US2144355A - Superheterodyne receiver - Google Patents

Description

Jail. 17, 193s).v A T, Wm; 2,144,355

SUPERHETERODYNE RECEIVER Filed Aug. 22, 1956- INVENTOR A A.T. WITTS ATTORN EY I Patented Jan. '17, 1939 PATENT OFFICE 2,144,355 SUPERHETERODYNETRECEIVER Alfred Thomas Witts, CheamySurrey, England, assignor to Radio Corporation of America, a corporation of Delaware Application August 22, 1936," Serial No. 97,355

In Great Britain 7 1 Claim.

This invention relates to superheterodyne receivers and has for itsobject to provide improved means for overcoming a well known difficulty which frequently arises in known superhetero-" 5 "dyne receivers, namely that of image frequency reception.

It will be appreciated that when the local oscillator of a superheterodyne receiver is tuned to a 'which will combine withthislocal oscillator frequency to produce the predetermined interme diate frequency of the receiver, these twofrequencies being equidistant on either side of the local oscillator frequency. Accordingly it may ocf cur that when a -superheterodyne receiver is tuned to receive from a particular desired transmitter station there may be interference from another transmitting stationwhose frequency'is as much on one side of the local oscillator frequency as the frequency of the desired station is on the other. Numerous attempts have been made to overcome this well known difficulty, i. e., to provide for image frequency suppression, but known expedients for this purpose have not been entirely satisfactory. Known expedients for the purpose in question have been either unduly complicated and expensive or not satisfactorily operative over a wide range of receivable frequencies or have been such as to involve loss of ain in the receiver or have presented combinations of these disadvantages. For example, it is well known to seek to overcome image frequency reception to some extent by utilizing a high intermediate frequency, for example 450 kilocycles, but this expedient presents the disadvantage of limiting the overall gain and furthermore results in some loss of adjacent channel selectivity as compared with that which would be obtained were a lower intermediate frequency chosen.

The present invention seeks to provide a simple and efiiective image frequency suppression means not involving the disadvantages above mentioned and satisfactorily operable over a wide receiving range, e. g., the present broadcast range.

According to this invention image frequency reduction or suppression in a tunable superheterodyne receiver is accomplished by means of a frequency selective rejector circuit or device which is arranged to be varied as to its frequency when the tuning means of the receiver is varied and is associated with a radio frequency circuit or a frequency changer circuit of the receiver, said rejector circuit or device being so arranged as to reject, at any given setting of the tuning means of the receiver, incoming signals whose frequency given frequency there will be two frequencies September 21, 1935 (Cl. 25020); I

differs quantitatively from the local oscillator frequency bythe same amount. a'sthat by Which the frequency desired to be received (atthe said tuning setting) differs from the local oscillator frequency. I

Preferably the rejector circuit or device is a tuned circuit gang controlled with the high fre-' quency and local oscillator tuned circuits of the quency circuit.

Though not limited to its application thereto the invention is of particular advantage in superhetercdyne receivers not involving high frequency amplifier valves for it is in these receivers that, owing'to the limited high frequency selectivity obtained, image frequency, interference is most serious. v

The invention is illustrated in the accompanying drawing which shows diagrammatically one embodiment thereof. r

receiver and electronically coupled toa radio fre- V m Referring to the drawing in which only those parts necessary to an understanding of this invention are shown a superheterodyne receiver has a so-called" triode-pentode valve I which is used both as a local oscillator and as a frequency changer; Incoming signals are applied to the innermost grid GI of the pentode portion of the valve, this grid circuit containing the usual radio frequency parallel tuned circuit Ll Cl, the condenser Cl being, of course, a variable condenser for tuning purposes. Bias, for example automatically Varied bias for automatic gain control purposes, is applied in any convenient manner well known per se to this innermost grid. As shown automatic gain control bias is applied via lead A. V. C. and resistance Rl to grid GI, KI being a blocking condenser. The next grid G2 of the pentode portion of the valve is connected to a suitable source (not shown) of positive potential through a parallel tuned circuit L2 C2 in series with a resistance R2. The variable condenser C2 is gang controlled with the variable condenser Cl as indicated by the broken liine. The third or suppresser grid G3 of the pentode portion of the valve is connected to the cathode'C as in the usual way and the anode Al of the pentode portion is connected to the potential source through a parallel tuned circuit L3, C3, responsive to the predetermined intermediate frequency of the receiver and in series with a resistance R3. The cathode C is connected as shown to the potential source ends of the tuned circuits L2, C2 and L3, C3 through suitable condensers K2, K3. The triode portion of the triode-pentode valve is used as the local oscillator, the triode anode circuit containing a variably tuned circuit consisting of a variable condenser C4 shunting a coil L4 in series with a fixed condenser C5. The condenser C4 is gang controlled with the other two variable condensers Cl, C2 and the coil L4 is magnetically coupled to a coil L5 which is connected in series with a resistance R4 in the cathode lead. The junction point of L5 wanna is connected to the triode portion grid G4 through 1 suitable grid resistances R5 and the customary shunt condensers K4 are provided as shown. The relative setting of the three gang controlledcih cuits mentioned is such that in any given position the tuning of the local oscillator parallel tuned circuit L4, C4, C5, is midway between the tuning of the other two tuned circuits Ll, CI and L2, C2

se, e. g., by the utilization of small fixed con densersand/or by using variable condensers with specially shaped plates. Anode potential is-ap plied to the anode A2 of the triode portion of the gyalve through resistance R6. 7 I

The invention is notlimitedto the particular embodiment above described. For example a triode-pentode frequency changer-local oscillator need not be used, for obviously any type of frequency changer may be employed for example 2. h'eptode or triode-hexode'and in cases where one or more radio frequency amplifier valves precedethe frequency changer the image suppress- 40, mg variably tunable rejector circuit may be electronically coupled to any oneof these preceding high frequency valves.

Having now particularly described and ascer tained'the nature of said invention and in wha t manner the same is to be performed what is claimed is: r

In a superheterodyne receiver, a combined detector oscillator circuit comprising a tube having a detector section including a cathode, a signal grid, an auxiliary grid and an anode, and an oscillator section including said cathode, a grid electrode and an anode, a source of uni-directional current having a positiveterminaland a negative terminal, a grid circuit and an anode circuit for said oscillator section, said anode circuit including a.connection between the anode of said oscillator section and said positive terminal, said grid circuit and anode circuitbeing coupled together for producing oscillations and including a. tunable'circuit for controlling the frequency of theproduced oscillations, variable means-in said tunable circuit for tuning the circuit over a range of frequencies, an anode circuit for the detector section of said tube comprising a connection between the anode of the detector section andthe positive terminal of the source,

and a connection between the cathode of the tube and'the negative. terminal of the source, said anode connection including a circuit resonant to the intermediate frequency, means including, a tunable circuit for connecting the auxiliary grid of the detector section to the positiveterminal oi the source, said last named tunable circuit in. cluding variable means for tuning the circuit over a range of image frequencies, an input circuit for the detector section-of said tube; come prising a connection between the signal grid electrode and the cathode, said connection including a tunable circuit including, veriable means" for tuning the circuit overa range of signal ire-,-

quencies, and single means for varying said three variable means simultaneously and insuch a way that the oscillator frequency ismaintained half way between the frequencies to which the signal frequency circuit and the image frequency circuit are tuned throughout the range of operation of the system.

" ALFRED THOMAS WITTSL

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