBAL
+15V indicator. The cathode of LED DS2 output remains open.
Conversely, if the -15 volt input goes more negative by
approximately 1 volt, the unbalance condition causes the
comparator switch inverting input to go negative, grounding the
cathode of DS2and opening the DS1 cathode. Asa result, the-15V
unbalance indicator is lit.
3-21.
Laser Current Regulation
3-22. The laser current regulation circuit i s a control loop consistingof an error sensing circuit and the high
voltage power supply. A 390-ohm resistor on the A1 Connector Board provides a current path for the laser
tube cathode. The resulting voltage drop across this resistor provides a monitoring input to a difference
amplifier on the Connector Board. This difference amplifier functions as a voltage comparator. The other
comparator input is a reference voltage which is determined by the adjustment of potentiometer AlR11.
Laser current is adjusted by monitoring A7TP1 (i.e., the laser cathode current test point on the Control
Board) or AlTPl (adjacent to AlR11) and setting AIR11 for the appropriate reading. Once set, any change in
laser current results in a comparator error output signal. This error signal changes the conduction of driver
AlQ3. Transistor A1Q3 drives a series regulator Q1, which acts a s a variable resistance to control the amount
of drive to the High Voltage Power Supply A2.
3-23. The A2 High Voltage Power Supply consists of an oscillator and a high voltage multiplier circuit. The
oscillator is activated when -15 volts is supplied via safety switch AlS2. Oscillation is maintained by internal
switching transistors that alternately drive magnetic coretransformerT1 in and out of saturation at a rate that
depends on the amount of voltage delivered by series regulator Q1. Higher drivevoltage results in a higher
oscillator frequency and a higher peak-to-peak amplitude. Less voltage reduces oscillator frequency and
amplitude. The typical oscillator output range is 12.5 kHz at 50 volts (peak-to-peak) to 25 kHz at 125 volts
(peak-to-peak). The oscillator output signal determines the amount of high voltage dc output that is
produced by the high voltage multiplier circuit.
3-24. The high voltage multiplier consists of voltage doubler circuits that are wired in series to produce a
net high voltage output of up to 10K Vdc. This variable output is applied to the anode of the laser tube to
control tube current. The high voltage circuit responds to a variation in cathode current by providing a
change in anode high voltage. This high voltage change brings laser tube current back to the appropriate
level.
3-25.
Automatic P A Tuning
3-26. The laser tube is automatically fine tuned by a control loop, which consists of the Beam Splitter
Assembly (A4), Lock Reference Assembly (A5), and the PZT Power Supply Assembly (A6). These circuits
sample the output beam and provide PZT control voltages that maintain the appropriate dual-frequency
beam emission.
3-27. The Beam Splitter Assembly diverts approximately 20% of the laser tubeoutput beam and applies this
portion of the beam to a polarized beam splitter. The polarized splitter extracts the orthogonal frequency
components from the beam sample and provides separated f l and f, frequency inputs to the Lock Reference
Assembly photodetector diodes.
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