US2097937A - High frequency oscillating circuit - Google Patents

Description

Nov. 2, 1937. N. M. RUST HIGH FREQUENCY OSCILLATING CIRCUIT Filed Oct. 8, 1935 2 Sheets-Sheet 1 Jlll III INVENTOR NOEL MEYER R057 BY 0 a ,jx' p/wwx/ ATTORNEY HIGH FREQUENCY OSCILLATING CIRGUI-T Filed Oct. s, 1955 2 Sheets- Sheet 2 lNVENTOR ATTORNEY l atented Nov. 2, 1937 UNITED STATES] HIGH FREQUENCY osoILL G CIRCUIT Noel Meyer Rust, Chelmsford, Essex, England, assignor to Radio Corporation of America, a corporation of Delaware Application October 8, 1935, Serial No l4,045

' In Great Britain September 14, 1934 1 Claim." (01. 250-20) This invention relates to electrical high frequency oscillating circuit arrangements and has for its object to provide improved means whereby the tuning or natural frequency of a. high 5 frequency electrical oscillating circuit may be automatically varied in dependence upon departures from predetermined desired results in such manner as to correct for such departures,

The principal applications of they invention are 10 (1) to so-called homodyne receivers, i. e. re-

ceivers wherein a received modulated carrier wave is mixed with a locally generated carrier.

wave of like frequency for reception purposes, and (2) to what are sometimes termed self-tuning? l5 receivers, i. e. receivers wherein a tuning instrumentality is caused automatically to be varied in such manner as to maintain the receiver accurately tuned to a desired signal even if said signal drifts i. e. varies slightly, in. frequency; 20 The invention is. based upon the fact that .by' virtue of so-called Miller effect the space between the electrodes of an electron. discharge device may be arranged to present a substantially pure reactance of a magnitude which can be varied by 25 varying the operating constants of the device,,

and in carrying out this invention a reactance so presented is utilized as a variable reactance in' a high frequency tuned circuit to be automatically controlled; the operating constants of said de- 30 vice being varied in dependence upon departures from results desired to be obtained, such varia-' tions being such as to produce variations in tuning in a sense to compensate for the departures. The invention is illustrated in the accompany:

35 ing drawings which show diagrammatically dif-.

ferent embodiments thereof. Fig. 1 shows a homodyne circuit embodying the invention; Fig.

2 shows a portion of a superheterodyne receiver embodying the invention inan oscillator fre-. 4.0 quency control circuit; Fig. 3 illustrates a modification of Fig. 2. Referring to Figure 1 which showsone ar-' rangement in accordance with the invention as applied to a so-calledhomodyne receiver the local 45 oscillator of the said receiver in the particular embodiment illustrated includes certain of fthe electrodes of a so-called heptode valve l, i. 'e; a valve having a cathode 2, an anode 3 and five grids 4, 5, 6, I, 8, in succession between cathode 50 and anode. In the description which follows valve grids will, for the sake of brevity,be dew scribed in reference to their order as-counted, from the cathode; for example, the first'grid'is the grid 4 nearest the cathode, the second grid 55 is the next'grid 5 and so on. The first grid 4 is connected to thecathode 2 through a leak resistance 9 and is'also connected through a coupling condenser l and a parallel tuned circuit consisting of an inductance l I shunted by a variable condenser l2 to the negative terminal l3 of the anode potential source (not shown). The second grid 5 is connected through a coil 14 which is adjustably coupled to the coil II to a source of positive potential (not shown) connectedjat [5, the cathode 2 and the first and second grids 4 and 5 accordingly constituting the electrode system of a local oscillator which is utilized as the local oscillator of a homodyne receiver, A third coil I6 coupled to the coil i I is utilized to take off local oscillatory energy 15' to the homodyne detector (not shown). The third and fifth grids 6, 8, are connected together and to a suitable source of positive potential connected at I1. The fourth grid 1 is coupled through a coupling condenser l8 to a point (not shown) of the receiver from which received high frequency signal voltage (for locking purposes) is obtainable, the said grid 7 being also connected through a resistance l9 to a variable tapping point upona potentiometer resistance 2| one end ofwhich is connected to the negative terminal 22 of a suitable bias source and the other endof which is connected to the cathode 2. The said cathode is connected through a milliammeter 23in-series with a choke 24 and a second potentiometer resistance 25 to the terminal l3. The potentiometer resistance 25 is shunted by a condenser 26 and a second condenser 21 is shunted across the series connected choke 24 and resistance 25'the said choke and two condensers constituting a low pass filter across which is effectively shunted the resistance 25. An adjustable tapping point 28 upon the resistance 25 is connected through a further resistance 29 to the control grid'30 of a valve 3| which is utilized as 40 a variable reactance device, the grid 30 being also connected directly to the junction point between the parallel tuned circuit H, l2 and the condenser lllii In the embodiment illustrated a pentode valve is utilized as the variable reactance device and the, control grid 30 is connected to the anode 33 thereof through a variable condenser 32 which servesto increase the anode-grid capacity .of the valve and which-can be varied to adjust the value of the apparent input reactance of the valve 3|.

The 'second grid 34 of the valve 3! receives positive bias from any convenient source (as shown it is connected to terminal I!) and the cathode"35 of the valve 3| is connected as in the usual way to the third grid 36 thereof and also to an adjustable tapping point 37 on a resistance 3i? shunted between the positive and negative terminals H and I3 respectively. The anode 33 of the valve 3! is connected through frequency expected to be present and desired to.

be received;

Now it will be appreciated that with this ar-- rangement the grid cathode-circuit 30 ?35 of thevalve 3! is effectively in shunt across the parallel;

'. tuned circuit ll, 52, in the circuit of the first grid 4 of the heptode, and accordingly, the capacitative reactance presented by the control grid circuit of the valve 3! will form a smallcondenser in shunt across the normal condenser of the said tuned circuit.

The low pass filter will pass on to the second potentiometer 25, 23 (from which potential for the control grid of the valve 3! is" obtained) either slow beats due to the interaction, between the local oscillations and locking signal oscillations or--if the' 'local oscillator is operating accurately at signal frequency-a resultant direct current depending for its magnitude upon the relative phase between the local oscillations and the locking signal oscillations. This resultant-slow beats or direct currentwill be indicated by the milliammeter 23 and because of the fact'that a desired proportion of the said resultant is fed to the control grid 39 of valve 31', it is possible so to adjust'the apparatus that the tuning of the local oscillatorfwill be automatically varied soas to maintain it automatically substantially at the same frequency as the received locking signal frequency and in a predetermined phase relationship therewith. In other words, the arrangement effectively interlocks the local oscillation frequency with received signal oscillation frequency. 'It is found in practice possible so to make the adjustments asto secure any relative phase relationship required between local oscillations and received signal oscillations, such adjustments being made either by varying the position of the tapping point 28v or by manually adjusting the frequency of the parallel tuned circult ll, l2. Where the receivedsignals are weak the adjustments become more delicate and it is found better to move the slider 28 in such direction as to give maximum controlling action upon the pentode and then to adjust the tuning of the parallel tuned circuit ll, 12

An important advantage of the above described arrangement isto be found in the fact that not only is an easily adjustable and satisfactory looking action obtained between received, signal oscillations and local oscillations, but the local oscillations are only very weakly if at all. modulated by modulation energy from the received. signals, and this, of course, is of considerable importance in homodyne receivers. The plate circuit of the he-ptode may be used for anypurpose.for example it may be connected to an arrangement for indicating beats so as to facili tate adjustment of the'circuit. In Figure lsuch an arrangement is illustrated, and comprises a valve ll coupled as shown to the plate circuit of the heptode and giving anindicating output which may be taken from terminals 42.'

Figure 2rshows another embodiment .of the invention as applied to a superheterodyne receiver wherein self-tuning is resorted to. Here a point (not shown) in the intermediate frequency amplifier of the receiver and from which intermediate frequency voltage is obtainable, is coupled through a condenser 43 to one end of a two-branch tuned circuit, the branches being designated A and B. Branch A consists of a capacity 44a in series with an inductance 45a, and. branch B which is in parallel with branch A likewise comprises series elements Mb and 45b. The two branch circuit is tuned as a whole to the operating intermediate frequency, but one branch is arranged to be an acceptor circuit for a frequency above the operating intermediate frequency and; the other to be an acceptor circuit for a frequency a corresponding amount below the intermediate-frequency. For example, if the intermediate frequency is 100 kilocycles one branch may be tuned to 110 kilocycles and the other to. 90 kilocycles. The junction of the inductance 45a and the capacity Na in branch A is connected through a rectifier 46afor example a copper oxide rectifier in series with a condenser shunted resistance combination 47a i80L---to that end of the two branch circuit remote from the coupling condenser 43, and in a similar manner the junction point of inductance 452) with capacity 44b is connected through a second rectifier 46b and a second capacity shunted resistance combination llb 58b to the same end of the two branch circuit. Adjustable tapping points 49a and 4% upon the resistance 41a 41b respectively, are connected together through a further resistance 50,'and the mid-point of this. further resistance is connected through a still further resistance 51 to the first grid 3% of a valve 3| which is again a pentode and which acts as a variable reactance device. The rectifiers are connected in opposite sense so that one of the two adjustable tappingw points 4911 or will be positive and the other negative as a resultv of uni-directional currents obtained from rectification. The end of the two branch circuit remote from the intermediate frequency amplifier is connected to an adjustable tapping point 3'! upon the potentiometer resistance 38, which as in the previously described embodiment is shunted between the negative and positive terminals 83, ll, respectively, of a source of potential (not shown) the positive terminal of this source being connected, to the second grid 34 of the pentode andthe'tapping point 31 being connected to the cathode 35 of the pentode. The cathode 35 is also connected as in. the usual way to the third or suppressor grid 36 of the pentode. The anode 33 of the pentode is, as before, connected through a resistance 40 in series with a choke 39 to the positive terminal I 5 of a source of anode potential and a variable condenser 32 is connected between the control. grid 38v and the anode 33. The control grid-cathode space of the valve 3| is connected by means of. terminals I3; 60, across the normally provided coil (not shown) in the adjustable frequency determining circuit of the local" oscillator of the receiver.

In practice the local oscillator is manually adjusted as in the usual way, so as to beat with received signals to produce as nearly as possible the desired operating. intermediate frequency, but in a circuit as just described, owing to the interconnectionof the pentode with the local oscillator of thereceiver, if this manual adjustment is not accurately made, or if the received signal drifts somewhat in frequency, the local oscillator tuning will be, automatically varied in such, manner as to produce the correct operating intermediate frequency. In this connection it will be appreciated that so long as the intermediate frequency is accurately obtained at the point from which it is taken off to the two branch circuit, the potentials set up by the two rectifiers will be equal and the bias upon the first grid of the pentode will not be changed. faulty manual tuning or by reason of drift in the received signal frequency, the proper beat frequency is not obtained, the bias upon the pentode will be changed and this change will produce a variation in the effective reactance in the frequency determining circuit of the local oscillator.

in such direction as to rectify the fault. In this way the action of the signal itself is utilized to correct for frequency drift or for tuning inaccuracies. Accordingly, the invention enables a rela- 1 tively narrow band pass intermediate frequency amplifier to be employed since accurate local oscillator tuning is automatically obtained, while furthermore, in receivers having automatic gain control the tuning indicator means usually deemed necessary to assist accurate manual tun ing, can be omitted.

The last described embodiment is satisfactory so long as a normal fairly low intermediate or beat frequency is employed e. g. an intermediate frequency of the order of 100 kilocycles, but where very high beat frequencies are employed copper oxide rectifiers are not satisfactory and the modification illustrated in Figure 3 is preferably employed. In this figure, in which parts corresponding to like parts in Figure 2 are indicated by like characters, a two branch circuit is used as before and, also as before, one branch is tuned slightly above and the other slightly below the operating beat frequency which is now presumed to be of a high value of, for example, 2,000,000 cycles per second. For 2,000,000 cycles per second one branch might be tuned to 2,020,000 cycles and the other to 1,930,000 cycles. One end of this twobranch circuit is, as before, connected through a coupling condenser to the beat frequency ampliher; and the other end is connected to the cathode of a double diode triode iii. A point between the inductance and capacity in one branch of the twobranch circuit is connected through a capacity shunted resistance combination 41a, 48a to one diode anode iter of the double-diode-triode tube 3i-this anode ta' is also connected to the grid 6? of the double-diode-triode-while a corresponding point on the other branch of the twobranch circuit is connected through a capacity shunted resistance combination llb 48b to the second diode anode 4611, this second diode anode being connected through a resistance 50' to an If, however, either by reason of,

adjustable tapping point 68 upon a further capacity shunted resistance combination 69, 10, which is in series between the cathode H of the doublediode-triode and the negative terminal l3 of an anode potential source. The anode 12 of the double-diode-triode is connected through a resistance 13 to the positive terminal ll of the source of anode potential, and the mid-point of the resistance 50' is connected through a further resistance to the control grid 30 of the valve 3| which acts as a variable reactance valve. The cathode 50 of this valve is connected as before to an adjustable tapping point 3'! upon a resistance 38 shunted between the, terminals IS, H, the remaining connections being similar to those of Figure 2 The invention is not limited to its application to heterodyne receivers and the principles embodied in the self-tuning super-heterodyne receivers described are applicable in like manner to securing self-tuning action against slight inaccuracies of tuning or slight signal frequency drifts in other types of receiversfor example, receivers embodying ordinary radio frequency tuned amplifiers.

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:-

In a superheterodyne receiver of the type pro- Vided with an intermediate frequency energy circuit and a tunable local oscillator circuit whose frequency is to be automatically controlled, the improvement which consists of a network comprising a pair of resonant circuit-s, each resonant circuit including a rectifier, means for impressing the intermediate frequency energy upon said resonant circuits, one of the resonant circuits being tuned to a frequency differing from the intermediate frequency by a predetermined frequency value, the other resonant circuit being tuned to a frequency differing from the intermediate frequency by the same frequency value but in,an

' opposite direction, an electron discharge tube,

means, responsive to the current flow in one of said rectifier circuits, for varying the space current flow of said tube, an impedance in the space current circuit of said tube, and tube means connected to said impedance and the second of said rectifier circuits, and Whose gain is responsive to direct current voltage developed across said im pedance by the space current flow therethrough and the current flow in said second rectifier circuit, for providing a reactance of variable magnitude.

NOEL MEYER RUST.

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