Patent Publication Number: US-3968361-A

Title: Laser receiver anti-sun circuit

Description:
BACKGROUND OF THE INVENTION 
     In the field of photodetection some applications require exposure of the devices to bright ambient condition. One such application would by any of the systems designed to receive and/or detect laser beam or pulses in the field. That is, a laser receiver subject to sunlight exposure. Heretofore, no known system has been provided that will compensate for ambient daylight in a laser receiver. Such a system would be advantageous, and in some applications would be necessary, to avoid detector saturation. If the receiver is saturated by ambient conditions a pulse of interest received during the period of saturation cannot be detected by the receiver. As a result, important information may be lost. Therefore, if the effectiveness of a laser receiver is to be improved it must include means for preventing receiver saturation from ambient conditions. 
     The present invention is such a system. It senses the direct current resulting from the detector&#39;s exposure to ambient conditions and generates an equal but opposite current to counteract its effect. Thereby, the system is adapted to the conditions of its environment. 
     Also disclosed is a dual equalization circuit as an alternate to portions of conventional receivers to provide ultra-high system sensitivity. Increased sensitivity increases efficiency in any receiver circuit and is especially beneficial when the signal to noise ratio of the input is low, as is the case with high ambient noise levels. High noise levels are present in signals received by optical receivers exposed to bright sunlight, for example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an embodiment of the present invention including a transistor as the current source, and 
     FIG. 2 is a schematic diagram of an embodiment of the present invention including receiver components specially designed to increase receiver sensitivity. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The drawing shows a preferred embodiment of the present invention. The technique of the present inventon is to sense direct current I d  resulting from ambient conditions sensed by the photodetector and cause an equal and cancelling direct current I e  to flow into node 10, and thereby null the effects of current I d . Photodetection diode 12 and detection amplifier 14 may be components present in such as a laser receiver. Sense circuit 16 and current source 18 are the means by which equal but opposite current I e  is generated for the technique of the present invention. Sense amplifier 20 is an operational amplifier having its positive terminal coupled to the output of detection amplifier 14 and its negative terminal coupled through capacitor 22 to its own output for feedback comparison. Sensing circuit 16 senses deviations in the output of detection amplifier 14 from the desired direct current level and generates a control level output that is coupled to current source 18. The restorative current level provided by current source 18 is responsive to the output of sensing circuit 16. 
     Current source 18 may be a bi-polar transistor as shown in the drawing, in which case the output of sensing circuit 16 would be coupled to the base thereof as shown. It may, however, be any other current source, such as a field effect transistor (FET). Another alternative as a current source would be a photodiode wherein the output provided by sensing circuit 16 would be employed to control the output of an illumination source directed at the photodiode that would be connected in line with node 10. 
     Capacitor 22 and resistor 24 provide effective low frequency DC breakpoint, and establish a lower frequency of effective flat pass band of the entire FIG. 1 system frequency response. 
     Typical components that may be included in an operative embodiment of the present invention are operational amplifier LM308A, National, as sense amplifier 20, a 1 (one) microfarad capacitor as capacitor 22, and a 10,000 (ten thousand) ohm resistor as resistor 24. In such an embodiment current source 18 could be a 2N918 transistor. 
     The present invention operates as follows: A deviation in the desired (perhaps zero) output of detection amplifier 14, caused by d.c. current I d  from diode 12 of the photodetector circuit in response to ambient conditions such as sunlight, is sensed by sensing circuit 16 which generates a voltage level that is coupled to current source 18. The voltage level controlling the level of current I e  provided by the current source will cancel the undesired direct current level at node 10. Thereby, the detection circuit will not be saturated by ambient conditions and system output 26 will be indicative of detected pulses of interest. 
     FIG. 2 shows an embodiment of the present invention including specially designed receiver circuitry for ultra-high sensitivity. Most receiver amplifiers are designed to provide a flat overall transfer function by picking a bandwidth, and then picking a photodetector resistive load R L  such that R L  = 1/C Shunt  W max .. Optimum operation will occur, however, when system design results from the following. Since V noise  = K 1  √R L  and V signal  = K 2  R L   V  2/V N  = K 3  √R. 
     To maximize, R is maximized. As a result, maximum realizable signal to noise ratio is achieved as conventional analysis can easily show. With this design technique no direct attempt is made to achieve a flat frequency response at the photo diode output terminals. Instead, input equalization is made to maximize the signal to noise power ratio regardless of the effect on frequency response. After the initial gain is achieved, the signal is processed by the second equalizing circuit to flatten the frequency response over the range of interest. FIG. 2 shows an embodiment of the technique wherein equalization is provided for amplifier 14 and the input photo diode response. 
     In the embodiment of FIG. 2, current optically induced in photodiode 12 flows to negative source - V 1  through constant current transistor 18. Since the output impedance of transistor 18 is extremely high, the input time constant will typically be so long (due to total input shunt capacitance to ground) that the effective diode response will be falling at -6db per octave throughout the entire frequency band used. This will result, however, in the maximum signal to noise power ratio possible. Frequency flattening will be taken care of in a later stage. 
     The dynamic path of the signal is from the drain of amplifier 32 to the base and collector of amplifier 46. Then, through the high pass filter composed of resistors 64 and 68, and capacitor 66. The stray capacitance around the filter determines the maximum value of resistors 64 and 68. Namely, they are chosen such that the +6db per octave gain of the high pass circuit begins to diminish at the upper operating frequency required of the system. Amplifier 70 then amplifies the signal further and provides output isolation from the equalization network. 
     The two networks have a net frequency deviation of 0db per decade until the high pass filter begins to fall from its +6db per octave rise. Capacitors 40 and 42, and resistor 44 furnish power supply decoupling. Capacitors 34 and 36, and resistor 38 furnish further decoupling and a low AC impedance to ground for the source terminal of amplifier 32. Resistor 78 and its shunt capacitance provides low pass filtering of the source output voltage of transistor 32. 
     In this configuration the transistor 32 source will follow the gate voltage with a given offset, except that high frequency components will be dropped across amplifier 32. Any deviation of the source voltage of amplifier 32 from 0 (zero) volts will cause integration current to flow in resistor 24 and displacement current in capacitor 22 due to amplifier 20. This will cause negative feedback through transistor 18 and cause the source voltage of amplifier 32 to average 0 (zero) volts. Thus, the average current of diode 12 will flow through transistor 18 and not saturate amplifier 32, as would normally be the case with a strongly sunlit background, or the like, and conventional resistive load. 
     Source -V 2 , and resistors 38 and 44 determine the average current of amplifier 32. Resistor 50 and capacitors 52 and 54, and resistor 56 and capacitors 60 and 62 provide power decoupling. Resistor 48 provides load resistance to amplifier 32. Resistor 58 in conjunction with transistor 46, capacitor 66 and resistors 64 and 68 establish the second stage gain. Resistor 72 provides a balanced source resistance to ground, and capacitors 74 and 76 provide power decoupling, for amplifier 70. 
     The system&#39;s lower -3db frequency is usually determined by the values of capacitor 22 and resistor 24, and the gain of transistor 18 since the time constant of input node 10 is much lower in frequency than the so determined value. At high frequencies, sensing circuit 16 and current source 18 have virtually no effect on system signal gain. But, at the low frequency cutoff point circuit 16 and transistor 18 have a composite gain of unity, the system signal gain drops to -3db of the flat passband gain, and gain decreases 6db lower for every octave lower in frequency below cutoff frequency until node 10 breakpoint is reached. Then, low frequency rolloff changes to 12db per octave. 
     Implementation of the above technique may be accomplished by utilizing the following components in the exemplary circuit of FIG. 2. It should be noted that the present invention can be practiced by other designs employing the techniques disclosed herein, and that all of them are considered to be part of the present invention. 
     
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Symbol Component         Type or Value                                    
______________________________________                                    
12     Photodiode        PIN5D                                            
20     Operational Amplifier                                              
                         μA741                                         
30     Resistor          402Ω metal film (MF)                       
32     Field Effect Transistor                                            
                         2N4416                                           
34     Capacitor         0.1μf ceramic                                 
36     Capacitor         20μf                                          
38     Resistor          2670Ω MF                                   
40     Capacitor         0.1μf ceramic                                 
42     Capacitor         68/20 μf                                      
44     Resistor          100Ω                                       
46     Transistor        2N4457                                           
48     Resistor          301Ω MF                                    
50     Resistor          100Ω                                       
52     Capacitor         0.1μf ceramic                                 
54     Capacitor         68/20 μf                                      
56     Resistor          150Ω MF                                    
58     Resistor          32Ω MF                                     
60     Capacitor         0.1μ f ceramic                                
62     Capacitor         20μf                                          
64     Resistor          402Ω MF                                    
66     Capacitor         10pf silver mica                                 
68     Resistor          402Ω MF                                    
70     Operational Amplifier                                              
                         μA733                                         
72     Resistor          402ΩMF                                     
74     Capacitor         0.1μf ceramic                                 
76     Capacitor         0.1μf ceramic                                 
V,V.sub.1                                                                 
       Voltage Supply    50 volts                                         
V.sub.2                                                                   
       Voltage Supply    15 volts                                         
V.sub.3                                                                   
       Voltage Supply    15 volts                                         
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     To those skilled in the art it will be obvious upon a study of this disclosure that the present invention permits a variety of modifications in structure and arrangement and hence can be given embodiments other then particularly illustrated and described herein, without departing from the essential features of the invention within the scope of the claims annexed hereto.