Optical Chopper

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transistor places a pulse on top of every sixthpulse, per the chopper wheel design. Thissecond signal provides a signal at f/6, withknown phase, which is used to synthesize otherreference outputs.Logic signals at f & f/6 are generated by the dualcomparator U6. The 1/2 of U6 compares thephoto current signal to a fixed voltage of about3.6VDC in order to generate f. The 2/2 of U6compares the photo current signal to the peakvalue of the photo current signal, less two diodedrops. Comparing to the peak voltagecompensates for variations in LED &phototransistor sensitivity. Both comparatorshave about 0.2V of hysteresis to preventmultiple edges on logic transitions.FREQUENCY SYNTHESISThe outer row frequency, f, is measured directlyby one of the phototransistors. The differencebetween the outer & inner row frequencies, f/6,is measured by the other phototransistor. Theother frequencies are synthesized from these tworeferences by a phase-lock loop circuit.Name Frequency Descriptionƒ ƒ outer row5ƒ 5ƒ 5 x outer rowƒ inner 5ƒ/6 inner rowƒ sum 11ƒ/6 outer row + inner rowƒ diff ƒ/6 outer row – inner rowThe Dual 1:4 Multiplexer, U17, is used to pass for f/6 to the right hand reference output BNCper the setting of the REFERENCE MODEswitch, SW1. U17 also selects f of f/6 as asource to the PLL frequency synthesizer circuit.To generate 5f/6, the frequency of the inner rowof slots, U17 selects f/6 as the input to the phasedetector of U12, a CMOS phase-lock loop. Theoutput of the phase detector is filtered by R23,R24, and C3. The quad analog switch, U4, willalso switch in C2 or C1 if the 400Hz or 40HzMAX FREQ/SLOTS ranges are selected atSW2. The filtered output of the phase detectorcontrols the VCO frequency. The maximumfrequency of the VCO is set by R22. U4increases the maximum frequency by switchingin R20 or R21 when the 400 Hz or 4KHz rangesare selected. The output frequency of the VCOis divided by U18, which is programmed todivide by 5 or 11 per the setting of theREFERENCE MODE switch, SW1. Togenerate 5f/6, U18 loads 10 when the counterreaches 15 to divide by 5. In order for thefrequencies at the input of the phase comparatorto be equal, the VCO must run at 5xf/6.The same circuit is used to generate 5f,however, the multiplexer, U17, selects f (notf/6) as the input to the phase detector. Togenerate 11f/6, the sum frequency, f/6 isselected as the input to the phase detector, andU18 is programmed to divide by 11 bypresetting to 4 when the counter reaches 15. Inall cases, the output of the VCO is passed to theleft reference output BNC. This output isalways a square wave.FREQUENCY DISPLAYThe four-digit frequency display always showsthe frequency, f, of the outer row of slots,independent of the reference mode, which hasbeen selected. The time base for the counter is a32,768Hz crystal oscillator, which is sustainedby U10, a hex inverter. U11, a 14-stage binarycounter and the flip-flop 2/2 U14 are used todivide by 215 to generate a 1.000-second gate.In order to eliminate one-count flicker in theresult, the count latch 1/2 U14, is set by a fallingedge of the wave form which is to be counted.When the count latch is set, the reset to the timebase counters is released, starting the onesecond-countperiod. Also, the reset to U15, thefour-decade counter/latch/display driver isreleased to begin the count. The countaccumulates for 1 second, when, the falling edgeof the Q output from 2/2 U14 sets 1/2 U13 high.The Q output of 1/2 U13 stays high for about1/2 clock period (15µs) asserting the LatchEnable input on U15 to transfer the countercontents to the latch for display. Then 1/2 U13is reset, and 2/2 U13 is clocked high, asserting16
the Q-bar output to reset the count latch. Theentire count cycle begins with the next fallingedge from the opto pick-off comparator. If thewheel stops, so there are no more pulses tocount, the reset to U15 will be assertedindefinitely. After a few seconds, R36 willcharge C13 to assert the Disp Sel to U15,causing the contents of the counters (which arezero) to be displayed instead of the contents ofthe latches (which contain the last count). U15multiplexes the display information to the 4digit common cathode LED. Each of the digitsare selected in sequence by U15 saturating oneof the display driver transistors Q2-Q5.POWER SUPPLIESThe power transformer primary is tapped foroperation at 100, 120, 220, or 240VAC. Thepower entry module does the requiredswitching. Full wave bridge rectifiers are usedto rectify the 7.5VAC and 30VAC to 8.5VDCand 40VDC. The unregulated 40VDC is usedby U1 the motor driver transistor, and by U8 toprovide regulated 30VDC. The 30VDC is theDC supply to U7, a +15VDC regulator. U9regulates the 8.5VDC to 5.0VDC.17
Page 1 and 2: MODEL SR540OPTICAL CHOPPER
Page 4: TABLE OF CONTENTSSymbols used …
Page 10 and 11: APPLICATIONSSINGLE BEAM EXPERIMENTI
Page 12 and 13: amplitude of the 5th harmonic term
Page 14 and 15: DETECTION AT THE SUM &DIFFERENCE FR
Page 18 and 19: SR540 PARTS LISTMotor and Motor rel
Page 20: SR540 PARTS LISTR 46 4-00107-402 10

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