US2488359A - Frequency-modulated carrier signal receiver - Google Patents

Description

Nov. 15, 1949 H. A. WHEELER FREQUENCY-MODULATED CARRIER SIGNAL RECEIVER Filed March 11, 1940 ATTORNEY Patented Nov. 15, 1949 FREQUENCY-MODULATED CARRIER SIGNAL RECEIVER Harold A. Wheeler, Great Neck, N. Y., assigner, by mesne assignments, to Hazeltine Research, Inc., Chicago, Ill., a corporation of Illinois Application March 11, 1940, Serial No. 323,290

(Cl. Z50-20) 9 Claims. l

This invention relates to frequency-modulated carrier-signal receivers and more particularly to such receivers of the type including means for adjusting the selectivity of the receiver in accordance with a characteristic of a received signal.

Receivers for the reception of frequency-modulated carrier waves are subject to a certain amount of background noise which is more noticeable during the reception of a carrier Wave having a weak frequency modulation thereon when the Width of the frequency-modulation band is considerably less than the normal Width of the pass band of the receiver selecting means which ordinarily has a xed pass band that is sufciently wide to pass a frequency-modulation band of the maximum Width corresponding to the strongest modulation. Also, if the pass band of the receiver is greater than that required by the frequency-modulation band at any time, noise pulses are relatively great and are more noticeable in the reproduced signal.

Therefore, it is an object of the present invention to provide an improved frequency-modulated carrier-signal receiver which overcomes the above-mentioned disadvantages and undesirable features of such receivers of the prior art.

It is a further object of the invention to provide a frequency-modulated carrier-signal receiver including an automatic band width control system, whereby the band width is maintained just sufficient to pass the frequency-modulation band of the received signal without distortion.

In accordance with the invention, therefore, a frequency-modulation receiver, of the type including an intermediate-frequency amplifier and a frequency-modulation detector, comprises an intermediate-frequency feed-back circuit connected between the output and input circuits of the amplifier, means for deriving from the detected voltage a direct-current voltage Whose magnitude is substantially proportional to the degree of modulation of the frequency-modulated waves, and means for varying the feedback through the feed-back circuit in response to the magnitude of the direct-current voltage to adjust the band Width of the intermediate-frequency amplier in accordance with the degree of Inodulation of the frequency-modulated Waves.

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

The single iigure of the drawing is a circuit diagram, partially schematic, of a complete superheterodyne receiver adapter to receive a frequency-modulated carrier signal and comprising therein a control system in accordance with the invention for controlling the pass band of the receiver in response to and in accordance with the width of the frequency-modulation band of a received signal.

Briefly described, the receiver comprises a radio-frequency amplifier Il) having input terminals connected to an antenna system II, I2 and output terminals coupled to a frequency changer or oscillator-modulator I3. Connected in cascade with oscillator-modulator I3, in the order named, are an intermediate amplifier I4 of one or more stages, an adjustable band-pass selector I5, described more fully hereinafter, an intermediate-frequency amplier and limiter I6, a frequency-modulation detector I'I, an adjustable volume control I8, an audio-frequency amplifier I9 of one or more stages, and a sound reproducer 20. Each of the elements or units Ill-M, inclusive, and I6-2I, inclusive, may be entirely conventional in construction so that a detailed description is considered unnecessary herein.

Considering rst the operation of the receiver as a Whole, without regard to the details of the arrangement of the present invention, per se, a desired received frequency-modulated signal is selected and amplified in the radio-frequency amplifier I0 and converted to an intermediate-frequency frequency-modulated carrier signal by the oscillator-modulator I3. The signal as thus converted is further ampliied in intermediatefrequency amplifier I4, translated by the bandpass selector I5, further amplified and limited to a constant amplitude by intermediate-frequency amplier and limiter I 5, and detected by the frequency-modulation detector I'I, thereby deriving the audio-frequency modulation components which are, in turn, translated by the adjustable volume control I8, amplified by audiofrequency amplifier I9, and reproduced by the sound reproducer 20.

Referring now more particularly to the details of the portion of the receiver embodying the present invention, the adjustable band-pass selector I5 comprises a tuned input circuit 22, 24, the inductance 22 of which is inductively coupled to an inductance 2| in the output circuit of amplier I4, and a tuned output circuit 26, 21, the inductance 21 of which is inductively coupled to an inductance 28 in the input circuit of the amplifier and limiter I6. The two tuned terminal circuits are coupled `in the forward direction by a vacuum tube 25 having input electrodes connected to the tuned terminal circuit 22, 2d and output electrodes coupled to the tuned terminal circuit 26, 21. The tuned terminal circuits 22, 24 and 26, 21 are also coupled in a backward direction by means of a vacuum tube 30, preferably similar in characteristics to the forward coupling tube 25 andA having input electrodes inductively coupled to the tuned circuit 25, 21 by an inductance 29 and an output circuit inductively coupled to the tuned circuit 22, 24 by means of an inductance 3|. A common unidirectional potential source, such as a battery 32, is provided for the screen-grid and anode circuits oi"v vacuum tubes 25 and 3, while the grid circuits oi theVV tubes 25 and 3S are returned to a common cathode terminal through a high-frequency by-pass4 condenser 42.

In order to derive a control effect, such as a control-bias voltage, by which to eiiect adjustment of the pass band of selector |25, there is coupled to the outputy circuit of the. detector l1, independently of the volume control I8', an audiofrequency buffer 4amplifier 35 to which is coupled, by means of a transformer 36, a diode rectifier 31 provided with a load circuitincluding a resistor 39 shunted by a carrier-frequency by-pass condenser 4B. The voltage developed across the load resistor 39 is applied positively, through an audio-frequency filter comprising a series resistor 38 and shunt condenser 4| to aA control conductor 34 in oppositiontoia iixed bias from a suitable source such as a battery 33. The control conductor 34 is effective to apply the resultant bias to the control 'gridof each of the tubes 25 and 30. The load circuit of rectier 31 has a relative fast charging rate corresponding to a small time constant of the order of l/ao second and a slow discharging rate corresponding Vto a, large time constant of the order of one second so that the useful rectifiedoutput voltage is substantially unaffected by transient noise pulses to which the rectiiier 31 may be subjected.

In considering the operationl of that portion of the system comprising the present invention, it will be assumed that there is being received a signal having a wide frequency-modulation band correspond-ing to a strong modulation signal, which is accompanied by a large amplitude audiofrequency output from the frequency-modulation detector I1, which, after amplification in the buiier amplifier 35, is impressed on rectifier 31. The large rectified Vvoltage output of rectifier 31- is sufficient to reduceto a minimum the negative bias from battery 33 applied to the grids of tubes 25 and S, so that the transconductances of these tubes, the effective couplingbetween the tuned terminal circuits, and the pass band of the selector I are all at maximum values. When the width of Vthe frequency-modulation band decreases in correspondence with a weak modulation signal, the rectified voltage output orrectier 31 decreases, increasing-the negative bias from battery 33 applied to the grids oftubes'25 and 30, `to decrease their transconductances to a minimum. The coupling between the tunedterminal circuits is correspondinglydecreased, reducing to a minimum the pass band of selector I'5. The pass band of the selector I5 preferably is maintained only slightlywider than the 4width ofr` the frequencymodulation band of a received signal so that the selector is eiiectiveto reduce noise to a maximum degree consistent with the signalY being received; it is particularly effective during the reception of a weakly-modulated signal and 'inthe'absence of 4 any desired signal modulation, when noise is most objectionable. However, the pass band of selector I5 should never be reduced below twice the highest required audio frequency of modulation of a received signal.

It has been proposed in certain wide-band frequency-modulation systems to deviate the` signalcarrier frequency a maximum of 75 kilocycles on either side of the mean carrier frequency, resulting in a maximum frequency-modulation band of somewhat greater than kilocycles, and to allot to each channel a frequency band about 200 kilocycles wide. It has also been proposed in such systems to transmit lan audio-frequency range having an upper limit of about 15 kilocycles which requires a minimum frequency-modulation band `somewhat greater than 30 kilocycles to maintain fidelity of reproduction. Therefore, if the receiver or the invention is utilized in such a system, the pass band of selector "I5 should be preferably adjustable between about 40 kilocycles and 200 kilocycles in response to variations in the strength of' modulation of a received signal. However, in some cases it may be desirable to sacrifice the fidelity of reproduction in order to secure further reduction of noise during reception of a weaklymodulated signal. In such case, the minimum band width of the selector l5 may be reduced to 20 kilocycles or even to 10 kilocycles during reception of a very weakly-modulated signal. The maximum band width of vthe selector l5 in all cases, however, must be somewhat greater than the maximum width of the modulation band it is desired to reproduce.

The adjustable volume control I8 is provided in the main channel of the receiver for adjusting the loudness of reproduction of the reproducer 2H as desired. However, the auxiliary audio-frequency amplier 35 for developing the control bias is. coupled to the detector i1 in advance of the volume control I8, so that the band-width control bias` is entirely independent of the adjustment of the volume control I8. Another desirable feature oi the receiver described is that the rectifier 31 for developing the band width control bias is coupled to detector I1 through the buier amplifier 35 so that it does not constitute a load on the main signal-translating channel, 'which would distort the audio-frequency signal inthe main signal-translating channel. The use of like tubes 25, 3G in the forward and backward paths and the application of the same bias on the grids of both tubes provides the effect of a pair of coupled circuits with adjustable coupling by which the band width may be varied at least in the ratio of four-to-one, without substantially altering the gain of the selector.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without vdeparting from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention. i

What is claimed is:

1. A frequency-modulated carrier-signal receiver comprising, a carrier-frequency signaltranslating channel including a band-pass selector, means for adjusting said band-pass selector to control the width of the pass band thereof, a frequency-modulation' detector coupled to said channel, an adjustable volume control and signalreproducing deviceV coupled to said detector,

means coupled to said detector independently of said volume control and responsive to the detected signal amplitude for deriving a control eifect, and means for utilizing said contro'i effect to control said adjusting means to adjust the pass band of said selector directly in accordance with the degree of modulation of a received carrier signal.

2. A frequency-modulated carrier-signal receiver comprising, a carrier-frequency signaltranslating channel including a band-pass selector comprising two terminal circuits and a pair of vacuum tubes individually coupling said terminal circuits in the forward and backward directions, means for adjusting said band-pass selector to control the width of the pass band thereof, a frequency-modulation detector coupled to said channel, rectifier means responsive to the detected signal amplitude for deriving a control-bias voltage, and means for applying said control-bias voltage to both of said vacuum tubes to adjust the pass band of said selector directly in accordance with the degree of modulation of a received carrier signal.

3. A method of frequency-modulation reception which includes collecting frequency-modulated waves in a circuit, amplifying a wide band of collected Waves, establishing feedback of amplied waves during said amplification in such a manner as to Widen the effective band width of the circuit, deriving modulation voltage from the amplied waves, deriving a unidirectional voltage from the modulation voltage Whose magnitude is substantially proportional to the modulation amplitude, and controlling the said feedback with said unidirectional voltage.

4. A method of frequency-modulation reception which includes collecting frequency-modulated Waves in a circuit, amplifying a wide band of collected waves, establishing feedback of amplified Waves during said amplification in such a manner as to Widen the effective band width of the circuit, deriving modulation voltage from the amplified waves, deriving a unidirectional voltage from the modulation voltage whose magnitude is substantially proportional to the modulation amplitude, and controlling the said feedback with said unidirectional voltage, and simultaneously controlling said amplification with said derived unidirectional voltage.

5. In a frequency-modulation receiver of the superheterodyne type including a signal collector, a converter, at least one intermediate-frequency amplier, an amplitude-modulation limiter, means for deriving modulation voltage from the amplified frequency-modulated, intermediate-frequency voltage; the improvement which comprises means operatively associated with said intermediate-frequency amplier for adjusting the effective pass band width of said amplier, means for deriving from the said modulation voltage a unidirectional control voltage Whose magnitude is directly proportional to the modulation voltage amplitude, means utilizing said control voltage for regulating the effectiveness of said band width adjusting means, said band width adjusting means comprising an intermediate-frequency voltage feed-back circuit connected between the output and input circuits of said amplier, and said utilizing means applying said control voltage to said feed-back circuit in a sense to produce maximum pass-band width with maximum control voltage.

6. In a frequency-modulation receiver of the superheterodyne type including a signal collector,

a converter, at least one intermediate-frequency amplifier, an amplitude modulation limiter, means for deriving modulation voltage from the amplified frequency-modulated, intermediatefrequency voltage; the improvement which comprises means operatively associated with said intermediate-frequency amplifier for adjusting the effective pass-band width of said amplifier, means for deriving from the Said modulation voltage a unidirectional control voltage Whose magnitude is directly proportional to the modulation voltage amplitude, and means utilizing said control voltage for regulating the effectiveness of said band Width adjusting means, and additional means for applying said control voltage to said intermediate-frequency amplifier for increasing the gain thereof simultaneously with widening of said pass-band width.

7. In a frequency-modulation receiver of the type including an intermediate-frequency ampliiler and a frequency-modulation detector, an intermediate-frequency feed-back circuit connected between the output and input circuits of said amplifier, means for deriving from the detected voltage a direct-current voltage whose magnitude is substantially proportional to the degree of modulation of the frequency-modulated Waves, and means for varying the feedback through said feed-back circuit in response to the magnitude of said direct-current voltage.

8. In a frequency-modulation receiver of the type including an intermediate-frequency amplifier and a frequency-modulation detector, an intermediate-frequency feed-back circuit connected between the output and input circuits of said amplifier, means for deriving from the detected voltage a direct-current voltage whose magnitude is proportional to the degree of modulation of the frequency-modulated Waves, and means for varying the feedback through said feed-back circuit in response to the magnitude of said direct-current voltage, said deriving means including a rectifier having an input circuit coupled to the detector output circuit.

9. In a frequency-modulation receiver of the type including an intermediate-frequency amplifier and a frequency-modulation detector, an intermediate-frequency feed-back circuit connected between the output and input circuits of said amplifier, means for deriving from the detected Voltage a direct-current voltage whose magnitude is proportional to the degree of modulation of the frequency-modulated waves, and means for varying the feedbackthrough said feedback circuitl in response to the magnitude of said direct-current voltage, said deriving means including a rectifier system having an input circuit coupled to the detector output circuit, and said rectifier system including a time-constant network constructed and arranged to permit development of said direct-current voltage at a rapid rate and discharge of the network at a relatively slow rate.

HAROLD A. WHEELER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,819,508 Hansell Aug. 18, 1931 2,167,400 Farrington July 25, 1939

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