This invention relates to an AM-FM detector circuit, i. e. a detector circuit which is capable of detecting either the AM-IF (amplitude-modulation intermediate frequency) signals or ^* the FM-IF (frequency -modulation intermediate frequency) signals developed by the IF amplifier of a superheterodyne broadcast radio receiver. The capability of handling either AM or FM signals by the detector circuit, and also by the IF amplifier, is achieved by sharing of circuit components for the AM and FM operations. In this specification reference will be made to the following literature:

United States Patent Corresponding British Specification No. Patent Specification No,

AA. 3, 936, 750 1, 511, 041

McFadyen and Peil 1, 511, 042 1, 511, 043

BB. 3, 999, 138 1, 503, 171

McFadyen and Peil 1, 503, 172

1, 503, 173

CC. 4, 050, 022 McFadyen and Peil

DD. Publication: Service Information for Model 7-2810 F FM/AM Portable Radio, published by General Electric Company, Audio Electronics Technical Services, Utica, New York, U. S.A., June 1976

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Subsequent reference to the above-tabulated literature will be simpl AA, BB et cetera.

In particular, the invention provides improvement over the FM/AM detector circuit disclosed in BB, and also in CC, in respect to bias and output voltage variations in AM and FM operation. While BB and CC disclose an AM-FM detector circuit of highly satisfactory operation well suited for integrated circuit (IC) fabrication, un¬ avoidable discrepancies in the processing of the IC's create differences in the electrical properties of the IC' s active and passive components from one circuit to another, which are the result of differences relating to geometry of the semiconductor components. These discrepancies, however slight, tend to produce bias and output voltage variations which reduce the s -called yield, i. e. the ^* percentage or ratio of the fabricated circuits falling within acceptable, limits, to -the - . total number of fabricated circuits...- ; In addition, in operation the detector circuit of BB or CC may exhibit additional bias and output signals variations with changes in ambient temperature. It is difficult to compensate integrated circuits fox temperature instability, which results in drifts even in FM ^' operation, and perhaps even to greater extent in AM operation. The AM-AGC

(automatic gain control) voltage has a tendency to drift with temperature change, which requires a tightening of the tolerances and thereby acts to further reduce the yield.

It is therefore an object of the invention to provide an improved AM-FM detector circuit wherein bias and output voltage variations due to unavoidable IC process changes and due to ambient temperature changes, are significantly reduced.

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The AM-FM detector of the present invention meets this objective. It is, generally speaking, organized in a manner siπiilar to that of BB or CC, and as such it comprises at its input, a different¬ ially connected emitter follower pair which in FM operational use functions to pass the greater, or more dominant of two FM-IF input signals, which are phase displaced at the FM-IF carrier frequency by 90 , with the phase displacement varying continuously about the 90 , in accordance with the modulation. In AM operational use, that emitter follower pair operates on incoming AM-IF signals as a full-wave peak rectifier. Onward of the emitter follower pair is an FM-channel, which comprises a zero crossing detector, and an output, stage which provides the recovered audio modulation and an AFC (automatic frequency control) signal.

The emitter-follower pair, and the FM channel hereof incorporate substantial improvements vis-a-vis BB or CC, in the interest of stability. The AM channel hereof incorporates sub¬ stantial departure from that of BB or CC, also in the interest of stability. As such it features a linear differential amplifier, which . feeds into the same output amplifier as the FM channel; the latter output amplifier is fully active in both FM-mode and AM-mode, and in the latter mode provides the recovered audio and an AGC signal. Other objects, advantages, and features of the invention will appear in the following, more det ailed specification which includes the appended Claims and Abstract, with reference to the accompanying drawings, in which:

Figure 1 is a block diagram of a superheterodyne radio receiver circuit embodying the inventive AM-FM detector circuit;

Figure 2 is a primarily schematic and partly block circuit diagram of the AM-FM detector circuit, and of associated circuitry in accordance with the invention;

Figure 3 is a series of graphs employed in the description of the circuit of Figure 2.

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With reference to Figure 1, shown therein is the FM-AM detector circuit which is the subject of the present invention, incorporated in a typical superheterodyne radio receiver circuit, the components of which are presented in block form. An antenna arrangement 2 is capable of receiving FM or AM signals at their respective radio frequencies (RF), and couples them to an AM-FM converter 3, which converts the signals to the intermediate frequency (IF) for AM or FM, depending upon the selected mode of operation. The IF signal, having a center frequency nominally of 455KHz for AM and 10. 7MHz for FM, is coupled to an IF amplifier 4 which provides adjacent channel at¬ tenuation and functions to amplify the AM-IF signals or the FM-IF signals. AM-FM converter 3 and IF amplifier 4 may be known com¬ ponents such as disclosed in AA orCC. Indeed, since the C C disclosure incorporates the disclosures of AA and BB, with some improvements to the radio receiver circuits shown in AA and BB, it will be convenient to assume that the block diagram of Figure 1 hereof and the circuitry of Figure 2 hereof, are those of CC with the exception of the FM-AM detector circuit which is the subject of the present invention, and with some other minor exceptions pointed out hereinafter. The disclosure of CC is incorporated herein by reference. One such minor exception is given here; it concerns the AM-FM converter 3. The AM portion of the converter 3 may be assumed to be that of CC. As to the FM portion of the converter 3, it is remarked here that the FM converter is usually "off" the IC chip, even to the extent that its transistors ' and resistors are off the IC chip. Therefore, a variety of such FM converters have been utilized, depending upon the particular kind of radio receiver desired (table model, portable etc. ) . Hence, the FM converter within block 3 of Figure 1 could be that of CC or that of AA or that of DD, or other similar FM-converter.

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The output of the IF amplifier 4 is coupled via line 16 to one input of the AM-FM detector 1, and via branch line 16A ^' to the input of the AM-FM filter 5. The lines 16 and 16A carry the same signal in either FM or AM operation. The filter 5, at its input contains an FM parallel resonant circuit (significant in FM, insignificant in AM) and an AM parallel resonant circuit (significant in AM, insignificant in FM). These resonant circuits insure that the signal on line 16

(and also on line 16A) is maximized in amplitude. The signal on line 16, i. e. ingoing to the AM-FM detector 1 , is hereinafter referred to as the "in-phase signal" or being of "0° phase" or "reference phase" relative to the signal outgoing from the AM-FM filter and applied as second input signal to the AM-FM detector 1.

The AM section of the filter 5 produces its output signal — irignfrg via line 18 into the second input of the detector 1 — as a 180° phase shifted version of the in-phase signal on line 16. The FM section of the filter ^' 5 produces its output signal _. -* -ingoing via line 18 into the second input of the detector 1 — with a phase shift, relative to the in-phase signal on line 16, which is a linear function of the instantaneous frequency of the FM-IF signal; at the nominal center frequency of 10.7MHz the phase shift is about 90 .

The detector circuit, which will be described in detail when considering the schematic circuit diagram illustrated in Figure 2, operates to automatically detect either the AM or FM/IF signal. The circuit is particularly suited to IC fabrication for providing improved yields through significantly reduced bias and output voltage variations in the presence of unavoidable IC process and ambient temperature changes, and also provides extended dynamic range of AM signal response. The detected signal at the output of the detector 1 is coupled to a de-emphasis network 6 and thence to an audio amplifier 7 and speaker 8. The detected signal is also

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coupled via line XL to an automatic gain control-automatic frequency control (AGC-AFC) network 9. The network 9 is referred to in AA and BB as "bias supply" or "controlled current source" or "adjustable current source" or "AFC-AGC amplifier" . For present purposes, the network 9 may be assumed to be as composed of the following components of CC: transistors Qll, Q17, Q22, Q23 and the circuitry associated with these transistors.

The output of the AGC-AFC network or amplifier 9 is coupled via "primary bus" 131 to the IF amplifier 4 and the converter 3, to effect automatic gain control in AM operation and automatic frequency control in FM operation; also to apply greater input current and voltage to the IF amplifier 4 in FM operation than in AM operation, and hence provide greater IF amplification in FM operation than, in AM operation. The primary bus 131 corresponds to the primary bus 31 of

CC; for other components hereinafter mentioned and identified by reference numeral which is in the range of from 120 to 199, it may be assumed that these correspond respectively to components of CC having the same two least significant digits. The dβemphasis network 6 is the same as that shown in BB or CC, as will be explained hereinafter. The audio amplifier 7 may be of conventional circuitry; however, if it is desired to provide the audio amplifier in integrated circuit format, integrated together with the other integrated circuit components of Figure 1 on a single IC chip, the audio amplifier may be of the type shown in U. S. Patent Specification No. 4, 025, 871 Peil or its corresponding British Patent Specification No. 1, 85, 682.

Referring to Figure 2, shown therein is the penultimate stage (Q7, Q8) and the final stage 135 (Q9, Q10) of the IF amplifier 4, which are replicas of the corresponding IF amplifier stages of CC, and utilizing the same identification as in CC. As to Q7, Q8, noteworthy

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here are the collector circuit connections to the primary bus 131, the secondary bus ^* 132, these being typical also for still lower preceding ^■ stages, plus the feedback resistor R9 to the base of Q8, which is the origin of the secondary bus 132. As to the final stage 135, the current source Q12 of Figure 2 hereof is intended not only to reflect the transistor Q12 of CC, but also the circuitry associated with transistor Q12 of C C. Input IF signal from the preceding IF amplifier stage Q8 ' is applied to the base of Q9, whereas the base of Q10 is connected to the secondary bus 132 . In AM operation, the bus 132 provides AGC voltage to _the bases of -Q8 and Q10 as well as to the bas.es of some transistors of the preceding IF amplifier stages. In FM operation, the bus 132 provides higher dc potential to the bases of Q8 and Q10 and of other transistors, to match the higher d. c. potential provided to the bases of Q7 and Q9. The manner of operation of the IF amplifier is more fully described in AA. There is one difference as between the final stage 135 hereof and the final stage 35 of CC. As shown in Figure 2 hereof, the transistor Q9 has its collector supplied by the d. c. voltage V ₁ which is + 3 volts, in contrast to CC where the collector supply voltage for Q9 is + 6 volts. The reason for this is that Q10 hereof is also supplied by the + 3 volts as will be presently seen, in contrast to Q10 of CC which is supplied by + 6 volts. The usage of the lower + 3 volts for the final IF amplifier stage 135, rather than + 6 volts, does not affect the performance of the IF amplifier per se, nor in a direct sense. However, the usage of the + 3 volts for supplying the transistors Q9 and Q10 has an import__nt impact on the operation of the FM/AM detector 1 of the present invention, which then in an indirect way affects the performance of the entire IF amplifier 4 in a beneficial manner, as will be described hereinafter. The AM/FM filter 5 is essentially the same as shown in

DD or BB , and in effect is also equivalent to the AM/FM filter shown in CC.

As shown in Figure 2, the FM filter section 10 of filter net¬ work 5 includes a first parallel resonant tank circuit 11 at the input and a second parallel resonant tank circuit 12 shunted by a resistor 13 at the output. Input and output are capacitively coupled by a capacitor 14 which co operates with resistor 13 and circuit _2to produce the requisite 90 phase shift at the resonant frequency of 10. 7MHz. The collector of Q10 is connected to the conductor 16, and to the inductor 11A of the input tank circuit, the output of the FM filter section being coupled via a transformer 15 to the conductor 18. The AM filter section 20 includes a transformer 22 having a tuned primary winding 22A,with the untuned secondary winding 22B providing a phase inverting output connection to conductor 18. The + 3 volts d. e. is supplied to the collector of Q10 via the primary winding 22A of the AM transformer 22, thence through its center-tap, thence through the input winding 11A of the FM resonant circuit. The + 3 volts d. c. , and also the IF signal (FM or AM) appearing at the collector of Q10, are propagated via conductor 16 to the base of transistor 24, which in terms of Figure 1 is the first input of the AM-FM detector 1. The + 3 volts d. c. is also passed via the serially connected secondary windings 22B and 15B of the AM transformer, and the FM transformer, and via conductor 18, together with the phase shifted IF signal (FM or AM) to the base of transistor 26, which in terms of Figure 1 is the second input of the AM-FM detector 1. The "dots" associated with the windings 22A, 22B and 15A, 15B are in line with the usual dot convention. As to the AM transformer 22, they reflect the required phase reversal (180 phase shifty . As to the FM transformer, the dots have special significance discussed hereinafter. The filter sections 10 and 20 provide the phase shift functions required for

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the hereinafter described detection process, as well as additional adjacent channel attenuation.

The FM-AM detector circuit 1 has at its input a differen¬ tially connected emitter follower pair of the previously mentioned transistors 24 and 26 employed in both AM and FM signal detection. The inphase IF signal is applied to the base of transistor 24, and the phase shifted signal is applied to the base of transistor 26. The collectors of these transistors are joined and connected to a second regulated d. c. voltage supply V ₂ which is typically + 6 volts, i. e. of substantially greater d. c. voltage than V. applied to their bases, for reasons to be considered hereinafter. Note that in BB or CC, the bases and collectors of their corresponding transistors were all supplied by + 6 volts; this points to an important distinction in the generally similar differentially connected emitter follower pairs. As in BB or CC, the emitters of transistors 24 and 26 are joined and connected to the collector of a first current source comprising NPN transistor 28, to the base of an NPN transistor 30 which with NPN transistor 32 comprises a zero crossing detector 33 that is operative during FM detection, but the devices 28 and 33 differ substantially from the corresponding devices of CC or BB. Moreover, and this is new vis-a-vis BB or CC, the commoned emitters of transistors 24 and 26 are additionally connected to the base of an NPN transistor 34, which with NPN transistor 36 comprise a linear differential amplifier 37, which provides a first stage preamplification of the AM audio signal, and of the dc AGC signal. The emitter of the first current source transistor 28 is connected via a current determining resistor 54 to a "mode switching control bus " 133, which in turn is connected to a mode setting switch 46 having one fixed contact 47 at ground for FM operation, and another fixed contact 49 at a dc voltage V_ , for example + 1.2 volts,

in AM operation. The dc voltage V ₃ maybe fixed at typically + 1.2 volts, as is shown in AA or BB, or it may be provided from the secondary bus 132 (as in CC), in which case the mode switching control bus 133 corresponds to the mode switching control bus 33 of CC.

The zero crossing detector transistor 30 has its collector electrode connected through a diode 38, poled as shown, to V and to the base of transistor 32, the collector of which is coupled to the collector of PNP transistor 40 of an output amplifier network 41. The emitters of transistors 30 and 32 are connected to the collector of a second current source comprising NPN transistor 42. The d ual c onn e c tio ns between transistors 30 and 32 pro¬ vide, for regenerative, trigger-type action*. The emitter of transistor 42 is connected through a current deter_____t_ing resistor 44 to the mode switching control bus 133. Connected to a temperature stable reference voltage reference point 59 are the bases of the following transistors: first current source transistor 28, second current source transistor 42, and third current source transistor 60 discussed hereinafter, and an "FM-AM switch" transistor 70 also discussed hereinafter. The temperature stable reference voltage point 59 is provided by a pair of resistors 48 and 50 and a trnprs-iTpe <______- pensating diode 52 (poled as shown) , one side of the resistors being connected to the regulated dc voltage supply V. and the other side connected through diode 52 to ground. The reference voltage point 59 is established at the junction of resistors 48 and 50. Current sources or switching transistors, generally speaking somewhat analogous to transistors 28, 42 and 60, appear also in BB and CC, but important differences exist.

The zero crossing detector 33 generates output current pulses with a duty cycle as a function of FM signal modulation.

* Note that the collectors of transistors 30 and 32 are isolated, i. e. not intercoupled by a resistance, as in BB or CC.

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Transistors 30 and 32 have current conduction properties closely matched to each other and to those of emitter pair transistors 24 and 26, which in IC fabrication is done through precise control of the base-emitter junction geometry of the devices. In the present circuit the transistors are all of minimum junction geometry, except as otherwise noted. Minimuin junction geometry, or minimum geometry should be understood to mean the minimum area assigned to the transistor junctions, for a given IC process, necessary to provide requisite small signal properties on the transistor. As will be more fully described, by setting the currents in current source transistors 28 and 42 through resistors 54 and 44, respectively, which currents are made temperature stable, and by controlling device geometry, the current ratio between transistors 30 and 32 can be properly established to insure that with the radio receiver set to the FM-mode, there is no steady-state shift of the dc voltage level on the AFC line XL (Figures 1 and 2) upon transfer from any one, to any other, of the following conditions:

(1) The "static" condition of "no FM-IF input signal is present". This static condition exists in an operational radio receiver, when the receiver is detuned from all broadcast stations. The static condition is also deliberately produced during testing and line-up of the radio receiver as an important criterion for these purposes, as will be evident hereinafter.

(2) A first ' ^■ dynamic" condition of unmodulated FM-carrier (10.7 Mhz) injected into the FM-AM detector 1, and hence propagated via transistors 24 and 26 to transistors 30 and 32. This first dynamic condition is an important criterion for testing and line-up of the radio receiver. In actual FM operational use, the first dynamic condition might exist when the radio receiver is tuned in to receive an FM broad- cast station, which right then there is transmitting, but is observing a brief "audio silence", as a symbol of mourning for example.

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(3) A second "dynamic" condition of modulated FM -carrier (10.7 Mhz) injected into the FM-AM detector, and hence similarly progagated. This second dynamic condition exists in an operational radio receiver, when it is tuned in to receive a broadcast station so that the broadcast audio signal is reproduced in the loud ^* speaker. The second dynamic condition is produced for testing and line-up purposes, where it is an important criterion, by injecting into the FM-AM detector 1 . FM-IF carrier (10. 7 Mhz) frequency -modulated with a "sweep- signal", i. e. one that repeatedly sweeps through the useful AFC-band, as will be explained hereinafter.

Maintaining the same dc voltage level on the AFC line XL is important in regard to (1) maintaining stability in the AFC loop, and consequently maintaining stability of tuning, and indeed maintaining stability of the receiver, and (2) minimizing audio distortion in FM- reception. This will be evident from the subsequent description. The circuit configuration of the zero crossing detector network 33 insures that the current ratio between transistors 30 and 32 remain constant and that audio distortion be minimized even in the presence of any changes in value of total current carried by transistors 30 and 32, which changes may occur due to IC process variations or ambient temperature changes.

With respect to linear differential amplifier 37, its circuit configuration provides the generation of an output current that accurately reflects the AM signal modulation independently of IC process variations and ambient temperature changes. Transistors ^" 34 and 36 have matching current condition properties. Their emitters are coupled through the series connection of equal value resistors 56 and 58 (each typically 3 K), the junction of which is coupled to the collector of the third current source NPN transistor 60. The collector of transistor 34 is connected to the +6 volt line V„ and to the collector of bias current NPN transistor 62, the base of which

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is connected to the + 3 volt line V\ and the emitter of which is connected to the base of transistor 36. The collector of transistor 36 is connected to the collector of PNP transistor 40 of output network 41. Transistor 62 has a base-emitter junction area twice that of transistors 24 and 26 so as to provide equal current conduction in transistors 34 and 36 in the absence of AM-IF signal. The emitter of transistor 60 is connected through current determining resistor 64 to ground, and its base is connected to the reference point 59 as previously stated. The emitter of transistor 60 is also connected to an AM-FM transistor switch arrangement 65 comprising PNP transistors 66, 68 and NPN transistor 70. Transistor 68 has twice minimum junction geometry. Transistor 66 is connected as a diode. The emitters of transistors 66 and 68 are connected to the + 3 volt line V. . The base of transistor 66 is connected to its collector to form the diode, and also to the base of transistor 68 and to the collector of transistor 70. The collector of transistor 68 is connected to the emitter of transistor 60. The emitter of transistor 70 is connected through a resistor 72 to the mode switching control bus 133, as previously stated. The output amplifier network 41 comprises afirstP!S!P i-puttr_rs- istor 40,a current multiplier or output PM ⁵ transistor 74, these two having matching current conduction properties, and a • second PNP i n p u t transistor 76. The three PNP transistors 40, 74 and 76 are active in FM mode as well as in AM mode, noting that connected together are the collectors of FM-zero crossing detector transistor 32 and of AM-differential amplifier transistor 36. In addition to other functioiis described hereinafter, the transistors 40 and 76 perform the following "d. c. level shifting" function. The base-collector junction of transistor 40 constitutes a first diode which couples to the base of the output transistor 74 the collector of either transistor 32 (in FM) or of transistor 36 (in AM). The

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base-emitter junction of transistor 76 constitutes a second such coupling diode, which is matched to the first coupling diode but poled in reverse thereto. Thus with either more positive or more negative voltage at the collector of either transistor 32 or transistor 36, transmission to the base of transistor 74 occurs via one coupling diode or the other, but with dc level shift which stems from the inherent voltage difference across one or the other coupling diode.

The transistor 74 has a base-emitter junction area twice that of transistor 40, and hence provides a current multiplication of two in the output current carried by transistor 74. The emitters of transistors 40 and 74 are connected through resistors 78 and 80,* respectively, to the + 6 volt line V„ . The bases of these transistors are joined and connected to the emitter of transistor 76, the collector of which is coupled to ground. The base of transistor 76 is coupled to the collectors of transistors 36, 32 and 40. The base to emitter capacitance of transistor 74, and the integrative effects associated with the drift time required for carriers to reach the collector of transistor 74 , provide integration of the FM output current, and to a small extent of the AM output current. The output current is taken from the collector of transistor 74, which is connected through an output resistor 82 to ground and to output terminal 84 of the detector circuit from which the detected AM or FM audio signals are derived, and from which the AM-AGC dc voltage or the FM-AFC dc voltage are applied (together with the detected audio signals) via line XI to the AFC-AGC amplifier 9. In correspondence with the AFC- AGC amplifier of CC (its Q17, Qll, Q22, Q23), or of BB (its transistors Q15, Q16, Q17 and its diode D6), the emitter of one mode- switching transistor within the block 9 — corresponding to Q23 of CC or Q17 of BB — is connected via a resistor (internal to block 9) to the mode switching line 133.

* Consonant with the current multiplication of two, the resistors 78 and 80 are in the ratio 2:1, being typically 1.2k and 600 ohm - ^* ..

The matters set forth in this arid in the next few paragraphs, will be more evident from the subsequent description of the operation of the detector 1 in AM-mode and in FM-mode. In AM-mode, of the transistors of the detector 1 shown in Figure 2, the following are active: input transistors 24 and 26, linear differential amplifier transistors 34, 36 and 62 as well as their current source transistor 60, and Output amplifier network transistors 40, 76 and 74. All other transistors are inactive.

The AM-AGC sense of the detector 1 is as follows. Assume that initially the radio receiver is tuned to receive a relatively strong AM-RF broadcast signal. This requires that the voltage on the primary bus 131 attain a minimum (composite static plus dynamic AGC) positive value, so that the gain of the IF amplifier 4 and of the AM _. section of the AM-FM converter 3 be minimum; see the description of AA.

Next assume that the receiver is tuned to receive a very weak AM-RF broadcast signal. This requires that the composite static plus dynamic AGC voltage on the primary bus 131 should attain a relative high positive value. * In consequence of such weak AM-RF signal reception, the AM-IF carrier signal developed by the IF amplifier would be relatively small, and hence at the commoned emitters of the input transistors 24 and 26 there would be developed a dynamic AGC dc voltage of relatively small positive value, and of course the demodulated audio signal would be of relatively small amplitude.

* The maximum positive value on primary bus 131 is attained in the "static" AM condition , i. e. the radio receiver is turned on, but is detuned from all broadcast stations, so that no AM-RF signal and hence no AM-EF signal is present; again see AA.

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The linear differential amplifier 37 preserves the sense of the dynamic AGC dc signal so that at the collector of transistor 36 there is developed a positive dynamic AGC voltage of relatively small value. The output PNP transistor 74 at its collector (i. e. output terminal 84 and line XL) however, inverts the sense, so that the dynamic dc AGC voltage developed thereat is negative and is of relatively small magnitude. Hence the composite static plus dynamic AGC dc voltage on line XL is relatively high. Since for weak AM signal the composite voltage on primary bus 131 is required to be relatively highly positive, it follows that the AGC-AFC amplifier 9 __x>u]d roduce __nevean-rnber cf sense- inversions; this is the case for the AGC-AFC amplifier of BB or CC, which produce two such inversions.

In FM-mode, of the transistors of the detector 1 shown in Figure 2, the following will be inactive: AM linear differential amplifier transistors 34, 36 and 62, as well as their current source transistor 60. All other transistors will be active. The FM-AFC sense of the detector is as follows. Assume first that the radio receiver is turned on, but is detuned from all FM-broadcasting transmitting stations; this corresponds to the static FM condition ("condition 1") previously mentioned. Certain static level voltages exist at the collector of zero-crossing detector transistor 32, and at the collector of the PNP output transistor 74, i. e. at terminal 84 and line XL. Correspondingly, there exists a static level voltagp on primary bus 131, which may be typically +2.4 ^" volts as described in AA or DD. Next assume that the radio receiver is turned on, and is tuned to receive an FM-RF signal from a broadcast station, which right then and there is transmitting, but is observing a brief "audio silence" , as a symbol of mourning for example. Assume further, that the initial tuning is perfect, so that the frequency of the FM local oscillator is exactly 10.7 Mhz above the FM radio frequency. Under

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these circumstances, the IF signal injected into the detector 1 is exactly 10. 7 Mhz, and moreover is unmodulated ; this corresponds to the aforementioned first dynamic FM-condition ("condition 2") utilized for test purposes. In consequence, there will be developed at the commoned emitters of the input transistors 24 and 26, a signal having the waveform shown in Graph B of Figure 3 (solid line curve 94), which is negative (relative to the horizontal time axis) for 25%, and positive 75%, of one cycle of the 10. 7 Mhz IF. In consequence, at the collector of the zero-crossing detector transistor 32 there is developed a signal having the waveform shown in Graph C of Figure 3 (solid line curve 96), i. e. a narrow negative pulse for 25%, and a wide positive pulse of 75% of one cycle of the 10.7 Mhz IF.

The waveform 96 necessarily contains a d. c component, which nevertheless has the same value as in the FM static condition, and in consequence the terminal 84 (line XL) will carry the same dc potential as it had in the FM static condition ("condition 1"), and hence the primary bus 131 will also be at its static potential of say + 2.4 volts; this is important out of considerations of maintaining stability of the IF amplifier 4, and indeed stability of the radio receiver, as has been stated before.- Prevalence of the same dc levels in "condition 1" and in "condition 2" , for the collector of transistor 32, for the collector of transistor 74 (line XI), and for primaiy bus 131, is attained when the receiver is aligned at the factory, by adjustment or tuning of FM-transformer (windings 15A and 15B) ; the alignm ait procedure is explained hereinafter.

Continue to assume perfect tune-in to the FM-RF broadcast station, but assume that audio silence has ended, so that the FM detector 1 is now producing audio output signal, which however n ecessarily has no dc component. This is the aforementioned second dynamic condition for FM mode ("condition 3"). It is quite evident,

that the dc conditions given in the immediately preceding paragraph still prevail, so that again the dc levels are the sa ne as in the static condition ("condition 1") for the collector of transistor 32, for line XL, and for primary bus 131 (typically + 2.4 volts). Next, assume again tune-in to an FM broadcast station, but with imperfect tuning, e. g. the initial tune-in causes the FM local oscillator frequency to be lower than required for perfect tune- in, and hence the FM-IF injected from the IF amplifier 4 into the detector 1, is less than 10. 7 Mhz, say 10.6 Mhz. The consequent events described in this and the following two paragraphs, are predicated on the "dot connection" for the FM transformer as shown in Figure 2. Again assume at first, FM-RF transmission, but observance of audio silence. Under these conditions, the negative excursions in Graph 94 (waveform 94) will be wider than 25%, and the positive excursions will be narrower than 75% (of one cycle of 10 Mhz), than in the case of perfect tune-in. Similarly, in Graph C (waveform 96) the negative pulses will be wider than 25%, and the positive pulses will be narrower than 75%, than in the case of perfect tune-in. Hence, the dc voltage at the collector of transistor 32 will be lower than in the static condition, and lower than in the condition of perfect tune-in. At the collector of PNP output transistor 74, i. e. at terminal 84 or line XI, there occurs an inversion of the dc AFC signal sense; the dc voltage is higher than in the static condition. Since the AFC-AGC amplifier 9 subjects the AFC dc signal to two inversions (as stated above), the voltage on primary bus 131 will analogously be greater than + 2.4 volts. Again assuming imperfect tune-in, but now presence of audio modulation, it is evident that the AFC dc relations are nevertheless the same, since the audio does not affect the dc relations. The behavior described in the immediately preceding

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paragraph, is reflected by Graph D of Figure 3, which is the "S-curve" or "FM-discriminator characteristic" produced by the FM -detector 1, at the output terminal 84. The S-curve is obtained by sweep-frequency modulating a 10.7 Mhz FM-IF test signal carrier, injecting the so modulated signal into the detector 1, i. e. subjecting it to the aforementioned second dynamic condition ("condition 3"), and observing the resultant output on a cathode ray oscilloscope, which is connected to terminal 84; this procedure is explained in greater detail detail in DD. The following comments are noteworthy in regard to the S-curve: (1) The horizontal axis is not merely indicative of frequency, but also reflects the dc level at terminal 84 in the static FM condition ("condition 1") or the first dynamic FM condition ("condition 2") , (2) The S-curve is linear within + 75 khz of the 10. 7 Mhz point 97, this linear range being required to avoid audio distortion. (3) The S-curve is symmetrical about the 10. 7 Mhz point 97; this is desirable to achieve the same dc voltage at terminal 84 in the static condition as in the two dynamic conditions. Symmetry implies that the positive characteristic in the second quadrant be a mirror image of the negative characteristic in the fourth quadrant. (4) The peaks 98A and 98B are each separated by about 200 Khz from the 10. 7 Mhz point 97. (5) The end points 99A and 99B , which define the limits of the useful pull-in range or hold-in range, are each separated by about 400 Khz from ϋzie 10.7 Mhz point 97. To achieve such wide band, for AFC purposes, a resistor might be inserted in shunt with inductor 11A, as shown in DD.

Recall the circumstances set forth two paragraphs herein- above, i. e. the unmodulated FM-IF signal is at less than 10.7 Mhz , so that the voltage on the primary bus 131 is increased above the + 2.4 volt value. To increase the frequency of the FM local oscillator (within block- 3), so that the IF be forced to 10.7 Mhz, that FM local

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oscillator must have the characteristic of incre-_sed voltage υή primary bus 131 produces increased FM local oscillator frequency, which is the case for the two rather dissimilar FM local oscillators of CC and DD. However, radio receivers of the kind contemplated herein, often contain FM local oscillators which have exactly the opposite FM-AFC response, namely assuming that the FM local oscillator frequency is too low, and hence the FM-IF lower than 10. 7 Mhz, voltage on the primary bus 131 should be decreased, so that the local oscillator frequency increase, and the FM-IF be forced back to

10.7 Mhz; an example of such opposite FM-AFC response is the FM local oscillator of AA, this despite its apparent similarity to the FM local oscillator of DD. The opposite FM-AFC response is easily implemented in the arrangement of Figure 2, simply by connecting conductor 18 to the terminal 15 Bl instead of terminal 15 B2, and vice versa for the conductor shown to be incoming (in Figure 2) to the terminal 15B1 of the FM- transformer secondary 15B. When such interchange of conductors is made, the following will be true for the conditions of FM local oscillator frequency too low, and the FM-RF broadcast station observes "audio silence" : (1) In Graph B, the narrow negative portions are narrowed and its positive portions are widened, rather than vice versa (as before) . (2) In Graph C , the narrow negat ive pulses are narrowed, and its wide positive pulses are widened, rather than vice versa (as before), and the dc voltage at the collector of transistor 32 is more positive, rather than -more negative (as before) . (3) The voltage at the terminal 84 (i. e. line XL) is decreased rather than increased (as before). The S-curve (Graph D) is accordingly inverted, being positive in the first quadrant and negative in the third quadrant. The subsequent description will be primarily for the dot connection as actually shown in Figure 2, i. e. the conductors incoming to terminals 15 Bl and 15 B2 being as

shown in Figure 2, and the consequences of interchange of these conductors can be readily inferred.

Before proceeding to describe the operation of the circuitry of Figure 2 in detail, the description of the structure of that circuitry will now be completed. The resistor 82 has a value typically of 7.5 Kohms (as is the case for the corresponding resistors of CC or BB); externally of the integrated circuit chip which Figure 2 reflects, the resistor 82 is shunted by a 0.01 microfarad capacitor (as also shown in CC or BB) to constitute the de-emphasis network 6 of Figure 1. The primary and secondary busses 131, 132, and the mode switching control bus 133, are bypassed to ground by off-the-IC-chip capacitors (not shown) which may be assumed to have the same values as in CC. The entire arrangement of Figure 2 may be assumed to be integrated together, with the exception of the mode-switch 46 and the AM-FM filter 5.

In one exemplary embodiment of the AM-FM detector circuit of Figure 2, the following circuit components and values were employed which are given principally for the purpose of illustration and are intended not to limit the invention:

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Transistors 24, 26, 28, 30, 32 Minimum Geometry NPN 34, 36, 42, 60, 70

40, 66 Minimum Geometry Lateral PNP 68, 74 Two Times Minimum Geometry Lateral PNP 62 Two Times Minimum Geometry NPN 76 Minimum. Geometry

10 Substrate PNP

Diodes 38, 52 Minimum Geometry NPN Type

Resistors 56, 58 3 Kohms

78 1200 ohms

80 600 ohms

15 82 7. 5 Kohms

48 6 Koh s

50 1 Kohm

44 870 ohms

54 1. 3 Kohms

20 64 1. 35 Kohms

72 930 ohms ige Source v. + 3 volts dc

+ 6 volts dc

+ 1.2 volts dc

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Ope ration -FM Mode

Referring to the operation of the detector circuit of Figure 2, the basic detection operation is the same as in BB and is fully described therein. In this discussion primaiy emphasis will be placed on the operation insofar as the present invention is concerned. During FM signal detection the mode select switch 46 is at ground which causes first and second current source transistors 28 and 42 to conduct since the reference voltage at the junction 59 of resistors 48 and 50 is at a positive value which is determined by V, and by the voltage drops across resistors 48 and 50, and diode 52. In addition, transistors

66, 68 and 70 (contained in the switch network 65) conduct;this develops a voltage across resistor 64 that prevents the third current source transistor 60 from conducting. Accordingly, the "AM" differential amplifier 37 is made inactive. Conduction of the current source transistors 28 and 42 make the zero crossing detector 33 active and determines the currents flowing in it and in the differentially connected emitter follower pair of transistors 24 and 26. The inphase FM input signal coupled to the base of transistor 24, having the waveform- - shown by curve 90 in Graph A of Figure 3, and the phase shifted FM input signal coupled to the base of transistor 26, having the waveform shown by curve 92 in Graph A, cause one or the other of these transistors 24, 26 to be conductive in accordance with the greater or predominant one of the input waveforms 90 , 92 to provide an output waveform as shown by curve 94 in Graph B. The output waveform 94 is a voltage appearing at the commoned emitters of transistors 24 and 26; it has variable width and variable amplitude positive and negative going portions. Significantly, the width or duration of the negative going portions is , proportional to the in¬ stantaneous frequency deviation. The negative going portions are approximately 90 wide at zero frequency deviation (10.7 Mhz) ,

corresponding to a 90° phase displacement of the second input signal (base αf transistor 26), from the first input signal (base of transistor 24) varying between 65 to 70 for negative 75 khz frequency deviation, and 110 to 115 for positive frequency 75 Khz deviation from 10.7 Mhz, for the wiring of the incoming conductors to secondary winding 15B as shown in Figure 2 (and vice versa, were these incoming conductors interchanged, for the purpose of accommodating the FM local oscillator of for example AA, as previously mentioned). As a result of the regenerative, trigger- type action of transistors 30 and 32, the zero crossing detector 33 provides, at very low input signal levels more or less linear amplificarion , but provides a limiting action even at below normal signal levels. Thus, the negative going portions of waveform 94 are converted into corresponding variable width rectangular pulses of current, such as shown by waveform 96 in Graph C, having a width or duty cycle also proportional to the instantaneous frequency deviation and an amplitude that is constant or limited; the latter proportionality and the latter limiting are necessary to avoid distortion in the audio signal. Current conduction in the transistors of the zero crossing detector for the static FM condition ("condition 1") is established by the first and second constant current sources of transistors 28 and 42, respectively, and the application of the same + 3 volts voltage V, to the bases of transistors 24 and 26 and to the anode of diode 38. The sum of the base-emitter voltage drops of transistors 26 and 30 must equal the sum of the voltage drops across diode 38 and the base emitter of transistor 32. Since the current flowing in diode 38 must substantially equal the current in transistor 30, by maMng the junction area of diode 38 and the base-emitter junction areas of transistors 24, 26, 30 and 32 equal, the voltage drop across

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diode 38 must equal the base-emitter voltage drop of transistor 30. Thus, the base-emitter drops of transistors 26 and 32 must be equal and the currents flowing in these transistors must be equal. By setting the currents in the first and second current source transistors 28 and 42 to some prescribed value, a desired current ratio between the current in transistors 30 and 32 can be readily established, so that the dc or average output current in the network 41 does not change in the transitions from the static to the first dynamic to the second dynamic FM condition. This assures generation of the symmetrical S curve at the output terminal of the detector, shown in Graph D. In the exemplary circuit under consideration, the static currents in the first and second current sources (28, 42) were set at approximately 0. 1 and 0.8 I, respectively, with 0.2 I current in each of transistors 24 and 26, 0. 6 I current in transistor 30 and 0.2 I current in transistor 32.

The currents in first and second constant current source transistors 28 and 42 are temperature stable to a high degree due to the coupling of their bases to the junction 59 of resistors 48 and 50 of the temperature compensation network. Any change in voltage due to change in ambient temperatures across the base- emitter junctions of the current source transistors 28 and 42, which have a negative temperature coefficient, is compensated by a matching negative temperature coefficient of diode 52. Any change in resistance due to change in ambient temperature of the current source emitter resistors 54 and 44, which have a positive temperature coefficient , is also substantially compensated by inherent similar . resistance changes in the circuits involved. While small changes in absolute current can be tolerated, faithful, distortion free detected output signals depend primarily on the proper ratio of the currents in transistors 30 and 32. This ratio is initially

*~~ Buϊ the current source (28, 42) emitter resistors 54, 44 are in the ratio of 6*4, being typically 1.3k and 870 ohms, because of the unequal base-emitter volt drops in transistors 28, 42, due to their unequal currents.

re¬ established by the current sources (28 and 42) and the precise control of device geometry in the fabrication of the current sources. Device geometry is an IC property that can be controlled to a high degree of accuracy for a given IC. Once the ratio is established it will remain substantially constant since any variation in device properties of one of the transistors 30 or 32 in its fabrication or in its operation due to ambient temperature change will be compensated by equal variation in device properties of the other transistor. A similar compensation in device properties is effective for transistors 40 and 74 of the output network 41 (and for transistors 34 and 36 of the linear differential amplifier 37, which is active in AM mode).

For the first dynamic FM condition (unmodulated IF carrier present), a positive polarity signal present at the base of either transistor 24 or 26 will correspondingly increase the voltage at the commoned emitters above their static voltage level. According¬ ly, for purposes of illustration, it may be assumed that the base of transistor 30 will respond to a positive going voltage for 3/4 cycle of the FM-IF and to a negative going voltage for 1/4 cycle. Since the zero crossing detector exhibits a high gain due to the dual, regenerative coupling connections between the transistor 30 and the t r an s i s tor 32, during the 3/4 cycle transistor 30 rapidly increases its conduction to become fully saturated (one limiting condition), and transistor 32 rapidly decreases its conduction to become non-conduc tive (the other limiting condition . Conversely, during the 1/4 cycle transistor 30 rapidly becomes non-conductive and transistor 32 fully saturated.* Integrative effects contributed by the output network 41, as well as the de- emphasis network 6, picvife the same dc tμ current in the first dynamic FM condition, as in the static FM condition (no IF signal present). In response to an FM signal modulated with audio (the second dynamic condition) the duty cycle operation of

* The 3:1 ratio{3/4 cycle to 1/4 cycle) corresponds to the 0. 6 I: 0. 2 I current ratio, in the static FM condition, of transistors 32, 30, and gives rise to the same dc output current for both FM conditions.

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the transistors 30 and 32 varies continuously fro cycle to cycle of the FM-IF.

The output current of the zero crossing detector 33 which is the current in transistor 32, flows substantially solely through transistor 40 of network 41. For this consideration, the additional base current of transistor 76 is negligible. By virtue of the current multiplication provided by transistors 40 and 74, an amplified FM detected output current flows into the load resistance 82. The multiplication factor is made extremely precise by controlling the base-emitter junction geometry of transistor 74 in relation to that of transistor 40 and by proportioning their emitter resistors 78 and 80. ]_a this example the junction area of transistor 74 is twice that of transistor 40 and the value of resistor 80 is one half of that resistor 78, to provide a current multiplication of two. Transistor 76 serves to effectively buffer the current c£ transistor 74 from that of tiHjS-Stor 2 (or 36 in AM) by reducing that portion of the transistor 74 base current flowing to faa-sstor32 pr 36 __ιAM)by the current gainfactor of transistor 76, and also to provide the enabling collector voltage for transistor 32 in FM-mode ( or for transistor 36 in AM mode). The precisely controlled circuit configuration of the output amplifier network 41 assures that no significant change in output current shall occur due to IC process variations or ambient temperature changes. Thus, there is provided from the output of network 41 a more faithful, distortion reduced, temperature stable FM detected output signals and correspondingly accurate AFC signals.

The procedure for aligning, at the factory, the IF amplifier and FM-AM detector 1 for FM-operation is similar to, but more precise than the procedure given in DD. It is briefly summarized here. One begins with the radio receiver in the static condition, and measures dc voltages which exist at the pads of the IC chip,

* Typical values are 600 ohms and 1.2k.

and at other accessible points external to the IC chip, in order to determine whether they fall within requirements. Of particular interest here is the dc voltage at the output terminal 84 (line XL) , which for FM mode is typically + 1. 7 volts. It will be assumed for convenience in description, tltat the measured voltage is exactly + 1.7 volts, and that the dc voltmeter remains connected to terminal 84 for the remainder of the procedure.

The FM - section of the AM-FM converter 3 contains an FM-mixer transistor (see DD, CC or AA), whose base in operational use as radio receiver, accepts FM-RF signal from the antenna 2 and also FM -local oscillator injection. FM-IF output (10.7 Mhz carrier) is developed, in operational use as radio receiver, at the collector of the FM-mixer transistor, by virtue of the fact that the latter collector is connected to a first FM-IF variable-inductance tuneable circuit, which is coupled to a second FM-IF variable-inductance tuneable circuit, which in turn is coupled to the on-the-IC-chip part of IF amplifier (see DD, CC or AA).

For factory alignment purposes, there is no FM-RF injection as yet; the radio receiver is deliberately set to an FM radio frequency which is outside of the 88 to 108 Mhz FM broadcast band. Instead, swept frequency modulated IF carrier (10.7 Mhz) is injected into the base of the FM-mixer transistor, from an FM Sweep Generator ^" , which also provides a marker signal at 10. 7 Mhz. A cathode ray oscilloscope (CRT) is connected through an intervening detector pad (see DD) to the output of the second FM-IF tuned circuit. The first and second FM-IF tuneable circuits are adjusted usually repeatedly, to obtain on the CRT a display reflecting maximum gain, and best symmetry about 10.7 Mhz which is indicated on the CRT by marker. In the next step, the FM Sweep Generator remains connected

to the base of the FM-mixer transistor, but the CRT is now connected through a resistor (rather than a detector probe) to terminal 84 (Figure 2).

The "third" FM-IF tuned circuit 11 (Figure 2) is tuneable by adjustment of the inductor 11A, and the "quadrature" FM-IF tuned circuit 12 is tuneable by adjusting the FM transformer 15, and thereby _ac ι_ justing inductors 15A and 15B. The tuned circuits l ^' l and 12 are - ≡ted, usually repeatedly, and subsequently the tuned circuit 12 is adjusted alone, to obtain the following criteria, as displayed on the CRT with the 10.7 Mhz marker as indicative of the FM-IF, in the CRT display: (1) Maximum gain consistent with linearity in the + 75 khz range centered about 10. 7 Mhz. (2) Symmetry of the displayed S-curve centered about the 10.7 Mhz marker, consistent with cont inued indication, by the voltmeter, of + 1.7 volts dc, as under static conditions. As an additional verification of the alignment, in the FM sweep generator, the amplitude of the modulating signal may be reduced to zero, and the voltmeter should again indicate + 1.7 volts dc _; this is the first dynamic condition.

The FM-RF alignment, and the entire AM -alignment, are essentially as described in DD.

It has been mentioned in this Section "Operation-FM Mode, " that the diode 52 has a negative temperature coefficient, whereas re¬ sistors 54 and 44 have a positive temperature coefficient. It is, of course, usually true that IC resistors in general have positive tem- perature coefficient, whereas IC diode junctions or transistor base- em itter junctions have negative temperature coefficient, meaning that with increase in temperature there is a decrease in the volt-drop across such junctions.

Qperation - AM Mode

During AM signal detection the mode select switch 46 is at the potential of V„ (about + 1.2 volts); this renders transistors 28, 42, 66, 68 and 70 non-conductive, and third current source transistor 60 conductive. Zero crossing detector 33 is inactive and linear differential amplifier 37 active. The inphase AM signal coupled to the base of transistor 24, having an input waveform shown by curve 100 in Graph E, and the 180 phase shifted signal coupled to the base of transistor 26, having an input waveform shown by curve 102 in Graph F, are full wave rectified and formed into a peak detected output voltage at the commoned emitters of transistors 24 and 26 through integration provided by the total inherent circuit capacitance at the base of transistor 34. The peak detected waveform is shown in Graph G as curve 104. The bases of transistors 24 and ' 26 are dc-wise coupled to V. (+ 3 volts) and the base of transistor 62, which has a base-emitter junction area twice that of each of transistors 24 and 26, is also coupled to V . Thus, for the condition of no AM- IF signal present, equal bias currents are supplied to transistors 34 and 36 to provide equal conduction therein, each transistor conducting one half of the current of third constant current source • transistor 60. In response to an AM-IF signal applied to transistors 24 and 26, the voltage at the base of transistor 34 increases according¬ ly to increase conduction in this transistor _ τ ^χ ~ reduce conduction in transistor 36, in proportion to the amplitude of the AM-IF signal modulation. The output current of the differential amplifier 37, which is the current in transistor 36, flows substantially solely through transistor 40 of output network 41. Transistor 74 provides current multiplication of the detected AM signal in similar manner described with respect to the FM signal detection operation, so as to generate an amplified AM detected output current that flows into load

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resistance 82.

The current in constant current source transistor 60 Is highly temperature stable due to the coupling of its base to the junction 5δ of resistors 48 and 50 of the temperature stable network, as previously discussed. Furthermore, as a result of the generation of an output current by the symmetrical circuit arrangement- of the differential amplifier transistors 34 and 36, any change in device properties in one of the transistors that may occur due to IC process variations will be compensated by a comparable change in properties of the other transistor so as to nevertheless generate an output current that accurately reflects the AM signal modulation. The dc AGC signals are similarly temperature-stabilized.

In addition to temperature stabilization, and compensation for variations due to the IC chip manufacturing process, the M¬ detector described herein exhibits other significant improvements vis-a-vis that of BB or CC. Two such significant improvements are:

(1) The capability of the AM detector 1 of this invention, and more particularly the full -wave rectifier (transistor 24, 26), to accept from the IF amplifier's final stage 135, via the AM filter 20, AM IF signals of greater amplitude and without distorting the AM IF signals, than could be handled by the counterpart circuits of BB or CC. The base of each transistor 24, 26 can accept as much as 2 volts peak to peak AM IF carrier modulated by as much as 100% — meaning 4 volts peak to peak maximum — , without distorting the incoming IF signal nor the outgoing audio signal, and with ample safety margin for the specified 2 volts peak to peak (4 volts peak to peak maximum). In contrast, the limiting values for the corresponding transistors of BB (its Ql, Q2) or CC (its Q18, Q19) were 1 volt peak to peak AM IF carrier or 2 volts peak to peak maximum (for 100% modulation) , and with very little safety margin.

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(2) Improved AGC characteristic, i. e. extended dynamic AGC range, vis-a-vis that of CC (or BB or even AA). The more limited dynamic AGC range of typically CC, discussed in this para¬ graph, and the more limited capability of accepting at best 1 volt peak to peak AM IF carrier (2 volts peak to peak maximum) , men- tioned in the preceding paragraph, go hand in hand; they stem from one and the same limitation of the AM full-wave rectifier detector of typically CC (its Q18, Q19), namely that the bases and also the col¬ lectors of Q18, Q19 (of CC) were energized by +6 volts dc. The consequences of this will now be discussed; note that in contrast to CC the bases of transistor 24, 26 are energized by +3 volts, which is appreciably less than the +6 volt energization applied to their collectors.

The base-collector junction of an NPN transistor such as 24, 26 thereof, is under normal conditions a reverse biased diode. By normal condition is meant that the collector is at higher poten¬ tial than the base. As a reverse biased diode, the base-collector junction presents a very high impedance to signal incoming to the base. If however the base potential is permitted to approach and then even slightly exceed the collector potential — a phenomenon which in BB or CC might occur were the +2 volt maximum positive peak exceeded — the base-collector junction becomes a forward- biased diode which is virtually a short-circuit to the signal incoming to the base. In consequence the incoming IF signal is clipped or limited, which results not only in audio distortion, but also introduces a new rectified dc component which in effect causes the dc AGC voltage to limit . As so limited, the AGC dc voltage is propagated through the AM preamplifier (herein transistors 34, 36, 40, 74 ) , the AFC -AGC amplifier (herein 9 ), to the primary and secondary buses ( herein 131, 132 ) , and then into the IF amplifier ( herein 4 ) and the AM mixer

stage and even the AM local oscillator stage (both contained in block 3 hereof) . Therefore in CC , the AGC dynamic range extended to the point where the AM RF signal attained such a magnitude, as to cause the IF amplifier to deliver to the bases of the detector-rectifiers (its Q12,Q19) AM-IF signal which exceeded the 2 volt peak to peak maxi¬ mum. The receiver of CC was designed, by constraints imposed on the AM mixer and the IF amplifier, to insure that the 2 volt peak to peak maximum was normally not exceeded.

It should be understood that the dynamic AGC charac- teristic of a given AM radio receiver has a definite lower limit — not too significant here — and an upper limit. At the lower end, i. e. low amplitude AM-IF signal is insufficient to develop adequate AGC dc signal. At the upper end the amplitude of the received AM-RF signal might be -bnormally high for the given receiver — as an extreme, the receiver might be positioned very close to the trans¬ mitting antenna — so as to overload the most vulnerable stage in the AGC dc loop, e. g. the AM mixer, the IF amplifier, the rectifier- detector, the AM preamplifier or even the AGC amplifier. If for example the IF amplifier is overloaded, not only is the audio output distorted, but the dc rectification which results from such overload causes breakdown of the AGC performance. In CC, owing to the +6 volt dc energization of the collectors, and also the bases of the rectifier-detector stage (its Q 18, Q19), the breakdown occurred when the incoming AM IF signal exceeded the 2 volt peak to peak maximum; this condition normally did not come about, because of constraints on the AM mixer and IF amplifier, as mentioned above.

I the AM rectifier-detector stage of the present invention (transistors 24, 26), forward biasing of the base-collector junctions of these transistors will not occur, even with 4 volt peak to peak maximum applied to either base; this because the collectors receive +6 volts dc, and the bases +3 volt dc. .

Another distinction of the FM-AM detector 1 vis-a-vis that of BB or CC, is that PNP transistor 74 and its associated circuit components, is utilized more efficiently than its counterpart in BB or CC; this, because transistor 74 is active in both FM mode and AM mode. Considering BB as exemplary also for CC, in BB two corres¬ ponding PNP transistors were utilized, namely Q10, active in FM mode and inactive in AM mode, and Q5, inactive in FM mode and active in AM mode, with their associated circuit components partly shared in FM and AM mode. The circuitry depicted in Figure 2 possesses a further important advantageous feature, which however was true also of the circuitry of BB or CC. For purposes of this paragraph, assume that we are considering AM broadcast stations for which the AM-RF signals received by the receiver are of normal signal strength, i. e. not weak ⁿ °r unusually high, with approximately equal modulation per¬ centage. As the radio receiver is successively tuned in to these AM stations, approximately the same audio signal strength will be deve¬ loped at the output terminal 84 due to AGC action, so that the listener need not reset the audio volume control. Next, make the same assump- tion, but for FM broadcast stations. As the radio receiver is successively tuned in to these FM stations, again approximately the same audio signal strength will be developed at terminal 84, but for another reason, namely that this is inherent in the frequency modulation concept itself, so that again the listener need not reset the volume control. The important advantageous feature is that in switching reception fro one such AM broadcast station to one such FM broadcast station or vice versa, again approxi¬ mately the same audio signal strength will be developed at terminal 84, so that again the listener need not reset the volume control. The cir- cuitry of Figure 2, inclusive of the AM-FM detector 1, was designed to accomodate this feature as well.

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