US2390209A - Radio receiving system - Google Patents

Description

Dec. 4, 1945. P. A. DORIO ET AL RADIO RECEIVING SYSTEM Filed June 19, 1944 65:35 0 t mw W w 3 M TUH. M R MAW v m V m m m m a J S, d z m M W N GE 5 ill |1||||I l mokomhwo mmE32 024 883% a m e Patented Dec. 4, 1945 RADIO RECEIVING SYSTEM Pat A. DOrio, Oak Park, and John W. Huff, Elm- Wood Park, 111., assignors, by mesne assignments, to Belmont Radio Corporation, Chicago, 111., a corporation of Illinois Application June 19, 1944, Serial No. 541,046

(Cl. ZED-20) 7 Claims.

This invention relates to a combined amplitude modulation and frequency modulation detector and to a receiver embodying the same and particularly to a receiver of a type designed with a view to maximum simplicity for marketing in the lower price ranges.

In prior art superheterodyne receivers designed for reception of the standard broadcast band of approximately 500-1500 kilocycles and the frequency modulation band of 41-50 megacycles, it has been the practice to provide both a conventional detector for the amplitude-modulated intermediate-frequency signal and in addition thereto a discriminator or frequency detector for detecting thefrequency-modulated intermediatefrequency signal. This use of two detectors increases the complexity and cost of the receiver and has prevented marketing them in the lower price ranges.

It is an object of the invention, therefore, to

provide an improved detector capable of demodulating either an amplitude-modulated or a frequency-modulated intermediate frequency signal thus avoiding the use of a separate frequencymodulation discriminator required in the prior art arrangements.

It is another object of the invention to provide an improved superheterodyne receiver adapted selectively to receive amplitude-modulated signals or frequency-modulated signals and including a combined amplitude-modulation and frequency-modulation detector of the type described in order substantially to reduce the complexity and cost of the receiver.

In accordance with the invention, in a superheterodyne wavessignal receiver adapted selectively to receive amplitude-modulated signals or frequency-modulated signals and including means for deriving from a selected received signal an intermediate frequency signal having a nominal carrier frequency of one value for amplitude-modulated signals and a substantially different value for frequency-modulated signals, there is provided a combined amplitude-modulation and frequency-modulation detector comprising a single filter having a substantial response over the modulation band of the amplitude-modulated intermediate-frequency signals and a substantially sloping response characteristic over the modulation band of frequencymodulated intermediate-frequency signal, together with a single modulation detector coupled to the filter.

Also in accordance with the invention a superheterodyne wave-signal receiver adapted selectively to receive amplitude-modulated signals or frequency-modulated signals comprises in the order named a radio-frequency selecting and converting means for deriving from a selected received radio-frequency signal an intermediatefrequency signal having a nominal carrier frequency of one value for amplitude-modulated signals and of a substantially different value for frequency-modulated signals. The receiver also includes an intermediate-frequency selector and amplifier for translating the intermediate-fre-- quency signal, a combined amplitude-modulation and frequency-modulation detector of the type described, and a signal reproducing means coupled to the detector.

For a better understanding of the invention, together with other and further objects thereof, reference is bad to the following description taken in connection with the accompanying drawing, while its scope will be pointed out in the appended claims.

Referring now to the drawings, Fig. 1 comprises a circuit diagram, partially schematic, of a complete superheterodyne wave-signal receiver embodying the invention, while Fig. 2 represents an operating characteristic of the low-pass filter embodied in the circuit of Fig. 1,

Referring now to Fig. 1 of the drawing there is represented, partially schematically, the circuit of a complete superheterodyne wave-signal receiver adapted selectively to receive amplitudemodulated signals or frequency-modulated signals. This receiver comprises a radio-frequency selecting and converting means consisting of an RF selector and amplifier it) having its input circuit coupled to an antenna-ground circuit l l and its output circuit connected to an oscillator-detector l2, these two units being effective to derive from a selected received radio-frequency signal an intermediate-frequency signal having nominal carrier frequency of one value for amplitudemodulated signals and of a substantially difierent value for frequency-modulated signals. The units ID and I2 may be of conventional design with suitable range switches for selectively receiving amplitude-modulated signals or frequencymodulated signals. However, in the preferred embodiment illustrated the RF selector 10 includes a composite tuned circuit [3 including the coils I311, I31) and associated condensers I30, l3d, the circuit being permeability-tuned continuously over both frequency bands by means, of an adjustable ferromagnetic core 13c, while the oscillator-detector 12 includes a tuned circuit l4 comprising the coils Ma and Mb tunable by means of a variable tuning condenser I40, the coil I4b being arranged to be short-circuited by a switch ltd at a predetermined point in the adjustment in the condenser I4c. Thus the circuit I4 is also tuned over the two frequency bands by continuous adjustment of the tuning condenser Me. The adjustable core I36, thetuning condenser I40 and the switch I411 are mechanically interconnected by the unicontrol mechanism indicated in dashed lines at I5. By this expedient the tuning dial of the receiver may be marked with a continuous scale covering both of the frequency bands, preferably with a slight gap separating the two bands and corresponding to the portion of travel of mechanism I5 effective to close the switch Md.

The receiver also includes an intermediatefrequency selector and amplifier I6 for translating the intermediate-frequency signal of either type. The unit I6 may be of conventional design but in the preferred arrangement shown it constitutes a two-stage two-band composite selector and amplifier the first stage including the two-band composite selector I1 and amplifier I8 and the second stage comprising the ltwo-band composite selector I9 and amplifier 20. These elements are of conventional design, each of the selectors I! and I9 comprising a pair of secondary fixed-tuned circuits IIa, Ill) and I9a, I9b, respectively, tuned to the nominal carrier frequency of the amplitude-modulated and frequency-modulated intermediate frequency signals, respectively, together with separate primary windings I'Ic, IId and I90, I9d respectively, for the two tuned circuits. The series impedance of each of the component tuned circuits is negligible at the intermediate carrier frequency to which it is not intended to respond so that the two tuned circuits effectively comprise an automatic switching arrangement for selectively receiving either the amplitude-modulated or frequency-modulated intermediate-frequency signals. Preferably, the unit I6 also includes an arrangement for reducing the gain of the intermediate-frequency amplifier I8 for signals of undesired frequencies appearing in the primary circuits of selector II. To this end, the lowpotential terminal IIe of the secondary highfrequency winding of circuit Ila is connected through a switch IIj either to the AVG bus described hereinafter, for operation in the frequency-modulation band, or to ground through a condenser Hg, for operation in the broadcast band. The high-frequency secondary winding of circuit Ho and condenser I'Ig are designed to resonate at the nominal frequency-modulation intermediate-frequency carrier frequency, thus forming a trap for spurious signals falling in such frequency range. The switch I1] is connected for unicontrol with the band switch I411, as indicated.

The receiver also includes a combined amplitude-modulation and frequency-modulation detector 2I consisting of a single low-pass filter 22. which may be of conventional design, and a single modulation-signal detector coupled to the filter 22 and consisting of diode 23 and a conventional resistance-capacitance load circuit 24. The filter 22 is designed to have a response characteristic such as represented in Fig. 2, from which it is seen that the response is substantially uniform over the frequency range below the cut-off frequency ,fc which includes the modulation band of the amplitude-modulated intermediate-frequency signal having nominal carrier frequency ijm which may be, for example, 465 kilocycles. The response characteristic of the filter is substantially linear and sloping over the frequency range above the cut-off frequency fc and this linear sloping portion is centered with respect to the nominal value ifr'm of the carrier frequency of the frequency-modulated intermediate-frequency signal.

A signal-reproducing means, such as an AF amplifier 25 and loud speaker 26, is coupled ';o the load circuit 24 of the detector 23 for reproducing the modulation signal. Preferably an automatic amplification control or AVC bias is derived from the load circuit 24 and applied to the control electrodes of one or more of the tubes of the units I0, I2 and I6 automatically 7 to control the amplification therein inversely in accordance with the carrier-frequency intensity of the received signal. The AVC bias is supplied to the control grid of tube I8, during operation in the broadcast band when the direct current path to such grid is otherwise broken at switch I'If, through a direct-current path including an intermediate-frequency choke or, as shown, a grid resistor I 1h, which should be selected with a view to avoiding any time-constant circuit connected to the grid of amplifier I8 which would cause it to operate as a grid-leak detector in the broadcast band.

Considering now the operation of the receiver described above, the units Ill and I2 are effective to derive from a signal picked up by the antenna circuit I I an intermediate-frequency signal which for the frequency-modulation band is determined primarily by the constants of the tuned circuit I3a, I3c of unit I0 and the circuit Ida, I40 of the unit I2 when the unicontrol mechanism I5 is adjusted to close the switch Md and tune the receiver over this portion of its operating frequency range. The constants are such that an intermediate-frequency signal of constant nominal carrier frequency, for example 4.3 megacycles, is developed at the output circuit of the unit I2 and applied to the intermediate-frequency selector and amplifier I6. Simultaneously the switch I'If is connected to its left-hand contact I 72' so that this intermediate-frequency signal is selected in the high-frequency portions Ha, Ha and I90, I 9a of the composite selectors I I and I 9, respectively, and amplified by the tubes I8 and 20 and applied to the combined amplitude modulation and frequency-modulation detector 2|. Due to the fact that the nominal carrier frequency of this signal ifrM falls approximately at the center of the linear sloping portion of the response characteristic of filter 22, as shown in Fig. 2,-this filter and its associated detector 23 operate as a frequency-modulation detector in a manner well understood in the art. At the same time, when switch I1 is connected to the low-potential terminal I'Ie of the secondary circuit I'Ia of selector I1 for operation in the fre quency-modulation band, undesired signals at the nominal amplitude-modulation intermediate-fiequency carrier frequency are highly attenuated by the amplitude-modulation secondary tuning condenser of circuit I'Ib.

When the receiver is tuned by the tuning means I5 to operate over the amplitude-modulation broadcast band, the switch Md of unit I2 is open, the adjustable core I3e becomes associated with the coil I3b of unit I0 and the amplitude-modula tion signal selected by the unit IQ is converted by the unit I2 to an intermediate-frequency signal having a nominal carrier frequency which may be 465 kilocycles. This signal is selected and amplified by the low-frequency portions 11d, Nb and 19d, I9bof the composite selectors I1 and [9, respectively, and tubes l8 and 20 of unit l6 and applied to the detector unit 213 Since its normal carrier frequency 'ifAM falls on the uniform response portion of the characteristic of the filter 22, it is translated in'a conventionalmanner, but without selectivity, to the detector 23 wherein it is converted to an audio-frequency signal-which is amplified in unit 25 and reproduced in loud speaker 26. Simultaneously the switch Hf is connected to its right-hand contact I1 and any spurious signals at the nominal frequency-modulation intermediate carrier frequency that might appear in the primary circuit of selector H are absorbed by the series-resonant trap circuit including the high-frequency secondary winding of circuit Ila and condenser I'lg. Either the fundamental or harmonics of the local oscillator may fall in this frequency range when the receiver is tuned through the broadcast band.

Thus it is seen that the superheterodyne receiver described is simple andinexpensive in design due in part to the avoidance of the need of a separate frequency discriminator for detecting frequency-modulated signals, adapting it for marketing in the lower price ranges and yet it is effective selectivelyto receive either amplitude-modulated signals or frequency-modulated signals.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit or scope of the invention.

What is claimed as new is:

1. In a superheterodyne wave signal receiver adapted selectively to receive amplitude-modulated signals or frequency-modulated signals and including means for deriving from a selected received signal an intermediate-frequency signal having a nominal carrier frequency of one value for amplitude-modulated signals and a substantially different value for frequency-modulated signals, a combined amplitude-modulation and frequency-modulation detector comprising, a single filter having a substantial response over the modulation band of the amplitude-modulated intermediate-frequency signals and a substantially sloping response characteristic over the modulation'band of the frequency-modulated intermediate-frequency signals, and a single modulation detector coupled to said filter.

2. In a superheterodyne wave signal receiver adapted selectively to receive amplitude-modulated signals or frequency-modulated signals and including means for deriving from a selected received signal an intermediate-frequency signal having a nominal carrier frequency of one value for amplitude-modulated signals and a substantially different value for frequency-modulated signals, a combined amplitude-modulation and frequency-modulation detector comprising, a single low-pass filter having a substantial response over the modulation band of the amplitude-modulated intermediate-frequency signals and a substantially sloping response characteristic over the modulation band of the frequencymodulated intermediate-frequency signals, and a single modulation detector coupled to said filter.

3. In a superheterodyne wave signal receiver adapted selectively to receive amplitude-modulated signal or frequency-modulated signals and including means for deriving from a selected received signal an intermediate-frequency signal having a nominal carrier frequency of one value for amplitude-modulated signals and a substantially different value for frequency-modulated signals, a combined amplitude-modulation and frequency-modulation detector comprising, a single filter having a substantially uniform response-over the modulation band of the amplitude-modulated intermediate-frequency signals and a substantially sloping response characteristic over the modulation band of the frequencymodulated intermediate-frequency signals, and a single modulation detector coupled to said filter.

4. In a superheterodyne wave signal receiver adapted selectively to receive amplitude-modulated signals or frequency-modulated signals and including means for deriving from a selected received signal an intermediate-frequency signal having a nominal carrier frequency of one value for amplitude-modulated signals and a substantially different value for frequency-modulated signals, a combined amplitude-modulation and frequency-modulation detector comprising, a single filter having a substantial response over the modulation band'of the amplitude-modulated intermediate-frequency signals and a substantially linear sloping response characteristic over the modulation band of the frequencymodulated intermediate-frequency signals, and a single modulation detector coupled to said filter.

5. In a superheterodyne wave signal receiver adapted selectively to receive amplitude-modulated signals or frequency-modulated signals and including means for deriving from a selected received signal in intermediate-frequency signal having a nominal carrier frequency of one value for amplitude-modulated signals and a substantially different value for frequency-modulated signals, a combined amplitude-modulation and frequency-modulation detector comprising, a single low-pass filter having a substantially uniform response over a frequency range below its cut-off frequency and including the modulation band of the amplitude-modulated intermediatefrequency signals and a, substantially linear sloping response characteristic over a frequency range above its cut-off frequency and centered with respect to the nominal value of the carrier frequency of the frequency-modulated intermediate-frequency signal, and a single modulation detector coupled to said filter.

6. A superheterodyne wave-signal receiver adapted selectively to receive amplitude-modulation signals or frequency-modulated signals comprising in the order named, radio-frequency selecting and converting means for deriving from a selected received radio-frequency signal an intermediate-frequency signal having a nominal carrier frequency of one value for amplitudemodulated signals and of a substantially different value for frequency-modulated signals, an intermediate-frequency selector and amplifier for translating said intermediate-frequency signal, a

single filter having a substantially sloping re-- sponse characteristic over the modulation band of the frequency-modulated intermediate-frequency signals, a single modulation-signal detector coupled to said filter, and signal reproducing means coupled to said detector.

'7. A superheterodyne wave-signal receiver adapted selectively to receive amplitude-modulated signals or frequency-modulated signals comprising in the order named, radio-frequency selecting and converting means for deriving from a selected received radio-frequency signal an intermediate-frequency signal having a nominal carrier frequency of one value for amplitudemodulated signals and of a substantially dif-- ferent value for frequency-modulated signals, a two-band composite intermediate frequency selector and amplifier for translating said intermediate-frequency signal of either type, a single filter having a substantial response over the mod- PAT A. DORIO. JOHN W. HUFF.

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