the block diagram of fig2 of the drawing reveals the broader combinational aspects of the improved receiver , and may enlighteningly be compared with the block diagram of fig1 depicting a conventional prior art receiver for the same application . in the improved receiver diagrammed in fig2 certain blocks are identified with reference numerals identical to ones employedin fig1 ( i . e ., the antenna 10 , the light circuits 40 and the speaker or phones 54 ), and , as to those elements , it should be understood that the corresponding elements of the improved receiver may be identical to those previously described for the conventional prior receiver of fig1 . similarly , it will be noted that the tuner of the improved receiver is identified in fig2 by the reference numeral 20 &# 39 ;, and it should be understood that the tuner 20 &# 39 ; of the improved receiver is identical to the tuner 20 previously described in connection with the conventional prior receiver of fig1 except that the tuner 20 &# 39 ; has no need for and omits the external connection 62 with the internal a . g . c . circuit 38 required in the tuner 20 . the first element of the improved receiver that departs from prior practice in marker beacon receiver equipment is the radio frequency attenuator 70 that has been added , which permits elimination of the &# 34 ; false &# 34 ; a . g . c . input conventionally heretofore employed . the attenuator 70 has a radio frequency signal input terminal coupled with the antenna 10 as at 111 , a radio frequency signal output terminal coupled with the radio frequency input terminal of the tuner 20 &# 39 ; as at 113 , and a control voltage input terminal as at 115 . internal details of the attenuator will be subsequently described with reference to fig3 . the audio output signal terminal of the tuner 20 &# 39 ; is coupled with the signal input terminal of the light circuits 40 as at 141 , and the light circuits 40 may have associated therewith a test circuit as at 47 , 48 and 49 , all in the same manner as previously described for the corresponding parts of the conventional receiver of fig1 . the next difference between the improved receiver and prior conventional marker beacon receivers is that , whereas the latter typically employed a conventional two - terminal audio amplifier 52 for receiving the audio output from the tuner 20 , amplifying all of the received audio signals and delivering the same to the speaker or phones 54 ( and had no reason to do otherwise ), the improved receiver uses a three - terminal operational amplifier 80 for not only performing an audio signal amplifying function , but also to serve as an electrically controllable gating device for either delivering or blocking the delivery of audio signals to the speaker or phones 54 , depending upon whether or not a control voltage is being applied to a control voltage input terminal 181 of the op amp 80 . the audio signal path from the output terminal of the tuner 20 &# 39 ; to the audio frequency signal input terminal of the amplifier 80 is identified by the reference numeral 183 , and a path from the signal output terminal of the amplifier 80 to the speaker or phones 54 is labeled 185 in the drawings . it will be noted that a normally closed on - off switch 187 is also depicted as provided in series with the signal path 185 , which bears a functionally superficial similarity to the switch 57 of fig1 and which is not surprising since the switch in question is physcially identical and typically is present as a part of the regular audio signal switching panel in the aircraft . what is different , however , is that , whereas the switch 57 must be manually and successively opened and reclosed by the pilot to accomplish audio muting with conventional prior marker beacon receivers , the switch 187 of the improved receiver will remain continuously closed whenever the improved marker beacon system of the aircraft is being used ( with audio muting being automatically accomplished in the manner to be further described ). further details of the amplifier 80 itself will also be subsequently discussed with reference to fig3 . the next new element of the improved receiver , from the combinational viewpoint , is a peak detector 90 , which converts an audio frequency output of at least a selected minimum magnitude from the amplifier 80 into a direct current trigger signal . a branch 189 of the output lead 185 from the amplifier 80 delivers the audio frequency output from the latter to the signal input terminal of the trigger producing peak detector 90 , and the direct current trigger signal output of the latter is delivered at output terminal 191 thereof . the internal details of the peak detector 90 will be further discussed in connection with fig3 . the next additional element in the improved receiver is a timed control voltage generator 100 having a trigger input terminal coupled with the trigger output terminal 191 of the peak detector circuit 90 as at 103 , and a direct current control voltage output terminal 105 at which a control voltage output of predetermined duration is presented in response to a trigger signal from the peak detector 90 . the internal construction of the control voltage generator 100 , including certain additional terminals thereof used for timing purposes and not shown in fig2 will also be further discussed with reference to fig3 . the next aspect of the improved receiver to be observed from fig2 is that the direct current control voltage output , when presented at the output termianl 105 of the timed control voltage generator 100 , is delivered both to the control voltage input terminal 115 of the attenuator 70 via leads 117 , 119 and 121 and to the control voltage input terminal 181 of the audio frequency gating amplifier 80 via leads 117 , 119 and 123 . it will be understood that it is such control voltage applied to the attenuator 70 and the gating amplifier 80 which respectively serves to activate the former to reduce the sensitivity of the receiver and to activate the latter to block the passage of audio signals to the speaker or phones 54 . there remains one other aspect of the improved receiver , as depicted in fig2 to be mentioned . that relates to the apparent presence of an &# 34 ; auto - hi &# 34 ; switch 150 , which really should not be needed when the improved receiver is functioning properly and may be regarded as optional . as will be apparent , if such a switch 150 is used , it will normally be left continuously in its &# 34 ; auto &# 34 ; position ( which effects no connection ), although it could be shifted to its &# 34 ; hi &# 34 ; position to shunt any control voltage from the terminal 105 of the control voltage generator 100 to ground ( leaving the attenuator 70 in its non - attenuating or higher sensitivity state and leaving the speaker or phones 54 active ). however , there are two reasons why it might be desirable to provide such a switch 150 . first , it is conceivable that a particular pilot might prefer to always operate the marker beacon receiver at maximum sensitivity and with the speaker or phones 54 active , at least under adverse radio reception conditions or the like ; in essence , if the switch 150 is provided and ever moved into its &# 34 ; hi &# 34 ; position , the improved receiver would then operate quite similarly to the conventional receiver of fig1 . secondly , and probably of more pratical significance until pilots become accustomed to automatic control of marker beacon receivers , is the fact that pilots have for many years been using the &# 34 ; hi - lo &# 34 ; switch 61 of conventional receivers , and it may be reassuring for the improved receivers to provide a similarly appearing switch ( even though there really should probably never be any need to actually use it ); in the latter context , such a switch 150 could be given an even more similar appearance to the typical switch 61 if provided with a third position for implementing the temporary lamp test function , as is frequently done with the switches 61 of conventional prior receivers . reference is now made to fig3 for the purpose of further explaining certain of the more detailed aspects of the currently preferred construction for the improved receiver , including the internal circuitry of certain of the modules involved and some observations with respect to proper adjustment of various operating parameters . it will be noted that , in fig3 a system ground is indicated at various places by the reference numeral 6 , and that a source of operating potential designated &# 34 ;+ v &# 34 ; is represented at various points by terminals 8 . the radio frequency input signals from the antenna 10 to the input terminal 111 of the radio frequency attenuator 70 are fed to a pair of oppositely grounded parallel resistances 71 and 72 , the latter of which is provided with a variable tap 73 . the resistances 71 and 72 present a resistive input impedence to signals from the antenna 10 , which are typically fed to the receiver through a 50 ohm coaxial tranmission line . the tap 73 is coupled with the radio frequency signal output line 113 of the attentuator 70 for applying the received radio frequency signals to the signal input terminal of the tuner 20 &# 39 ;, either with or without the predetermined attenuation provided by the portion of the circuit next to be described . a variable resistance 74 is coupled with the collector electrode of a solid state , electronically controlled switching component 75 having its emitter electrode grounded , so that the resistance 74 and the collector - emitter internal half of the device 75 provide a shunt path from the output lead 113 to ground for attenuating radio frequency signals on the line 113 to an extent determined by the setting of the tap 76 of the resistance 74 when the collector - emitter path of the device 75 is rendered conductive . the device 75 is thus operated in its switching mode with its base electrode serving as the control terminal thereof . the control voltage input terminal 115 of the attenuator 70 is coupled with the base electrode of the device 75 through a resistance 77 , and a grounded decoupling capacitor 78 is provided for the base electrode of the device 75 . it will be understood that , in the absence of application to the base electrode of the switching device 75 of a control voltage of predetermined level , the collector - emitter path of the device 75 will remain nonconductive and the shunt path through the resistance 74 and the collector - emitter path of the device 75 will not be attenuating radio frequency signals upon the line 113 , thereby maintaining the receiver in a high sensitivity condition . when an appropriate control voltage is applied to the base electrode of the device 75 , however , the collector - emitter path of the latter will be switched to a conductive state , so that attenuation of radio frequency signals upon the line 113 occurs in an amount determined by the setting of the adjusted value of the resistance 74 . suitable values for the resistances 71 , 72 , 74 and 77 respectively are 100 ohms , 500 ohms , 500 ohms and 10 , 000 ohms . the decoupling capacitor 78 may be about 470 picofarads . the device 75 may be any suitable switching transister adapted for handling radio frequency signals . in adjusting the improved receiver for operation with existing marker beacon transmitter systems , the tap 73 of the resistance 72 should be set to provide a threshold for activating light circuits 40 when a radio frequency signal of about 200 microvolts is applied to the antenna input terminal 111 . the tap 76 of the variable resistance 74 is then set to provide a &# 34 ; light threshold &# 34 ; for activating the light circuits 40 of about 1 , 000 microvolts of radio frequency input signal when the device 75 is switched into and operating in its conductive state . the audio frequency gating amplifier 80 utilizes as its primary component a solid state operational amplifier device 81 having an inverting input terminal marked &# 34 ;-&# 34 ; a non - inverting input terminal marked &# 34 ;+&# 34 ; and an output terminal . the op amp circuitry for the gating amplifier 80 conventionally employs a resistance 82 coupled between the inverting and output terminals of the device 81 , a grounded resistance 83 coupled with the output terminal of the device 81 , and an output coupling capacitance 84 coupled with the output terminal of the device 81 . audio frequency signals are delivered from the output terminal of the tuner 20 &# 39 ; to the inverting input terminal of the device 81 via a line 183 and through an input coupling capacitor 85 . a reference potential is applied to the non - inverting input terminal of the device 81 through a resistance 86 and lead 87 . provision is made for also applying a control voltage input from the control input terminal 181 to the non - inverting terminal of the device 81 through a blocking diode 88 . suitable values for the resistances 82 , 83 and 86 are respectively 470 , 000 ohms , 1 , 000 ohms and 1 megohm . the output capacitance 84 may be about 1 microfarad , and the input capacitance 85 may be of comparable value depending upon the particular operational amplifier component employed to implement the device 81 . the level of the reference potential applied to the non - inverting input terminal of the device 81 is , of course , chosen to permit the device 81 to pass and amplify audio frequency signals applied to its inverting input terminal when no control voltage from the terminal 181 is being applied to its non - inverting input terminal , but to cause the device 81 to block the passage of audio frequency signals applied to its inverting input terminal through to its output terminal when a control voltage of predetermined level is applied to the control input terminal 181 of the amplifier 80 . any audio frequency output signals passed by the gating amplifier 80 are delivered to the speaker or headphones 54 via the output line 185 and the switch 187 ( which will typically remain closed whenever the improved receiver is in operation ). any audio frequency output passed by the gating amplifier 80 is also delivered to the input terminal 189 of the peak detector circuit 90 for producing a trigger input signal in response to application to the input terminal 189 of audio signals of at least an adjustable predetermined level . within the peak detector module 90 , the audio frequency input signals at the input terminal or lead 189 are applied to oppositely grounded resistance 91 having a variable tap 92 . the tap 92 is coupled with the input electrode of a rectifying diode 93 , which serves to convert audio frequency signals from the tap 92 into a direct current potential at the output electrode of the diode 93 . such direct current potential is applied via a line 94 to the base electrode of a solid state direct current amplifier device 95 . the emitter electrode of the device 95 is grounded , and the lead 94 from the output electrode of the diode 93 to the base electrode of the device 95 is coupled with an oppositely grounded capacitor 96 and a resistance 97 . the collector electrode of the device 95 is coupled through a resistance 98 with the source of operating potential 8 and with the output terminal 191 of the peak detector module 90 via a lead 99 . suitable values for the resistances 91 , 97 and 98 are respectively 10 , 000 ohms , 1 megohm and 47 , 000 ohms . the capacitor 96 may be about 0 . 1 microfarad . the device 95 may be implemented with any transistor adapted for use as a direct current amplifier at the signal levels involved . the variable tap 92 of the input resistance 91 should be adjusted such that the diode 93 remains conductive until the level of a received radio frequency signal conventionally modulated with marker beacon tone code groups reaches or excedes a threshhold value of about 250 microvolts . it should be understood , therefore , that the module 90 is adapted to produce a direct current trigger signal at its output terminal 191 as an aircraft is traversing the radiation pattern from a ground - based marker beacon transmitter and has penetrated such pattern to an extent such that the intensity of the marker beacon signals have increased to a level at which they may be reliably received by the improved receiver with the latter operating at reduced sensitivity through activation of the attenuator 70 in the manner next described . the timed control voltage generator 100 can most conveniently be implemented through the use of one of the commercially available solid state circuitry chip devices designed for providing a control voltage output of an appropriate level for a predetermined period of time established by an external resistance - capacitance time constant circuit coupled with terminals of the chip device in response to application to another terminal of the chip device of a direct current trigger signal of appropriate level . a number of such devices are available and well understood by those skilled in the art . for illustrative purposes , however , fig3 depicts such a device 102 as implemented with a signetics type ne555 timer chip or a functional equivalent thereof marketed by the same or another manufacturer . since the internal nature of such chip devices is both irrelevant to the improvement of marker beacon receivers claimed herein and a matter of public knowledge , it should be sufficient herein to describe the operation of the device 102 only in functional terms , it being noted that the pin terminal numbers for the type ne555 chip illustrated are shown in fig3 . pin 1 of the illustrative device 102 is grounded , and a source of operating potential 8 is coupled with pins 4 and 8 thereof via a lead 104 . pin 2 of the device 102 is coupled with the trigger input terminal 101 of the generator 100 , which is in turn coupled with the trigger output terminal 191 of the peak detector module 90 by a lead 103 . the resistance - capacitance time constant circuit for the device 102 , which controls the duration of the control voltage output provided by the generator 100 , includes the resistance 106 coupled between pins 4 and 8 of the device 102 and pins 6 and 7 of the device 102 , in conjunction with the capacitor 107 that is coupled between pins 6 and 7 of the device 102 and the system ground . the control voltage output generated by the device 102 is presented at pin 3 thereof , which is coupled through a resistance 108 with the control voltage output terminal 105 of the generator 100 . an oppositely grounded capacitor 109 is also coupled with the output terminal 105 of the generator 100 , which in conjunction with the resistance 108 provides a decoupling filter for any radio frequency signals that might be present upon the connection of the control voltage output terminal 105 of the generator 100 with the control voltage input terminal 115 of the attenuator 70 . it is currently preferred to utilize control voltage signals of approximately 15 seconds duration , and , to that end , suitable values for the resistance 106 and capacitor 107 respectively are about 2 . 2 megohms and about 6 . 8 microfarads . suitable values for the resistance 108 and the capacitor 109 are respectively about 10 , 000 ohms and about 4 . 7 microfarads . the control voltage output terminal 105 of the generator 100 is coupled via leads 117 , 119 and 121 with the control voltage input terminal 115 of the attenuator 70 for causing the switching device 75 of the latter to conduct and provide a predetermined amount of attenuation for radio frequency signals passing through the attenuator 70 , to reduce the sensitivity of the improved receiver during the period that a control voltage output is presented at the output terminal 105 of the generator 100 . the control voltage output from the terminal 105 of the generator 100 is also applied to the control voltage input terminal 181 of the gating amplifier 80 via leads 117 , 119 , 123 and a resistance 125 ( which may be about 10 , 000 ohms ) for changing the reference potential on the non - inverting terminal of the operational amplifier device 81 to block the passage of audio frequency signals through the latter and thereby mute the speaker or headphones 54 during the duration of the control voltage signal being applied . again , the previously discussed optional presence of the &# 34 ; auto - hi &# 34 ; switch 150 is depicted in fig3 with the pole element thereof coupled with the control voltage output terminal 105 of the generator 100 via leads 117 and 127 , while there is no connection to the &# 34 ; auto &# 34 ; contact 129 thereof , and the &# 34 ; hi &# 34 ; contact 131 thereof is coupled with system ground via a lead 133 . as earlier herein noted , with the improved receiver of the invention , it is anticipated that there should be little or no occasion for ever moving the switch 150 away from its &# 34 ; auto &# 34 ; position , which is essentially , non - functional . on the other hand , it may be noted that , if the switch 150 should ever be shifted into its &# 34 ; hi &# 34 ; position , the effect would be to shunt any control voltage output from the terminal 105 of the generator 100 to ground , thereby maintaining the improved receiver in a continuous state of high sensitivity operation with the automatic control functions of the gating amplifier 80 , peak detector 90 and control voltage generator 100 , as well as the attenuating capability of the attenuator 70 , deactivated . those skilled in the art will readily appreciate that a number of minor modifications or variations can be made to the currently preferred embodiment of the improved receiver disclosed for illustrative purposes without departing from the essence of the invention . accordingly , it is to be understood that the claims which follow should be construed and interpreted in accordance with their natural scope and as including a fair range of equivalents .