US2242661A - Frequency discriminator network - Google Patents

Description

y 20, 19 N. M. RUST 2,242,661

FREQUENCY DISCRIMINATOR NETWORK Filed Ndv. 14, 1939 4 Sheets-Sheet 1 n D. 0. Voltage I z r 16 NOE]. M Rl/S T n W by v I JYTTOBNEW May 20, 1941. N, RU T 2,242,661

FREQUENCY DISCRIMINATOR NETWORK Filed Nov. '14, 1939 4 Sheets-Sheet 3 Con/7x202 VOLTAGE Com-e01. VOLT/76E ca real V02 7965 Con/n30; Von/7a:

OF TUNE or TIM/E I by May 20, 1941. N. M. RUST FREQUENCY DISCRIMINATOR NETWORK 4 Sheets-Sheet 4 Filed Nov. 14, 1939 IN VfN TOE NOEL M. R415? MT'To/a/vEn Patented May 20, 1941 UNITED STATES PATENT OFFICE 2,242,661 FREQUENCY DISWATOR NETWORK Meyer Bust, Ellex, England, minor to n of America, New York, N. Y.,

a corporation Delaware Application mm 14, 1939, Serial No. 304,301 In Great Britain September 20, 1938 Claims. (Cl. 250-40) This invention relates to discriminator circuits for use in automatic frequency control (AFC) systems, i. e. systems for radio receivers and the like, whereby, once the receiveris tuned manually to within a predetermined degree of with the intended frequency which should ibe produced there, and provides a direct current voltage which, in polarity and magnitude, corresponds to the sense and extent of departure of said actually produced frequency from the intended frequency. This voltage, which maybe termed the controlvoltage, is utilized to correct the tuning of the receiver-usually by adjusting the local oscillator frequency in a superheterodyne receiver-so as to correct for the departure.

In the drawings: Fig. 1 shows graphically the discriminator charact ristic desired as compared to the known type,

Fig. 2 shows a known discriminator network.

Figs. 3 and 4 shows an analysis of the invention,

Figs. 5 and 6- show different embodiments of the invention,

Fig. 7 shows the characteristic of the network of Fig. 6,

Fig. 8 shows a further modification,

Figs. 9 to 12 show the characteristics of the modification of Fig. 14,

Fig. 13 shows the characteristic of the modification of Fig. 15,

Fig. 14 shows a further modification,

Fig. 15 shows still another modification, and

Fig- 16 shows yet another modification.

The type of operating characteristic curve obtained, in general, with a usual known discrim-- quency intervals below and above the origin, and

then falls away at first fairly steeply but then with increasingly less steepness outwardly of these maxima.

The main object of the present invention is to provide improved discriminator circuits having an operating characteristic curve differing from the usual curve above described, in that it falls away to zero fairly steeply outwardly of the maxima so that there are three points of zero ordinate value, namely the origin and two other points substantially equidistant from the origin on either side thereof. Such a curve is shown at 6, I, 2, 3, l, 8, 9 in Fig. 1. A discriminator circuit with such a curve has obvious operating advantages, among them being that the range of control of the AFC system is definite and 'predetermined. In practice, the curve may be allowed to cross the abscissa line outwardly of the two outer zero ordinate points as shown in Fig. 1 but, preferably, any rise of the curve outwardly ofv these values is kept as small as possible.

Discriminator circuits in accordance with this invention are of the phase discriminator type, and, in order that the present invention may be the better understood, a typical known circuit of this type will first be described with reference to Fig. 2. This known circuit chosen for description is an I F. circuit for use in a superheterodyne receiver-the usual case in practice.

This known circuit comprises a parallel tuned circuit having ends AC and sharply resonant at the intended I. F., this circuit being fed from an I. F. stage (not shown) of the receiver. The mid-point of the circuit is marked B. The end A is connected to the anode of one diode ID, and the and C is connected to the anode of another diode 2D. The two diode cathodes are connected together through two equal series resistances IR and- IR, each being shunted by a bypass capacity IC and 2C. The mid-point between the two resistances IR and 2R is marked E, and the two remaining resistance terminals are marked Dand F respectively. A direct current path must exist between the points B and E to enable the diodes to-conduct. Control voltage is taken oflE between D and F.

This known arrangement is such that, in the .in-tune condition the voltage AC is in quadrature with the voltage BE. Here, and in the following description, the voltage between any two points is designated by the names of those points: thus voltage AC is the voltage between A and C, and voltage BE is the voltage between B and E. The rectified voltages DE and FE are proportional. to the I. F. voltages AE and CE respectively. If the frequency fed to the tuned circuit (i. e. the mid-band frequency of the I. F. band) departs from the natural frequency of said tuned circuit, AC swings in amplitude and phase but BE remains substantially constant in amplitude and phase. For the sake of simplicity assume that BE is greater than AC. This assumption is not necessarily the correct one, but the making thereof will not invalidate the conclusions reached below and will simplify description. Then, with this assumption, the position for the in-tune condition may be represented vectorially as shown in Fig. 3. The voltage AC is represented by the line ABC with B as its middle point; the voltage BE is represented by the line BE at right angles to ABC and extend ing from B; the voltage AE is represented by the line BE. extending from B and making an angle with line BE on one side thereof; and the voltage CE is represented by the line BE which is of the same length as the line representing voltage BE and symmetrical therewith on the other side of line BE.

In the out-of-tune condition, which is represented vertorially in Fig. 4, the line ABC swings about B so as to be no longer perpendicular to BE, the line BE (representing voltage AE) either increases or decreases in length (as shown, the latter) and the line BE" (representing voltage CE) changes equally and oppositely in length. The line BE or BE" which increases in length is, of course, the one in the angle EBA or EBC which becomes acute as the result of the swi ing of ABC and obviously depends upon the direction of mistuning. The direct current voltages ED and EF (Fig. 2) depend upon the lengths of the lines BE and BE", and are independent of the phase of these vectors. Thus, for intune" ED=EF and DF (the control voltage) is zero, while for out-of-tune ED EF or ED EF (depending on the direction of mistune) and DF is finite and positive or negative. It is easy to show that if BE AC, DF is proportional to that component of AC which is in phase with BE; in more general terms, if either voltage (AC or BE) is considerably larger than the other the control voltage is proportional to that component of the weaker voltage which is in phase with the stronger. It is, also, true that, irrespective of AC and BE when the component of BE in phase with AC is zero, the control voltage DF is zero. In the usual known arrangement this condition is on y achieved in the in-tune condition.

According to this invention a discriminator circuit arrangement of the phase discriminator type, and suitable for use in an AFC system, comprises a resonant circuit the voltage across which is, in the in-tune condition, in quadrature with a voltage between two symmetrical points in said arrangement, one of said points being the mid-point of said resonant circuit or a point equivalent thereto, and the elements comprising said arrangement being so connected and dimensioned that the relative phase swing between said voltages, over the intended range of operation, exceeds 180 and there are two frequencies substantially symmetrically disposed with relation to that corresponding to the intune condition at which said voltages are in quadrature.

In other words, in accordance with this invention (using the terminology already employed) the phase of AC relative to BE is arranged to swing, with change of input frequency,.faster than in the known discriminator above described, and so that the relative phase swing can exceed 180, the control voltage vanishing to zero not 'only for the in-tune condition but at two symmetrically chosen out-of-tune conditions. For the in-tune condition AC and BE are at 180, and for the two symmetrically chosen conditions AC and BE are at and 270 respectively.

It will, of course, be appreciated that it is not essential that BE be of constant amplitude and phase and AC vary (the assumption made hereinbefore) for obviously, the relative phase of BE to AC being the essentialfactor, BE-may vary in phase or BE and AC may both vary in phase either in opposite directions and at the same or different speeds or in the same direction at different speeds. In general, however, it will be most economical and convenient to arrange for one of these voltages to be relatively constant in phase.

A number of practical embodiments or the invention will now be described, these embodiments being for a superheterodyne receiver with an intended I. F. of 450 k. c. which is the in-tune" frequency of the discriminator.

In the first of these embodiments, shown diagrammatically in Fig. 5, the input circuit of the discriminator is an I. F. tuned circuit I included between grid and cathode of a first valve 2 whose plate circuit is coupled by an I. F. band pass filter 3-1 of the coupled tuned circuit type to the anodes of two diode reotifiers 8, 9. The cathodes of the diodes are connected together through two similar capacity-shunted series resistances H), II. The control voltage is taken by leads from the outer ends of these two series resistances i. e., from between the diode cathodes. In the circuit of Fig. 5, the necessary 180 phase swing is produced by selective negative feedback. The first valve 2 has a suitable frequency selective circuitfor example two tuned circuits I5, l6 tuned respectively slightly above and slightly below the intended I. F. and connected in series inserted in its cathode leg circuit. A connection ll, including a blocking condenser I8, is taken from the grid of the first valve 2 to the mid-point I4 of the secondary-side I of the bandpass filter 3'|. As will be appreciated, in this embodiment the phase shift (for zero control voltage) between the grid of the valve 2 and the top of the secondary side 1 of the filter is There is inserted a radio frequency choke coil l'l' in the path between leads l1 and the junction of resistors l0 and II. In the case of the circuit of Fig. 5, the grid voltage of tube 2 is one of the component voltages. The voltage across circuit 3 is in phase therewith and that across circuit I is 90 displaced. Hence, at the mean frequency, the relations are correct, provided that circuits I5 and I are slightly detuned on opposite sides of the mean frequency so that the total cathodereactance is zero at the main frequency. As the frequency departs from the main frequency value,

the total cathode reactance becomes positive or negative in accordance with the sense of departure and this affects the phase relation between the plate current and the grid potential to an extent such that the phase of the voltage across circuit 1 may rotate by approximately thereby producing the desired vanishing of the AFC voltage when the signal departs a certain amount from the mean frequency. If desired, the arrangement of Fig. 5 may, with advantage, be modified by connecting the grid of valve 2-to a tap on the input circuit of said valvei.'e., tapping the grid down on said input between the grid of the first valve 2 and the top of the secondary I of the band pass filter 3-1. In Fig. 6 there is secured, in addition to the phase shift of the voltage across circuit I, a certain amount of phase shift in the voltage applied at point. l4. If this shift is in the opposite direction, the relative shift of the two component voltages will exceed 180, in accordance with the invention.

In these embodiments some ,of the circuits notably the band pass circuits (including the valves they couple) -may also form part of the I. F. signal channel of the receiver. In the embodiments so far described the components may easily be selected to give a resultant characteristic such as that shown in Fig. 7 with steep sides, and which has only very small humps outwardly of the outer zero control volts" points ZVI, ZVZ.

A further modification is shown in Fig. 8. In this modification the connection 22, including condenser 23, is provided between the anode of valve 2 and the center point H on circuit 1, and additional filter circuits 20, 2| are provided between circuits 3 and I as shown. In the circuit of Fig. 8 the plate voltage of tube 2 is directly impressed on point l4 as one component voltage, while the other voltage supplies three successive phase shifts of 90 each at the three mutual inductance couplings between successive tuned cir cuits. These three shifts bring the two component voltages at right angles so as to produce the desired central crossing, while the presence of three tuned circuits subsequent to the point where voltage is taken off for point l4 permit the relative phase shift toapproach a total of 270 as the frequency departs from the. center frequency.

All four circuits are tuned to the mid-band'frequency.

Still further improved results can be obtained by the application of reaction. The type of improvement attainable is illustrated graphically in Figs. 9 to 13 inclusive. Of these figures, Fig. 9 shows the characteristic obtained with an experimental embodiment as shown in Fig. 14, the reaction coil 24 being, however, omitted. This embodiment has four tuned circuits marked respectively A, B, C, D. By applying reaction to the first circuit A, the characteristic becomes as shown by Fig. 10. By applying reaction to the second circuit B (by means of coil 24) the characteristic becomes as shown by Fig. 11. plying reaction to the third circuit C the characteristic is as shown by Fig. 12. By applying reaction to the fourth circuit D-as shownin Fig. 15 :by means of the coil 25the characteristic becomes as shown in Fig. 13.

Fig. 16 shows a further modification difliering from Fig. 15 in that a double-diode-triode 211, having diode anodes 8a and 9a associated with load resistors Illa, a, replaces the separate, triode 2 and diodes 8 and 9, with their associated resistors Hi, H of Fig. 15. It will be seen that the circuits of Figs. 14, 15 and 16 are the same By apas that shown in Fig. 8, with the exception that a regenerative tube is connected across circuit B in Fig. 14, and across circuit D in Figs. 15 and 16. i 1

Specifically, Figs. 14, 15 and 16, like Fig. 8, each include four tuned circuits. These circuits are marked A, B, C and D, and correspond to the circuits marked 3, 20, 2| and I of Fig. 8. Energy is assumed to be fed into circuit A, while a pair of diodes is connected to opposite terminals of circuit D in Fig. 14, or to opposite ends of a coil coupled to circuit D in Figs. 15 and 16. In each of the figures thereis a direct connection through a blocking condenser from the input tuned circuit to the mid-point of the coil to which the diodes are connected. In Fig. 14 an auxiliary tube 2 has its input electrodes connected to circuit B, while its output is coupled back regeneratively to circuit B by mutual inductances between this circuit and plate coil 24. This tube, by its regenerative action, effectively sharpens the selectivity of circuit B, resulting in performance indicated in Fig. 11. If the auxiliary regenerative tube had been connected across circuit A the characteristic would have been as shown in Fig. 10. Fig. 15 shows auxiliary tube 2 connected across circuit D, with a resulting characteristic shown in Fig. 13. If the regeneration had ,been applied to circuit C the resulting characteristic would be as shown in Fig. 12.

In Fig. 15 the diodes 8 and 9 are not directly connected to the coil of tuned circuit D, but to a separate coil coupled thereto. The use of this separate coil permits grounding the lower end of circuit D, thus making it simpler to connect the regenerating tube 2 thereto. Fig. 16 difiers from Fig. 15 only in that the pair of diodes is included in the same tube structure along with the regenerative tube, and in that the output voltage is between a pair of leads (shown terminated by arrow points), neither of which is at ground potential as was the case in Figs. 14 and 15.

The operation of Figs. 14, 15 and 16 is qualitatively the same as that of Fig. 8. Each diode receives directly from the first tuned circuit through a blocking condensenshown as number 23 in Fig. 8, a certain component of voltage of the same phase as exists at the upper terminal of the first tuned circuit. In addition, the diodes receive further equal voltage components of relatively opposite polarity as a result of the transfer of energy from the first to the last of the tuned circuits by way of the reactive couplings between adjacent circuits of the cascade of tuned circuits. Since there are an odd number of these reactive couplings, the phase of the second-named components of voltage applied to the diodes will be different from the phase of the first-named component if the frequency of operation is exactly the resonant frequency of the various tuned circuits. If, however, the frequency changes, the relative phase of the two components shifts and may actually rotate more than as a result of a relatively small change in frequency. By increasing the selectivity of one or more of the various resonant circuits, the amount of frequency change required to produce a given relative phase shift between the voltage components at the diodes may be decreased. Thus, the characteristics representing the behavior of the circuits of Figs. 14, 15 and 16 show that a much larger control voltage per kilocycle change of frequency is obtained than in the case of Fig. 8.

Circuits in accordance with the invention may,

with advantage, be employed for the reception of frequency modulation transmissions. I

While I have indicated and described severa 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 organizations 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 is claimed is:

1. In combination with a source of wave energy whose frequency may vary, a rectifier system having an input and an output, a plurality of parallel resonant circuits each tuned to a common frequency arranged in cascade for transferring the wave energy from said source to said rectifier input, an odd number of reactive couplings between adjacent resonant circuits in said cascade, a direct coupling between'said rectifier input and said source of wave energy, 'and-a utilization circuit coupled to said rectifier output.

2. In combination, a pair of rectifier tubes having output electrodes connected in push-pull relation by impedances, said tubes having a common input circuit, a source of wave energy whose frequency may vary coupled to said input circuit, a plurality of circuits each parallel resonant at the frequency of said source arranged in cascade for transferring wave energy from said source to said rectifier input circuit in opposed phase relation, an odd'number of reactive couplings in said cascade, each reactive coupling being between each pair of adjacent resonant circuits a frequency independent coupling between said source of wave energy and the said input circuit, and an output circuit coupled to said impedances.

3. In a frequency discriminator network, a first resonant circuit, a second resonant circuit, at least one intermediate resonant circuit, each of said resonant circuits being a parallel resonant circuit tuned to a common frequency, couplings connecting said circuits in cascade, an odd number of said couplings being reactive, means for impressing wave energy the frequency of which may shift on said first circuit, means for combining a voltage derived from said first circuit with the voltage derived from said second circuit to form a vector sum voltage, means for combining a voltage derived from said first 01!! cuit with a voltage derived from said second circuit to form a vector difference voltage, sepatermediate point on said common input circuit and said first resonant circuit.

5. In combination with a pair of rectifiers having a common input circuit and a common output circuit, a first parallel resonant circuit, a plurality of parallel resonant circuits arranged in cascade between said first resonant circuit and said common input circuit, each of said parallel resonant circuits and said common input circuit being tuned to a common wave frequency, and a direct coupling connection between an intermediate point on said common input circuit and said first resonant circuit and means for increasing the selectivity of at least one of said parallel resonant circuits .by regeneration.

NOEL MEYER RUST.

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