US2171154A - Radio receiver - Google Patents

Description

l Aug. 29, 1939*.

G. M. WRIGHT ADIO RECEIVER Filed May 14, 1935 GEORGE /fzw WRIGHT ATTORNEY Patented Aug. 29, 1939 UNITED STATES RADIO' RECEIVER George Maurice Wright, London, England, assignor to Radio Corporation of America, a corporation of Delaware Application May 14,

1935, Serial No. 21,346

In Great Britain June 2, 1934 6 Claims.

This invention relates to radio receivers and more specifically to radio receivers of the socalled homodyne kind, i. e., of the kind in which received modulated carrier wave energy is mixed s with unmodulated carrier energy which should (for proper operation) be of the same frequency as the carrier frequency component in the received energy. For example, a typical known homodyne receiver comprises an aerial tuned to a desired frequency and coupled to a closed oscillatory circuit which is also tuned to the same frequency, there being introduced into this circuit a strong unmodulated carrier frequency derived from a local homodyne oscillator of the same frequency as the carrier component of the received energy and in predetermined phase relationship thereto. The combined carrier energy, i. e., that from the local source and that received are rectified by any suitable known rectifier and the resulting currents passed through a low pass filter to a loudspeaker or other utilization device. It is generally presumed that with a receiver as just described, if the homodyne oscillator be accurately maintained in correct adjustment, interference will be avoided and the Width of the band of frequencies passed to the loudspeaker or other utilization device will be controlled by the value of the cut-off frequency of the low pass filter and be numerically equal to twice this value. In practice, however, it is found that unwanted signals are in fact fed to the utilization device and the reason for this is to be found in the nature of the action of the rectifier to which the mixed received energy and locally generated energy is fed. Known rectiflers are not rectilinear as to their voltage (ordinates) current (abscissae) characteristics and in general the characteristic of a rectifier has a markedly curved part commencing at the origin, the curved part continuing into a substantially straight portion. Now it will be appreciated that although, as is generally the case, the amplitude of the locally generated homodyne oscillation will be sufficiently great to cause the rectifier to Work up over the substantially straight part of its characteristic, at each swing of the homodyne oscillator the curved part of the characteristic must be traversed and there are, therefore, periods corresponding to the times of traversal of the curved part of the characteristic, 50 during which the rectifier is able to rectify unwanted carriers and thus produce audio frequency output components which will lie within the band of frequencies passed by the low pass filter and will thus actuate the loudspeaker or other utilization device to give interfering results.

(Cl. Z-20) The object of the present invention is to reduce this effect and the said invention consists in principle in so arranging the homodyne oscillator portion of a homodyne receiver that the time per cycle of homodyne oscillator during which the rectifier can produce audio frequency components by rectification of unwanted carriers is substantially reduced. The invention may be carried into effect in any of three principal ways, (1) by utilizing in place of a homodyne oscillator giving a more or less sinusoidal output an arrangement giving synchronized pulses; (2) by the use of a homodyne oscillator giving a substantially square topped output wave; and (3) by the use of a homodyne oscillator in such manner that only the tips or peaks of the locally generated wave are utilized.

The invention is illustrated in the accompanying diagrammatic drawing.

Referring to the drawing which shows one way of carrying out this invention a homodyne receiver compris-es an aerial I which maybe tuned to the desired frequency and is coupled by means of a coil 2 to a parallel tuned circuit 3 tuned to the incoming radio frequency. One end of this tuned circuit is connected to the control grid 4 of a triode 5 which is adjusted to operate as a rectifier, the other end of the tuned circuit being connected through a resistance 6 toI the cathode 'I of the triode.

Device Il represents schematically an ordinary local oscillator having an output circuit connected to the input terminals of a limiter stage or device l2. The output terminals of the limiter are connected across a resistance 6. By reason of the interposition of the limiter stage the Wave form applied across the ends of the resistance 6 Will be of substantially square-topped form and accordingly the period of time per cycle during which the rectifier is operated upon the curved portion of its characteristic isv substantially re-l duced by reason of the square-topped formation of the local oscillation wave actually applied.

The limiter I2 may take the form of that described in Shore U. S. Patent No. 2,005,111.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed I declare that what I claim is- 1. In a receiver of the homodyne type in combination, a signal input circuit including means for tuning the circuit to the frequency of desired signal energy, a heterodyne frequency generator, a combining circuit including a rectifier tube and means for feeding the desired signal energy to the combining circuit, means for feeding the heterodyne energy to the combining circuit, said last named means including a limiter device interposed between the heterodyne frequency generator and the combining circuit, said limiter device being arranged so as to distort the wave form of the energy produced by the heterodyne frequency generator so that the energy impressed upon the combining circuit through the limiter device has a non-sinusoidal wave form.

2. In a radio receiving apparatus, a combining circuit including a detector tube provided with a tunable signal input circuit and an output circuit, a signal energy pick-up device coupled to said input circuit, a local oscillator, means for coupling the local oscillator to said input circuit, said last named means including a device for attening the Wave form of the energy produced by the local oscillator to such an extent as to cause the detector output current to be substantially Zero during at least half of each cycle of the local oscillator energy and large and relatively uniform during the remainder of the cycle.

3. In a radio receiver, a combining circuit including a detector tube having a tunable signal input circuit and an output circuit, a signal energy pick-up device coupled to the input circuit, an oscillation generator, means for feeding the generated oscillations into the combining circuit, said means including a limiter device adapted to change the generated oscillations to substantially square topped waves.

4. In a radio receiver, a tunable signal input circuit, a local oscillator, a source of signal energy couple to the tunable input circuit, ay combining circuit including a detector tube having an input circuit and an output circuit, said tunable input circuit forming part of the detector input circuit, a local oscillator, and means for impressing the output of the local oscillator upon the input of the detector tube, said last named means including a device for controlling the characteristics of the generated oscillations so as to give the oscillations a substantially square topped wave form whereby the detector output current is Zero during at least half of each cycle and large and relatively uniform during the remainder of the cycle.

5. A method of radio reception which includes the steps of receiving signal modulated carrier energy of the desired frequency, producing substantially square topped oscillations of the same frequency and phase as the received energy, modulating the received energy by the produced oscillations and deriving the signals from the products of said modulation,

6. In radio receiving apparatus, a signal wave pick-up means, a utilizing circuit, a variable resistance coupling circuit for transferring signal Waves from the pick-up means to the utilizing circuit, and means electrically connected to said coupling circuit and including a device for generating substantially square topped waves for alternately and periodically varying the resistance of the coupling circuit substantially instantaneously from one value tol another at the same frequency and phase as the desired signalling energy whereby the coupling circuit presents a substantially constant resistance to one half of each signal wave cycle and a substantially constant but different resistance to the other half of each signal wave cycle.

GEORGE MAURICE WRIGHT.

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