Abstract:
A modulated laser light detector that converts laser light energy into electrical signals which exhibit a frequency that is substantially the same as the laser light modulation frequency, in which these signals allow the detector unit to determine a position where the laser light is impacting upon a photodiode array. A synchronous rectifier circuit is provided which, by use of a phase locked loop, converts a full-wave analog signal representative of the modulated laser energy into a half-wave analog signal, which is filtered and demodulated to generate a DC level that is indicative of the strength of the received laser light energy. At least two channels of these signals are used to determine the impacting position of the laser light energy.

Description:
TECHNICAL FIELD 
     The present invention relates generally to laser light detectors and is particularly directed to laser light detectors of the type which detect modulated laser light energy. The invention is specifically disclosed as a modulated laser light detector that converts laser light energy into electrical signals which exhibit a frequency that is substantially the same as the laser light modulation frequency, and then signal conditions the electrical signals in a manner that allows the detector unit to determine a position where the laser light is impacting upon a photodiode array. A synchronous rectifier circuit is provided which, by use of a phase locked loop, converts a full-wave analog signal representative of the modulated laser energy into a half-wave analog signal, which is filtered and demodulated to generate a DC level that is indicative of the strength of the received laser light energy. At least two channels of these signals are used to determine the impacting position of the laser light energy. 
     BACKGROUND OF THE INVENTION 
     Laser alignment systems have been used for years, and some of these systems use a rotating laser light source that periodically sweeps 360°, thereby creating a plane of laser light that can be received and detected by a laser light receiver. Other types of laser alignment systems use a transmitter that projects laser energy continuously in a reference cone, or a reference plane, of light that spreads in all directions about a 360° circle. The range of such detection units, especially when used in direct sunlight, is greatly increased by modulating the laser light source at a predetermined frequency. Such a laser light alignment system using a modulated laser light source is disclosed in U.S. Pat. No. 4,756,617. 
     In this &#39;617 patent, the laser energy in the alignment field is modulated at 8 kHz, and the signals produced by photodetectors of a laser light receiver are filtered and amplified. A split-cell photodetector is used, thereby producing two input signals that each have a particular strength, depending upon the position of the split-cell photodetector as it is being impacted by the modulated laser energy. After these two input signals are run through individual band pass filters and amplifiers having automatic gain control, the ratio of their signal strengths are compared (by a “ratio comparator” circuit 111) to determine the relative position that the laser light is impacting the photodetector receiver. 
     The synchronization aspect of the circuit in the &#39;617 patent uses a phase locked loop to generate an internal oscillator signal that exactly matches the phase and frequency of the modulated received signal. The phase locked loop circuitry generates a control signal 136 (see FIG. 4) that feeds a pulse generator 150 (also on FIG. 4). Pulse generator 150 has some built-in time delay, and it outputs a pulse 107 that will occur approximately in the middle of the peak of the “pulse” or pseudo sine/square wave signal that is being received at the photodetectors. (By the time the received input signal comes through the band pass filter with gain, the square wave has been essentially converted into a pseudo sine wave.) 
     The &#39;617 patent uses an “auto-correlator circuit” as its level detection means for the two channels of signals coming out of the two photodetectors. The auto-correlator uses an analog switch 106 a  for channel 1, and an analog switch 106 b  for channel 2. These two analog switches 106 a  and 106 b  are controlled by the output signal 107 of the pulse generator 150. When these analog switches 106 a  and 106 b  are turned ON (in their conductive state), they will pass the incoming signal from the band pass filter to two resistor-capacitor (RC) filter circuits, 108 a  and 108 b , respectively. The incoming voltage from the band pass filter will charge the capacitors of these RC circuits, and cause the capacitors to be charged to the “peak” voltage of the pseudo sine waves. Then the analog switches are turned OFF, while the incoming waveform is still near its peak voltage, so that the voltage will remain on the capacitors for a certain time interval after the analog switches have been turned OFF. This essentially acts as a synchronous sample and hold circuit. 
     This auto-correlator approach of the &#39;617 patent exhibits certain problems. One inherent problem is noise on the incoming sine wave. When the voltage waveform is sampled at its peak, any noise (e.g., from ambient light reflections) that occurs at that moment will tend to falsely charge the capacitor over its intended “peak” voltage for that input cycle. Therefore, the capacitor voltage will be incorrect, and this will result in elevation errors, and/or unstable elevation indications. Another inherent problem of this particular circuit design is that the sampled voltage that is placed on the capacitor will discharge over time into the ratio comparator 111. This is clear from the waveform 110 on FIG. 6. Therefore, the actual peak voltage will not be exhibited at the capacitor at the moment the ratio comparator samples the two incoming voltages stored on their respective capacitors on the downstream side of the analog switches 106 a  and 106 b.    
     One implementation problem with the auto-correlator circuit of the &#39;617 patent is that the analog switches exhibit “charge injection,” which causes some spurious charge to flow toward the outputs of these analog switches when they are turned OFF. This thereby charges the “hold” capacitors of the RC filters on the downstream side of the analog switches 106 a  and 106 b . This will, of course, tend to disturb the voltages on the capacitors, which therefore, may not be at the correct magnitudes. Another implementation problem of this auto-correlator circuit is when one of the two input channels has a bias voltage greater than the other. This will be perceived by the auto-correlator as a greater magnitude of the signal of interest, and actually occurs in devices manufactured according to this &#39;617 patent. 
     It would be an improvement to provide a level detection current that overcomes the inherent problems and implementation problems that are exhibited in the design of the &#39;617 patent, while at the same time being able to detect the relative magnitudes of modulated laser light signals that are received by the photocells of the receiver device. 
     It is known in the art to use synchronous demodulators or synchronous rectifier circuits in “lock-in amplifiers,” which typically are used to extract low amplitude signals that are modulated by a carrier signal in high-noise environments. Many of these lock-in amplifiers are used in laboratory settings, including with laser spectroscopy devices. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an advantage of the present invention to provide a modulated laser light detector that can reliably measure the peak magnitudes of received laser light signals at their respective photocells, and thereby reliably indicate the correct grade or elevation where the laser signal impacts the receiver photocell elements. 
     It is another advantage of the present invention to provide a modulated laser light receiver that can detect the relative magnitudes or strengths of the laser light impacting at least two photocell elements of the receiver unit, while using a synchronous rectifier circuit having low noise characteristics to more accurately detect the peak values of the laser light signals impacting the photocells. 
     It is a further advantage of the present invention to provide a modulated laser light detector that uses a synchronous rectifier circuit having low noise characteristics to detect the relative strengths of laser light beams impacting at least two photocells, in which two synchronous rectifiers use multiple-input multiplexers to synchronously control amplifiers having low noise characteristics and will more accurately measure the relative strengths of the incoming signals from the at least two photocells. 
     It is yet another advantage of the present invention to provide a modulated laser light detector that uses a pair of synchronous rectifier circuits to detect the relative strengths of laser light signals impacting at least two photocells of the receiver unit, while having the ability to use several different photocell arrangements to produce at least two channels of input signals that will be signal conditioned and directed to the synchronous rectifiers. 
     Additional advantages and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. 
     To achieve the foregoing and other advantages, and in accordance with one aspect of the present invention, a modulated laser light detector is provided, which comprises: (a) at least one laser light photosensor, which generates at least one first electrical signal when receiving modulated laser light energy having at least one predetermined range of wavelengths and at least one predetermined range of modulation frequencies; (b) at least one amplifier stage that receives the at least one first electrical signal and outputs at least one second electrical signal having a first waveform that exhibits voltage components of both positive-going and negative-going regions; (c) a phase and frequency detector that receives the at least one second electrical signal and generates a lock signal that corresponds to a zero crossing of the first waveform; (d) at least one synchronous rectifier stage that receives the lock signal and receives the at least one second electrical signal, and generates at least one third electrical signal of a second waveform that comprises a substantially rectified version of the first waveform; (e) at least one low pass filter that receives the at least one the third electrical signal and outputs at least one fourth electrical signal that comprises a substantially DC voltage; and (f) at least one signal strength detector that inspects the substantially DC voltage of the at least one fourth electrical signal, and determines a relative strength of the received modulated laser light energy. 
     In accordance with another aspect of the present invention, an electronic circuit is provided, which comprises: (a) a first amplifier stage that outputs a first electrical signal having a first waveform that exhibits voltage components of both positive-going and negative-going regions; (b) a phase and frequency detector that receives the first electrical signal and generates a lock signal that corresponds to a zero crossing of the first waveform; (c) a first synchronous rectifier stage that receives the first electrical signal and generates a second electrical signal of a second waveform, in which the second waveform substantially comprises a rectified version of the first waveform and exhibits a voltage component having substantially only a single one of a positive-going and a negative-going region; and (d) a first low pass filter that receives the second electrical signal and outputs a third electrical signal that comprises a substantially DC voltage. 
     In accordance with yet another aspect of the present invention, a synchronous rectifier circuit is provided, which comprises: (a) a phase and frequency detector that receives a first electrical signal of a first waveform and generates a lock signal that changes logic states upon a zero crossing of the first waveform, wherein the first waveform exhibits voltage components in both positive-going and negative-going regions; (b) a multiple-input analog signal multiplexer that: (i) receives the first electrical signal at a first input, (ii) receives a virtual ground reference signal at a second input, (iii) receives the lock signal at a control input, and (iv) outputs a second electrical signal that comprises substantially one of: (A) the first electrical signal and (B) the virtual ground reference signal, depending upon the logic state of the lock signal at the control input; and (c) a gain amplifier that receives the second electrical signal, the gain amplifier having an effective voltage gain of substantially +A v  when the lock signal is at a first logic state, and the gain amplifier having an effective voltage gain of substantially −A v  when the lock signal is at a second logic state. 
     Still other advantages of the present invention will become apparent to those skilled in this art from the following description and drawings wherein there is described and shown a preferred embodiment of this invention in one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different embodiments, and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description and claims serve to explain the principles of the invention. In the drawings: 
         FIG. 1  is a front elevational view of a hand-held modulated laser light detector, as constructed according to the principles of the present invention. 
         FIG. 2  is another front view of an alternative embodiment for a hand-held modulated laser beam detector, in which an alternative photodiode array is constructed of a number of individual photocell segments. 
         FIG. 3  is a diagrammatic view of another alternative type of photodiode array, having two large cells and two middle small cells, usable with the present invention. 
         FIG. 4  is a diagrammatic view of yet another alternative photodiode array, having two large cells and one middle small cell, usable with the present invention. 
         FIG. 5  is a diagrammatic view of a set of two photocells, in which there are two large cells with no intermediate smaller cell, usable with the present invention. 
         FIGS. 6–17  are an electrical schematic diagram of a portion of the electrical circuit used in the modulated beam detector of  FIG. 1 . 
         FIG. 18  is an electrical schematic of an alternative embodiment input stage, usable in the present invention, which uses a set of silicon substrates for the photodiodes that electrically form a single split-cell for a proportional modulated beam laser detector, as according to the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings, wherein like numerals indicate the same elements throughout the views. 
     Referring now to  FIG. 1 , a modulated laser beam detector unit is illustrated, generally designated by the reference numeral  10 . Its front outer surface at  12  is typically a plastic molded housing, which has some openings at  14  to allow for an internal beeper or other type of audio output device to be used. A set of pushbutton switches generally designated by the reference numeral  20  are located along the upper left-hand side of the case. These switches are designated by the reference numerals  22 ,  24 , and  26 , in which the top switch  22  acts as a dead band control switch, the middle switch  24  enables the beeper (or other type of audio output device) to be activated, and the bottom switch  26  is the unit&#39;s ON-OFF switch. 
     The modulated laser beam detector  10  also includes a liquid crystal display, generally designated by the reference numeral  30 . On this LCD display  30  there are several symbols that provide information to the user. The “grade” symbols are designated at the reference numerals  32 ,  34 , and  36 , in which the middle symbol  32  would be energized when the laser beam detector  10  is positioned “ON-GRADE” with respect to the plane of light being emitted by a modulated laser light source (not shown). The LCD “arrowhead” triangular-shaped indicators  34  and  36  are, respectively, indicators of above-grade and below-grade. In the illustrated embodiment of the laser detector  10 , there are three different above-grade indicators and three different below-grade indicators, thereby providing the user with an indication as to how close or how far the detector  10  is from being on-grade. 
     A battery symbol at  42  is illustrated on the LCD display  30 , which will be visible when the battery that powers the unit  10  has been depleted past a predetermined point. The “speaker” symbol  40  is also part of the LCD display  30 , which is visible when the beeper (or speaker) has been enabled (by pushing the button  24 ). The symbol at  38  on the LCD  30  provides an indication as to the width for the dead band setting. 
     On  FIG. 1 , the modulated laser beam detector  10  includes a set of photodetectors, generally designated by the reference numeral  50 . In an exemplary embodiment, these photodetectors comprise multiple photocells, such as photodiodes, and they are arranged in a vertical line (as seen in  FIG. 1 ), in which the upper photodiode is at  52 , the bottom photodiode is at  56 , and the middle photodiode is at  54 . The photodiodes in this array are used to intercept the plane of laser light being emitted by the laser light source (not shown), and these photodiodes can output an electrical current, and they are used in the electronic control circuit of the receiving device  10  in a manner that is described in a schematic diagram that starts on  FIG. 6 . As can be seen in  FIG. 1 , the middle photodiode  54  is actually comprised of six different photocell sections, each of which represents a separate electrical component, and which are also illustrated on the schematic diagram of  FIG. 6 . 
     In this patent document, the terms “photocell” and “photodiode” both refer to an optoelectronic device that produces some form of an electric signal when optical energy impacts such optoelectronic device. A photodiode is a type of photocell, which is a more generic term. The type of electrical signal produced by a photocell need not necessarily be independently sourced to the photocell itself. For example, if a photocell merely changes electrical resistance, then it would not by itself produce an electrical signal. However, if a bias current or a bias voltage is applied to such a resistance-sensitive photocell, then it truly would become an optoelectronic device of the type that could be used in the present invention. 
     It will be understood that the photodetectors  50  do not necessarily need to consist of photodiodes. Other types of photosensitive components could be used, including phototransistors and photovoltaic cells, which can be considered equivalents to photodiodes for use in the present invention. Essentially any type of optoelectronic component could be used in the present invention, so long as it is “sensitive” to electromagnetic radiation (i.e., such that it will vary its resistance, its current output, or its voltage output as it receives a varying quantity of photons). Such optoelectronic devices do not necessarily have to exhibit a linear response to the received electromagnetic radiation, although such a linear response is generally helpful when determining the strength of the optical energy (i.e., electromagnetic radiation) being received. It will also be understood that the photodetectors  50  do not necessarily have to be particularly sensitive to collimated light in situations where the present invention may be used to detect non-laser light sources. However, for most commercial applications of the present invention, a laser light source is generally preferred, and the photodetectors  50  would typically be designed to be sensitive to such laser energy. 
     Referring now to  FIG. 2 , an alternative embodiment for a modulated laser beam detector is illustrated, generally designated by the reference numeral  100 . This photodetector includes a front outer case at  112 , and a set of holes for a speaker or beeper at  114 . There is also an array of pushbuttons at  120 , a liquid crystal display at  130 , and an array of photodetectors at  150 . The pushbutton switches  122 ,  124 , and  126  have essentially the same functions as the similar switches  22 ,  24 , and  26  that were found on the device  10  illustrated in  FIG. 1 . The LCD display  130  also has similar capabilities and symbols, including the grade symbols  132 ,  134 , and  136 , the low battery symbol  142 , the speaker enabled symbol  140 , and the dead band status symbol  138 . 
     The main difference between this second embodiment  100  and the first embodiment  10  is the actual photodiode array  150 . As can be seen by inspecting  FIG. 2 , there are again three major photodiode sections at  152 ,  154 , and  156 . However, each of these individual photodiode sections is broken into many small photocell segments, which provides for a more accurate laser receiver that can reduce shading effects of the photocells. The principles of this type of photocell design are disclosed in detail in U.S. Pat. No. 6,133,991, titled MULTI-SEGMENT COMPOSITE PHOTOCELL DEVICE, which is commonly assigned to Apache Technologies, Inc. of Dayton, Ohio. 
     The different types of photocell arrangements of the embodiments  10  and  100  are merely possible examples for use with the electronics of the present invention. Other example photocell array patterns are disclosed on  FIGS. 3 ,  4 , and  5 . Referring now to  FIG. 3 , a four-segment photocell array is illustrated, generally designated by the reference numeral  160 . The top photocell is  162 , the bottom photocell is  168 , and there are two “middle” photocells  164  and  166 . This type of photocell configuration has been disclosed in U.S. Pat. No. 5,486,690, titled METHOD AND APPARATUS FOR DETECTING LASER LIGHT, which is commonly assigned to Apache Technologies, Inc. of Dayton, Ohio. 
     Referring now to  FIG. 4 , a three-segment photocell array is generally designated by the reference numeral  170 , which has a top photocell  172 , a bottom photocell  176 , and a smaller middle photocell  174 . As another example of the flexibility of the present invention, the photocell array  180  of  FIG. 5  only includes two fairly large photocell segments, at  182  and  184 . In this design, there are no smaller “mid-cells.” 
       FIGS. 6–17  illustrate a rather complex electrical schematic diagram, starting on  FIG. 6 . The input photocells are illustrated along the left-hand side of  FIG. 6 , and these photocells (e.g., photodiodes) as a single array are generally designated by the reference numeral  50 . On  FIG. 6 , there are two “outer” photocells at  52  and  56 , and also a set of six individual photocells that act as the “middle” photocell  54 , as seen on  FIG. 1 . There is a set of analog switches, generally designated by the reference numeral  200 , that receives the input signals from some of the middle photocells  54 . These analog switches are controlled by a memory gate integrated circuit  210 , which periodically turns on each of the analog switches, so that their input signals can be properly apportioned to a “channel  1 ” signal and a “channel  2 ” signal, which in turn provides an indication as to the position, or level, that the laser light is striking the photocell array  50 . 
     The electrical signals that are output from the analog switches are split into channel  1  and channel  2  signals, as noted above, and these are then directed to two band pass filters  212  and  214 , illustrated on  FIG. 7 . The outputs of these band pass filters  212  and  214  are directed to a set of low noise amplifiers at  216  and  218 , respectively, which also are depicted on  FIG. 7 . The output signals from the low noise amplifiers  216  and  218  typically have an output (or transfer) impedance of about 26.7 kOhms, and a maximum output voltage amplitude of about 420 mV peak to peak, under conditions where laser light is striking the photodiode array  50 . 
     The output signals from the low noise amplifiers  216  and  218  are respectively directed to another set of band pass filters at  220  and  230 , as illustrated on  FIG. 9 . The outputs of these two band pass filters are directed to automatic gain control circuits  222  and  232 , respectively. For channel  1 , the band pass filter  220  also acts as a gain amplifier, and the gain is dependent upon the multiplexer setting of a multiplexer  224 , which decides which set of resistor values to switch in as part of the automatic gain control circuit  222 . A similar multiplexer  234  is used to switch in the appropriate resistors  232  for the channel  2  automatic gain control circuit that act as part of the band pass filter and gain amplifier combination  230 . 
     In the illustrated circuit, the overall gain of the two first stages of the band pass filter  220  is about 0.8. The band pass filter  220  actually includes a 3-pole Bessel low pass filter stage, and a 3-pole Bessel high pass filter stage. In the illustrated embodiment, the voltage gain for each of these Bessel filters is 0.9, the low pass filter corner frequency is about 20 kHz, and for the high pass filter stage the corner frequency is about 3.9 kHz. When including the next amplifier stage, which provides a voltage gain of about 14.3, it will provide a 5 volt peak-to-peak maximum voltage at its output stage. These voltage measurements again depend upon the occurrence of laser light energy striking the photodiodes in the array of photodiodes  50 . 
     The automatic gain control circuit continues on to the next drawing,  FIG. 10 , in which two further amplifier stages  226  are used to provide a voltage gain of about 510. The first amplifier stage in the illustrated embodiment provides a voltage gain of about 23.1, while the second amplifier stage provides a gain of about 22.1. The output voltage will be about 0.79 volts peak-to-peak, if a minimum signal of 1.58 mV peak—peak voltage is input to this portion of the automatic gain control amplifier  226 . A similar set of amplifier components is found at  236  for channel  2 . The gain bandwidth characteristics for the channel  2  filter and gain amplifier  230  are about the same as for the circuit  220 . 
     The outputs of the gain amplifiers  226  and  236  are directed into a pair of synchronous rectifiers  240  and  250 , respectively. For the channel  1  rectifier  240 , there is a two-input analog multiplexer at  242 , which sends its output signal to a gain amplifier  244 . The multiplexer  242  is controlled by a signal “DEMOD” along a signal pathway  262 , which controls which of the two inputs will be sent through to the output of the multiplexer  242  and into the positive input of the operational amplifier  244 . The output signal of the op-amp  244  is then directed into another low pass filter  248  with a gain amplifier at  246 . 
     The DEMOD control signal  262  nominally consists of a square wave that is output from a phase locked loop, which is described below. When the DEMOD signal  262  is in its Logic 1 state, then the op-amp  244  acts as a voltage follower, thus having a designed optimal voltage gain of +1. When the DEMOD signal  262  is at its Logic 0 level, then a reference voltage level that is equal to about one-half of the power supply rail for these amplifiers is sent through the multiplexer  242  to the positive input of the op-amp  244 . Since the power supply rails for the op-amp  244  are at +5 volts DC and ground in this exemplary circuit, the reference voltage is at about 2.5 volts DC, and acts as a virtual ground for this op-amp stage. In this circuit, the two resistors RN  300 A and RN  300 B, by virtue of being substantially equal in resistance value, will make op-amp  244  into a gain amplifier having a designed optimal voltage gain of −1. 
     The overall effect of this circuit  240  is to act as a synchronous low noise rectifier and, by switching the state of the DEMOD signal in sync with the zero-crossings of the sinusoidal or “near-sine” wave output from the band pass filters  220  and  230 , the sinusoidal wave that is output by the gain amplifier  226  is synchronously rectified so that the output signal from the op-amp  244  exhibits a half wave peak-to-peak voltage that is approximately one-half of the peak-to-peak voltage of the full sinusoidal wave that travels through the multiplexer  242  from amplifier  226 . Since this sinusoidal wave is now rectified, there are twice as many positive-going one-half “sine” waves in the positive quadrant, and there will be virtually no negative quadrant “sine” wave voltages at the output of the synchronous rectifier circuit  240 . The output from the op-amp  244  is directed to another low pass filter  248  with a gain amplifier circuit  246 , which essentially demodulates the rectified “half-sine” wave into a DC voltage, having a magnitude that is related to the strength of the laser light energy received at the photocells. 
     A similar set of amplifiers at  236  receives a signal for channel  2 , and the output of this amplifier  236  drives into another two-input multiplexer  252  and another gain amplifier (or op-amp)  254 . These are some of the major components of the second synchronous rectifier, generally designated by the reference numeral  250 . The output from the op-amp  254  is directed to another low pass filter  258  with a gain amplifier circuit  256 , which also essentially demodulates the rectified “half-sine” wave into a DC voltage, and exhibits a magnitude that is related to the strength of the laser light energy received at the photocells. 
     The two “sine” waves that come into the synchronous rectifiers  240  and  250  are also directed to a summing circuit  260 . These signals travel through a set of resistors and a capacitor, before being sent as a “summed AC” signal  264  to a phase locked loop circuit, which will be described immediately below. 
     A phase locked loop circuit (PLL) and a “lock detect” circuit are illustrated on  FIG. 11 , and are generally designated by the reference numeral  270 . The summed AC signal  264  is used to provide information to the phase locked loop, and this AC-SUM signal  264  will provide some type of AC waveform if either one of the channels is outputting a voltage, regardless as to which exact set of photodiodes in the array  50  are being impacted by the modulated laser beam. The phase locked loop  270  is utilized to determine if the proper modulated signal is being received at the photodiode array  50 , and if so, attempts to bring the overall receiver circuit of the detector  10  into a synchronous lock situation with the incoming modulated laser beam signal. A phase error signal  278  is output from the PLL circuit  270 , and this phase error signal  278  is input to a microprocessor that is discussed below, and is used to indicate that the PLL has achieved a “lock” state. The PLL circuit  270  also creates the DEMOD signal  262  that is used to control the two multiplexers  242  and  252 , discussed above. 
     Referring now to  FIG. 12 , a microprocessor or microcontroller chip  280  is used to not only control the LCD display  30 , but also receives many of the control and waveform signals that run throughout the electronic circuit of the detector  10 . For example, the output signals from the synchronous rectifiers  240  and  250  are referred to as “DC- 1 ” and “DC- 2 ” for the two channels. These signals are directly fed to the microcontroller  280 , which has an on-board analog-to-digital (A/D) converter that can measure the magnitudes of these two input signals. In this manner, the software that executes on the microcontroller  280  will be used to determine the relative strengths (i.e., voltage magnitudes) of these two input signals from channel  1  and channel  2 . 
     Microcontroller  280  also has digital inputs, and these include inputs from the three pushbutton switches on the array of switches  20  on the front panel of the modulated laser detector  10 . Microcontroller  280  also controls the “gain” outputs that in turn control the firing gates of the integrated circuits  224  and  234 , which respectively control the automatic gain amplifiers  222  and  232  that are illustrated on  FIG. 9 . 
     In addition to the above, microcontroller  280  also controls the appearance of the liquid crystal display  294 , and it does so by sending control signals to an LCD driver chip, which is an integrated circuit generally designated by the reference numeral  290 , as seen on  FIG. 13 . The liquid crystal display  294  itself is shown schematically on  FIG. 14 , including the LCD driver chip  292 . 
       FIG. 15  illustrates some of the power supply circuit components, in which the first portion of the power supply is generally designated by the reference numeral  300 . This portion of the power supply  300  boosts the battery voltage from about 3 volts DC up to a regulated value of about +5.5 volts DC. The power supply control circuit is illustrated on  FIG. 16 , and is generally designated by the reference numeral  302 . The boost regulator  300  is designed to continue operating as the batteries begin to run down, even as low as 1 volt (instead of the nominal 3 volts). The power supply control circuit  302  is controlled by the ON-OFF pushbutton key  26 , as discussed above. There are multiple voltage regulator integrated circuits as part of the power supply, including regulators that provide an analog voltage source +VA (at reference numeral  304 ) and also a separate digital power supply rail +VD, as illustrated at the reference numeral  306 . 
       FIG. 8  shows several of the other power supply components, including power supply components  310 ,  312 ,  314 ,  316 , and  318 . Also on  FIGS. 8 and 17 , are illustrated a number of unused components  288  or gates  388  that are part of the integrated circuits used in the electronic control circuit of the laser detector  10 . Of course, these “extra” components could be available for other functions, if desired. 
     An alternative embodiment for the input circuit is illustrated as an electrical schematic on  FIG. 18 . This input circuit is generally designated by the reference numeral  400 , and includes a set of photodiodes that comprise a photodetector array, such as the array  150  illustrated on  FIG. 2 . The photodetector array  400  operates as a proportional modulated beam detector front-end, and electrically will look like three silicon substrates that form a single optical split-cell. The output voltages are at  410  and  420 , which are respectively referred to on  FIG. 18  as CH_ 1 -IN and CH_ 2 -IN. After running through a band pass filter (either  412  or  422 ), these channel  1  and channel  2  signals  414  and  424  become the familiar IC_ 1  and IC_ 2  signals that were seen on  FIGS. 7 and 9  which are output from the low noise amplifiers  216  and  218 . Once signals  414  and  424  reach this point on the schematic diagram of  FIG. 18 , they are passed on to circuit components that have been described above, such as the band pass filters with gain amplifiers  220  and  230 , respectively. 
     It will be understood that the logical operations involving the automatic gain control circuits  222  and  232 , the level detection functions, and the phase error detection functions that are performed by the microcontroller  280  could be implemented using sequential logic (e.g., using microprocessor or microcomputer technology), or using a logic state machine, or perhaps by discrete logic alone; it could also be implemented using parallel processors. One possible embodiment may use a microprocessor or microcontroller to execute software instructions that are stored in memory cells within an ASIC (an Application Specific Integrated Circuit). In fact, the entire microcontroller  280 , along with dynamic RAM and executable ROM, may be contained within a single ASIC, in one mode of the present invention. Of course, other types of circuitry could be used to implement the logical operations and analog circuit functions depicted in the drawings without departing from the principles of the present invention. 
     It will be further understood that the precise circuits depicted in the schematic diagrams of  FIGS. 6–18 , and discussed above, could be modified to perform similar, although not exact, functions and mathematical operations without departing from the principles of the present invention. The exact nature of some of the circuit elements and logical operations of these circuit elements in  FIGS. 6–18  are directed toward specific future models of modulated laser light detectors (those involving Apache Technologies, Inc. laser detectors, for example) and certainly similar, but somewhat different, steps would be taken for use with other types of laser detectors in many instances, with the overall inventive results being the same. 
     The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiment was chosen and described in order to best illustrate the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.