US2248229A - Frequency controlling system - Google Patents

Description

y 1941 E. A. GREEN 2.248.229

FREQUENCY CONTROLLING SYSTEM Filed Dec. 28, 1939 3 Sheets-Sheet 1 Fig-10 3nnentor Eric .22. Gmwm,

Gttorneg July 8, 1941. E. A. GREEN FREQUENCY CONTROLLING SYSTEM Filed Dec. 28, 1939 3 Sheets-Sheet 2 I Enventor Ema f. Green,

attorney July 8, 1941. A GREEN 2,248,229

FREQUENCY CONTROLLING SYSTEM Filed Dec. 28, 1939 s Sheets-:Sh eet 3 Enventor Eric J5. Green,

(Ittorneg Patented July 8, 1941 FREQUENCY CONTROLLING "SYSTEM Eric Alfred Green, Southall, England, assignor to Electric & Musical Industries Limited, Hayes,

ain

Middlesex, England, a company of GreatBrit- Application December 28, 1939, Serial No. 311,312 In Great Britain October 21, 1938' Claims. (Cl. 250-27) This invention relates to frequency responsive circuits for use in automatic frequency control systems for radio receivers of the superhetero-' dyne type or for frequency modulation detecting circuits.

In connection with automatic frequency control circuits for superheterodyne wireless receivers, it has been proposed to provide a discriminator circuit for deriving a direct current voltage from the intermediate frequency energy when the latter shifts from the assigned inter mediate frequency value. The direct current voltage so derived is utilised in various ways for controlling the frequency of the local oscillations generated and thus the beat frequency produced.

The object of the present invention is to provide various simple alternative forms of circuit arrangements which may be used for automatic frequency control or in frequency modulation detecting circuits.

According to the present invention a high frequency signal rectifying circuit comprises in combination two impedance networks connected in series, a source of input signals, means for feeding said signals to said networks, means for rectifying separately the voltages set up in said networks, means for combining said rectified voltages and utilising the difference between these rectified voltages or the difference between two voltages which are portions or multiples of these voltages.

The invention may be applied to a signal detecting circuit for modulated carrier waves in which case means for utilising the rectified voltages are constituted by low frequency reproducing apparatus.

According to a further feature of the present invention an automatic frequency control system includes a frequency discriminator arrangement containing at least two impedance networks connected effectively in series and means are provided for applying input signals whose frequency is to be controlled across one or more of said impedance networks, means also being provided for rectifying separately two voltages derived from these networks, further means being provided for combining said rectified voltages and utilising the difference between these rectified voltages or the difference between two voltages which are portions or multiples of these voltages respectively for effecting frequency control. In a particular system embodying the invention in which the input is applied across only one of the impedance networks some form of coupling is required such that voltages will be setup across the other impedance network Or networks. The impedances may be constituted by tuned circuits or by capacity which may be the capacity to earth of a diode anode, supplemented by an extra capacity if necessary, when such is used to constitute said rectifying means.

In order that the invention may be more clearly understood and readily carried into effect, certain discriminator arrangements embodying the invention wlil now be described in greater detail by way of example with reference to the accompanying drawings in which:

Fig. 1 showsschematically the invention,

Fig. 1a is a vector diagram explaining the discriminator of Fig. 1,

Fig. 2 showsanother embodiment of the invention,

Fig. 3 is the equivalent circuit diagram of Fig. 2,

Fig. 3a and 3b show vector diagrams of the circuit of Fig. 2,

Fig. 4 shows another modification,

Fig. 5 is the equivalent circuit diagram of Fig. 4,

Fig. 6 shows still another modification,

Fig. '7 illustrates still another modification,

Fig. 8 is the equivalent circuit diagram of Fig 7 Fig. 8a is a vector diagram for Fig. 8,

Fig. 9 shows another modification,

Fig. 10 is the equivalent circuit diagram of Fig. 9,

Fig. 11 is a vector diagram of Fig. 10,

Fig. 12 illustrates another embodiment,

Fig. 12a is the vector diagram of Fig. 12,

Fig. 13 shows schematically a frequency modulation (FM) receiver employing a detector of the type shown in Fig. 2.

Referring to Fig. 1 of the drawings, A and B are signal input terminals across which is connected a potentiometer consisting of impedances Z1 and Z2. C, D and E are output terminals from which can be taken the two voltages to be rectified, .thus V1 and V2, V1 and V3 or V2 and V3. In the case shown V1 and V3 are rectified. The voltage V1 is fed to terminals G and H connected to a diode rectifier I, and the voltage V3 is fed to terminals F and H connected to diode rectifier 2. The rectified voltages derived from the diodes are subtracted in load resistances 3 and 3' connected to the cathodes of the diodes l and 2. The vector diagram at the correct intermediate frequency forms an isoscles triangle in which V1 is equal to V3.

Referring to Fig. 2 of the drawings, an inter mediate frequency amplifying valve 4 is shown anode 9 to earth (Z1) and Z2 is constituted by the tuned circuit I0.

The equivalent circuit diagram is shown in Fig. 3. Explanatory vector di-' agrams are shown in Figs. 3a and 3b. rangement operates so that the voltages applied to the anodes 7 and 9 are equal in magnitude at the correct intermediate frequency. Thus, the voltage V1 across Z1 will lag 90 behind the ourrent and the voltage V2 across Z2 will lead or lag the current depending upon the adjustment of its resonant frequency. The locus of V2, when plotted to polar co-ordinates, can be represented by a point on a circle passing through the origin'as shown'in Figure 3a. By adding the voltages V1 and V2, V3' is obtained. Adjustmentof the resonance frequency of Z2 enables V1 to be made equal to V3, this occurring when the are shown dotted in Figure 3b intercepts thecircle. It will be seen as V2 is not at right angles to V1,'that the tuned circuit Z2 must be made resonant at a frequency other than the exact intermediate frequency, but the amount off resonance of the circuit is not sufficient seriously to affect the overall selectivity of the receiver;

Referring to Fig. 4 an intermediate frequency amplifying valve l I is shown coupled by two tuned circuits 1 2 and I 3 to a double diode rectifying valve M. No bypass condenser is provided between the circuits l2 and I3; There is mutual inductance shownbetween the coils in the tuned circuits [2 and I3, but this is not essential-as the stray capacity coupling is usually sufficient. The circuits l2 and I3 have to be slightly mistuned from the actual intermediate frequency. The diagram equivalent to Fig. 4 is shown in Fig. 5, and it will be seen in this case that the impedance Z1 is constituted by the capacity to earth of the diode anode IS, the impedance Z2 being constituted by the tuned circuit l3.

The arrangement described with reference to Fig. 2 may be modified by providing an actualcondenser for the impedance Z1 as shown in- Fig. 6 at C3. This will entail a greater degree-of mistuning of the circuit Hi. Excessive mistuning may be obviated by balancing a portion of the rectified voltage obtained from V1 at the correct intermediate frequency against a smaller portion of the rectified voltage obtained'from V2 at'the correct intermediate frequency. One method of effecting this is by employing two extra resistances R1 andR2. The required effectis then obtained provided R3 is less than R4.' In the case where the existing capacity is too high its effective value'at the correct'interrnediate frequency can bereduced by connecting a filtering system consisting of an extra choke coil CH1 and condenser C4 to the anode l1 instead of to the anode [8 which is the more usual point. 7

The connections of Z1 and Z2 may be reversed as shown in Fig. 7 in which Z2is provided by the stray capacity between the anodes of a double diode valve. Fig. 8; and the vector diagram in Fig..8a'.

Referring now to Fig. 9 an intermediate frequency amplifying valve 19 has connected inits.

anodelead two tuned circuits 20"a'nd 2 I which The ar- 7 The. equivalent diagram isshown in the double diode valve.

constitute respectively the impedances Z1 and Z2. These tuned circuits are shown coupled through condensers 22 and 23 to the anodes of a double diode valve 24. In this case the series connected circuits 20 and 2| are tuned to frequencies on .either sideof the correct intermediate frequency. The equivalent diagram is shown in Fig. 10 and the vector diagram in Fig. 11, this arrangement being operated with or without coupling between the tuned circuit 20 and 2|.

- Referring now to Fig. 12 of the drawings, an intermediate frequency amplifying valve 25 has a primary tuned circuit 26 in its anode lead and coupled through a condenser 21 to one anode 28 of a double diode 29. A secondary tuned circuit 30 is connected directly to the other anode 3| of This arrangement is an example of the particular case in which the input is applied to one impedance only, namely the tuned circuit 26 and some form of coupling has to be provided so that'a voltage can be set up across the other impedance, namely the tuned circuit 30; The arrangement operates in a mannerdifferent from that described with reference to the preceding figures. It will be seen that the voltages V1 and V2 are those set up across the primary and secondary tuned circuits respectively. These voltages are rectified separately in the load resistances 32 and 33, the A. F. C. lead being connected at the junction between the resistance 33 "and a choke coil 34 which serves to complete the D. C.v path between the anode 28 and its cathode.

Fig- 12 shows vectorially the relation between the voltage Vp across the primary circuit 26, and the voltage Vs across the secondary circuit30.

The secondary circuit 30 is tuned to a frequency slightly different from that of the correct intermediatefrequency. If the mistuning is represented by E kilocycles per second, the A. F. C. voltage'will be zero at 22 kilocycles per second ofi'the correct intermediate frequency as well as at the correct intermediate frequency. The exact degree of mistuning to be selected depends on the desired overall selectivity. Mistuning of about 4 kilocycles per second will be necessary but this is not a serious disadvantage when compared with usual forms of discriminator circuit which are themselves unselective.

While reference has been made to the use of the circuit arrangements described for automatic frequency control purposes, it is to be understood that theyare all operative as detectors of frequency'modulated waves.

The application to such a purpose of the arrangement shown in Figure 2 is illustrated in Fig. 13; In this figure an antenna 35 is shown connected to a receiver including a radio frequency amplifier '36, a'mixer 31, local oscillator 38, intermediate frequency amplifier 39 and amplitude limiter 40, and an intermediate frequency amplifying .valve corresponding to the valve 4 in Figure 2'. The choke coil 45, condenser 46 and the connections of the anodes 41 and 49 of the double diode 48"to the timed circuit 50 are as in Figure 2. Rectified' voltages are set up across the resistances 5i and 52 and these voltages are suitable for'reproduction." For this purpose the diode 48" is connected through a resistance capacity coupling 53, 54 to the control grid of an amplifying valve 55 connected to a loudspeaker 56.

It will be understood that the'forms of circuit for efie'ctin'gautomatic frequency control as defrequency modulated waves in a similar manner to that described with reference to Figs. 2 and 13. Further, it will be understood that the invention may find application to forms of circuit arrangements for effecting automatic frequency control or detection of frequency modulated waves other than those circuits specifically described.

I claim:

1. A high frequency signal rectifying circuit comprising in combination two impedance networks connected in series, a source of input si nals of said desired frequency, means for feeding said signals to said networks, means for rectifying separately the voltages set up in said networks, means for combining such rectified voltages, and means for utilising the difference between such rectified voltages.

2. A high frequency signal rectifying circuit comprising in combination two impedance networks connected in series, one of the networks being a tuned circuit off-resonance with a desired signal frequency, the second network consisting of untuned capacitative reactance, a source of input signals of said desired frequency,

means for feeding said signals to said networks,

means for rectifying separately the voltages set up in said networks, means for combining said rectified voltages and means for utilising the difference between two voltages derived from said rectified voltages.

3. A frequency discriminator arrangement comprising in combination two impedance networks connected in series, a source of input signals, means for feeding said signals to said networks, means for rectifying separately the voltages set up in said networks, said rectifying means comprising two diodes, the capacity between the anode of one diode and ground being one of said impedance networks, and a tuned circuit off-resonance with the signals being the second network, means for combining said rectified voltages and utilising the difference between two voltages derived from said rectified voltages.

4. In combination, in a modulated carrier system, a resonant input circuit tuned to a frequency difierent from a predetermined desired carrier frequency, a pair of diodes, a resistive path connecting the cathodes of the diodes, one end of the path being at a fixed alternating potential, an output connection to the opposite end of said resistive path, means connectingthe diode anodes to opposite sides of said input circuit, and a connection between an intermediate point on the resistive path and one of the diode anodes.

5. A network for detecting frequency modulated carrier waves, said network comprising a resonant circuit tuned to a frequency spaced from the mid-band frequency of said waves, a pair of diodes, the anodes of the diodes bein connected to points of opposite polarity of said resonant circuit, a resistive load path connecting the diode cathodes, one end of the path being at an invariable potential, the opposite end being connected to utilization means, and a connection between an intermediate point of the load path and one side of said resonant circuit.

ERIC ALFRED GREEN.

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