US2061611A - Multirange converter circuits - Google Patents

Description

Nov. 24, 1936. M. G. CLAY MULTIRANGE CONVERTER CIRCUITS Filed Oct. 20, 1954 AAAAAA I m QN Sill m VVVVVVV inbnnnn: 411111111 patented Nov. 24, 1936 UNITED MULTIRANGE CNVERTER GIRCUITSi Murray G. Clay, Hasbrouck Heights'fNf., 1assignor to RadioV Corporation 'of nierica; acorporation of Delaware Application Uctober 20, YSerialgNol :749,-186

2 claims.` (ci. 2'5o-`:'0)f

My present invention relates to converter networks of superheterodyne receivers, and is more particularly directed to a converter network having a local oscillator operative over a frequency range below the signal frequency range when the network is used to receive short wave signals.

Conventional superheterodyne receivers operative over the broadcast signal range (550 to 1500 k. c.) utilize a local oscillator frequency range which is above the signal range and constantly differs therefrom by the selected operating intermediate frequency. For example, when the intermediate frequency is of the order of 450 k. c., the oscillator range would be 1000 to 1950 k. c.

i It has not been considered advisable to operate the oscillator below the signal range because such operation necessitates an impractical oscillator frequency range ratio in order to reach the higher frequency end of the range.

This conventional broadcast band practice has been carried into short wave range reception. For example, in multi-range receivers utilizing converter networks capable of being switched from the broadcast band into a short wave band, say 8 to 23 megacycles, the local oscillator range has been selected to lie above the signal range in any selected band. While satisfactory for reception in the broadcast band, this mode of operation proves disadvantageous when operating over the short wave bands.

Now I have found that the undesirable effects, appearing as selectivity and conversion gain losses, which occur at the short wave ranges when operating with a local oscillator range above the signal range are due to the reactive nature of the load on the oscillator anode. Briefly, I have established the fact that when the oscillator frequency range is above the signal range in' the reception of short waves, the load on the oscil-l 1 lator anode is essentially inductive. The effect of this inductive load is to introduce degeneration into the signal input circuit with attendant decreased gain and broadened tuning, f

Further investigation revealed that the inductive nature of the oscillator anode load could be rendered capacitative by operating the oscillator over a range below the signal range for shortv wave reception. Such a capacitative load introduced regeneration into the signal input circuit, and resulted in increased gain and greater sharpness of tuning.

Accordingly, it maybe stated that it "is one of the prime objects of my present invention to pro-I vide a converter network adapted'for operation over a short wave band, the network havingits local oscillator circuit constants chosen so that the loaden the oscillator anode is essentially capacitative .cverfthe band whereby increased sharpness of .tuning of the signal input circuit ofthe networkais secured as well as increased gama.; Y

.Another important object of the present invention is to provide a converter network adapted for operation overthe broadcast band and a Shortwave band, the network having its local oscillatory circuit constants so chosen that the oscillator-frequency range in the broadcast band is above the signal frequency range, and the circuit constants ofthe local oscillator being so chosen for operation over the short wave band that the load under the oscillator anode is essentially capacitati've.

t Anothenobject of the invention is to provide a converter network for a superheterodyne receiverand particularly one of the pentagrid convertertype, vthe network being arranged for operation over; a plurality .offrequency rangesone offsaidranges covering .a band of the order of 500to 1500: kilocycles and the other range covering a 4band of frequencies of the order of 8 to 23 mega-cycles,y the circuit constants of the local oscillator-circuit of the converter network being chosen so. that the oscillator frequency range is aboveathedsignal frequency range in the second of saidfbandsi. 1

5 Still ,otherobjects ofthe invention are to improve Vrgenerally the efficiency of rst detector and local yoscillator circuits of superheterodyne rec.eivers,rand more especially to provide such circuitswhichare not only reliable in operation, but:` economically constructed and assembled in radio receivers.

`Thesnovel features which I believe to be characteristicsof .my Vinvention are set forth in particularity in theappended claims, the invention itselfl1owever,xas to.: both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby. my invention may be carried into effect. A j

.Referring now' to the accompanying drawing therel isishownfthe circuit details of the local oscillator .and first detector networks of a super.- heterodynereceiver. `The receiver is of the mnltirran'ge? type, but for the sake of simplicity ofrdiscldsure `the signal collector, .which may be anyfwell known type of antenna, 'and one or more stages of tuned radio frequency amplification have been omitted. Those skilled in the art will understand that the collected signal energy, whatever the frequency range operated in, will be amplified and then impressed upon the tunable signal input circuit of the converter tube It. The intermediate frequency energy output of tube I is transmitted to an intermediate frequency amplier II through the coupling network M1 which is tuned to the operating intermediate frequency. The amplified intermediate frequency energy is transmitted to a second detector, and the detected energy may then be ampiiied in one or more stages of audio frequency amplication, and finally reproduced.

rlhe tube I0, for the purpose of explaining the present invention, is of the well known pentagrid converter type. This type of tube includes a cathode K, shown as of the indirectly heated type, a plate P, and five grids I to inclusive disposed between the cathode K and the plate P. This type of tube, when using an indirectly heated cathode, is known by the designation RCA 2A'l'; when utilizing a filament cathode it is known by the designation RCA IAS. The signal input circuit of tube I0 comprises a. variable tuning condenser C1 having its high alternating potential side, or stator plates, connected to the signal input grid 4. The rotor plates of the tuning condenser are grounded.

A plurality of inductance coils are provided in the input circuit and a switching arrangement is used to connect a desired one of the coils across the tuning condenser Ci. In the interests of simplicity of disclosure, only two coils Li and L are shown. The wave band selector switch I2, connected to the stator side of the tuning condenser, is adapted to be connected to the high alternating potential side of either of the inductance coils. The low alternating potential sides of the coils are connected together, and the common connection point is connected to ground through a direct current blocking condenser I3. The low alternating potential side oi' Icoils L1 and L is also connected to a source of negative grid bias, and this source may be the usual automatic volume control (AVC) source employed to minimize fading effects.

This AVC network, as those skilled in the art know, is connected to the signal grid through a resistor I4, the condenser I3 usually being disposed between ground and the connection point through the AVC network. The plate P of tube I0 is connected to a source of positive voltage B, which may be the usual voltage supply resistor in the power supply network, through the tuned primary circuit I4 of the coupling M1.

The primary and secondary circuits. I4 and I5 of coupling network M1 are tuned to the operating intermediate frequency, and may have a band pass characteristic. Grids 3 and 4 function as screen grids, and are jointly connected to a source of positive voltage, and it will be understood that this source of voltage may be derived from the same supply resistor that the source B is derived from. The grounded cathode K includes in its grounded lead the usual biasing network I6, and it will be understood that the electrodes including the cathode K, the signal grid 4 and the plate P cooperate with their associated circuits to provide the converter, or rst detector, network of the receiver.

The local oscillator electrodes of tube I0 include the cathode K, the oscillator grid I, and the grid 2 which functions as the oscillator anode electrode. While the electrode 2 has been conventionally represented as a grid, it will be understood that its construction in actual practice will follow that usually employed in the commercial 2A1 form. The oscillator anode 2 is connected by lead 20 to a source of positive voltage, and this source of voltage may be the source B, the oscillator anode being connected to point B through a path which includes the resistor 2| having a magnitude depending upon the value to which it is desired to reduce the voltage from source B for use in connection with the oscillator anode. A pair of feedback coils Ls and L3 have a common terminal connected to lead 20, and their free Opposite terminals may be alternatively connected to resistor 2I through a selector switch 30.

The tunable local oscillator circuit comprises the tuning condenser 4i) which has its rotor plates grounded, and its stator plates connected to the oscillator grid I through a path which includes condenser 4I resistor Ro, the latter being shunted by the oscillator grid condenser Co.

The tunable oscillator circuit includes coils L2 and L2', the coils have a common terminal connected to the grounded side of tuning condenser 40, and the free terminals of the coils being adapted for selective connection to the stator side of condenser 40 by means of a selector switch 50. The rotors of the tuning condensers C1 and 40 are arranged for mechanical uni-control, and this is represented by the dotted lines designated by Tuner. Of course it will be understood that the rotors of the Various tunable radio frequency amplifiers, not shown, and which precede tube I 0 may also be uni-controlled with the rotors of the condensers C1 and 40.

The selector switches I2, 30 and 50 are the wave band changing switches, and they can be unicontrolled in any manner well known to those skilled in the art. It is sufcient for the purposes of this application to point out that when switch I2 is connected to coil Li, then switch 50 is connected to coil L2, and switch 30 is connected to coil L3. As shown in the drawing the wave changing switches, or range changing switches, have been adjusted to have the receiving network operate over the short wave range of the receiver. When the selector switches are connected to coils L', L2 and L3', then the receiver is to be understood as operating in a frequency range, such as the broadcast range which covers 550 to 1500 k. c. By way of example it is pointed out that the short wave range may be one covering 8 to 23 megacycles.

Of course any other desired type of multi-range construction may be utilized. The relative magnitudes of the inductance coils are chosen according to the frequency ranges which are to be covered. Merely by way of approximation it is pointed out that the ratio of the sizes of the coils in each of the circuits, for the ranges given above, is about 64 to 1.

The intermediate frequency to be employed may be varied to suit the desire of the set designer. By way of illustration let it be assumed that the operating intermediate frequency is of the order of 450 k. c. With this value of intermediate frequency, and assuming that the wave changing switches have been thrown into position to operate in the broadcast band of 550 to 1500 k. c., the frequency range of the local oscillator will be from 1005 k. c. to i950 k. c. Of course, the local oscillator circuit is adapted to properly track with the tunable signal circuit so that the energy produced in circuit I4 has a frequency which is substantially constant over the entire frequency range operatedover.-

Considering the operating of the frequency changer network shown overthe broadcast band, it is sufficient to point out that by virtue ofelec` tron coupling within tube I D a mixing function takes place. It is not believed necessary to include in this application Vthe theory of operation of a mixer tube of the 2A'7 type. It isbelieved sufficient to point out that byvirtue ofthe coupling between the oscillator anode electrode 2 and the tunable circuit of the oscillator grid .I. the electron stream flowing'from the cathode Kto the signal gridv 4 has impressed` upon it energy of the frequencyof the tunable oscillator circuit; The signal grid 4 has impressed upon it the desired signal frequency, and by virtue ofthe elec- Y tron coupling phonomenon .there is produced in the circuit I4 the difference frequency desired.V l g Over the broadcast frequency range it has been customary to operate superheterodyne local oscillator circuits at frequencies higher than'those ofthe desired signals in order to produce the intermediate frequency employed.` While it was well known that similar operation couldbe secured by operating the local oscillator below the signal range, it was realized that for `the reception of low frequency signals such va procedure was inadvisable. t v t It has been known that it required an impractical oscillator frequency range ratio in order to reach the higher frequency end of the band, when a range of oscillator frequencies was employed in broadcast band reception which is below the signal frequency range. For this reason local oscillator circuits in multi-range receivers have been operated above the desired signal frequency range through the broadcast band, and this practice has been carried into short wave operation under the impression that the practice is advantageous in the latter type of operation.

However I have found that this is not the case, and that what is apparently desirable and normal for reception in the broadcast band is detrimental and highly undesirable in the short wave ranges. Without entering into any analytical discussion, it can be stated that there is an appreciable mutual conductance between grids 4 and 2 of the order of possibly a hundred micro-mhos, and this is negative in sign. That is, a positive increment of voltage on the signal input circuit will decrease the current in the oscillator anode circuit. Hence, an inductive load under the oscillator anode will reflect positive conductance into the signal grid circuit, and a capacitative load will reflect a negative conductance into the same signal grid circuit.

Now, when the oscillator frequency is below the signal frequency in the short wave range the load under the oscillator anode is capacitative. In other words, when the receiver is adjusted to operate in a short wave range, say between 8 and 23 megacycles, the load under oscillator anode 2 is essentially inductive if the oscillator frequency is above the signal frequency; this load is capacitative in nature if the oscillator frequency is below the signal frequency.

It will therefore be appreciated that if the oscillator frequency is above the signal frequency in the short wave range, degenerative effects will be introduced into the signal input circuit because of the essentially inductive load under the oscillator anode 2. These degenerative effects will result in decreased gain and broadened tuning. On the other hand by maintaining the local oscillator frequency below the signalfrequenyjthe load under the oscillator is kept essentially cae pacitative with consequent regeneration into '--the signal grid circuit. This results in increased'gain and greater sharpness of tuning. 'r r 'Theload on the oscillator anode 2 which is con'- sidered as reflected into the signal grid circuit includes coils Lz and L3 tuning condenser 40, the oscillator grid resistor Ro andthe oscillatorgrid condenser Co.l Hence, andlassuming'that the operating lintermediate frequency is stillh450vk.. ci when the receiver is `operated'over the shortgwa've range, -the selector switchesbeing; connected-.as shown in the drawing; the local :oscillator 'free quency range will be from '7.55 mgc. (7550 k. c.); to.'22.55 mgc. (22.550vk.; c.) It is assumed that the signal frequency range is from 48 to'23 megacycles in the case of thelast named oscillator frequency range. Byoperating over this lower oscillator frequency range the load on the oscillator anode will be maintained essentially capacitative, and it has been found that the effective -conversion gain,ras well as the sharpness @oftuninggis considerably increased by employing thisrelat-ion between signal frequency range and oscillator frequency range. Since the percentage frequency difference becomes less for higherfrequency operation the advantageof the lower oscillator free quency range becomes much greater withincrease in frequency. Twenty meter, operation has shown an experimental advantage of about 4 to l in conversion gain, with a marked improvement in radio frequency selectivity, while, at the same time there was no appreciable difference in oscillator frequency coverage on the shorter wave bands.

It is to be clearly understood that the present invention is not limited to the case where operation may be over the broadcast bandand a band of the order of 8 to 23 mgc. In its essence the present invention comprises operating the local oscillator through a frequency range which is lower than the signal frequency range when the band over which the receiver operates is such that the load on the oscillator anode would be essentially inductive were the oscillator frequency range above the signal range. Again, the pentagrid converter tube I0 may be replaced by separate detector and oscillator tubes since the same beneficial effects of this invention have been obtained with separate converter and oscillator operation due to the inevitable coupling between the two circuits.

There also may be cases where the high leakage reactance in the oscillator circuit, due to low mutual between the feedback and tunable oscillator circuit coils, may cause the load under the oscillator anode to be inductive regardless of whether the oscillator circuit is tuned above or below the signal frequency circuit. However, even in this situation the degeneration and loading effects would be considerably less at the higher frequencies in the short wave range when the oscillator circuit is tuned through a frequency range below the signal frequency range. In general, the present invention will be seen to provide a method of minimizing to a great extent the degenerative effects introduced into the signal input circuit of a converter network by an inductive load on the oscillator anode, when the converter network is operated in the short wave range, and the method involves the simple step of operating the local oscillator circuit through a frequency range below the operating signal frequency range,

While I have indicated andr described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a frequency changer network for a superheterodyne receiver, a single electron discharge tube provided with an oscillator electrode section and a detector electrode section, both sections having a common cathode, a tunable signal input circuit connected to the input electrodes of said detector section, a tunable oscillator circuit connected to the electrodes ofr said oscillator section, said signal circuit including means for tuning it over a pair of Widely separated signal frequency ranges, means in the oscillator circuit for tuning it over a pair of correspondingly widely separated oscillator frequency ranges, the circuit elements of the local oscillator circuit having magnitudes such that the oscillator frequency range is above the lower one of said pair of signal ranges when operating in the lower signal range and the oscillator frequency range is below the upper one of said signal ranges when operating in the latter range.

2. In a superheterodyne receiver a tunable first detector circuit including means for rendering the detector circuit tunable over the broadcast band, a tunable oscillator circuit including means for rendering the oscillator circuit tunable through a range of frequencies spaced from, and above, the broadcast band and dii-Tering therefrom by the desired operating intermediate frequency, additional means operatively associated with the tunable detector circuit for rendering the detector circuit operative over a short wave range of frequencies, additional means operatively associated with said oscillator circuit to render the oscillator circuit tunable over a frequency range spaced from, and below, the short Wave range and diiering therefrom by the said intermediate frequency, an electron discharge tube having a cathode and cold electrode connected to the detector circuit, at least two auxiliary cold electrodes in said tube, one of said auxiliary electrodes and said cathode being connected to said oscillator circuit, said other auxiliary electrode being reactively coupled to said one auxiliary electrode, and said oscillator circuit introducing regenerative effects into said tunable detector circuit over the entire short wave range whereby the gain in said detector circuit is considerably increased and the selectivity is effectively increased.

MURRAY G. CLAY.

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