US2457013A - Angle modulated wave discriminator - Google Patents

Description

Dc. 2l, 1948.

G. L. USSELMAN v ANGLE MODULATED WAVE DISCRIMINATOR Filed. July 28, 1944 TO0-F VR IVETWO/PK .2

2 Sheets-Sheet 1 INI/EN To@ 6:90129@ L. Ufalrlmn E# M60/m A Toe/VE w Dec. 2l, 1948. G. L. Ussl-:LMAN 2,457,013

ANGLE MODULATED WAVE DISCRIMINATOR Filed July 28, 1944 2 Sheets-Sheet 2 0 O AMOA O O Patented Dec. Zlf 1948 pensate for the inherent pre-emphasis of the higher modulating frequencies.

Merely by way of megacycle (mc.) band, the latter band being the 'j presently-assigned FM broadcast band. course, the FM waves could be received from a transmitter operating in any other of the megacycle ranges. Where PM waves are being re-g ceived the present invention is readily adapted for reception of PM Waves Whose carrier frequency would be located in any of the known megacycle bands. So far as the frequency deviation range of the received FM signals is concerned, there is employed a 150 kilocycle (kc.) maximum deviation range in the l2--50 FM band. Here, again, the invention is not limited to that relatively wide deviation band, but may be employed for FM or PM waves with` a relatively narrow transmission channel.

Referring to Fig. 1, let it be assumed that the improved detector circuit is embodied in a super-j heterodyne receiving system. While my inven-l tion is in no Way restricted to any particular form of receiving system, it is preferred to specify thel superheterodyne receiving system since the latter is most widely employed at the present time in' the reception of angle modulated carrier waves.l

As is well-known to those skilled in the art of radio communication, the received FM carrier waves of a desired carrier frequency in the '425-50 mc. band are reduced to an intermediate frequency (I. F.). suited for the operating conditions at the point of reception, and a value of 4.3 mc. has been found suitable for the reception of FM waves in the 42-50 me. band. The 1.4F. energy is usuallysubjected to I. F. amplification in one or more amplier stages. In order to preventany ampli-` tudeyariations which may develop o n thereceived modulated carrier energy prior to the discrilminator circuits from affecting the subsequent detectonithas been the practice tol provide an amplitude limiting devicebetween the'source of angle modulated Waveenergy andthe discriminator section of the discriminator-detector ci`r` cuit. Limiter devices are'well-known, and generally take the form of amplifiers Whose output' amplitude is sube above a predetermined input stantially uniform. K c

Hence, in Fig. l I have shown a limiter tube of well-known formy located between the source of angle modulated wave energy and the discrimiv natorv network. AThe limiter tube I is shown spe-` cifically as of the pentode type, although it mayf be/of any other well-known construction. The input transformer 2 has its primary circuit 3 and secondary circuit 4 each tuned to the operating Of'.

I.F.*value, 4.3 mc. in the example given. course, lthe passband Width of transformer 2 will. be substantially in excess of the maximum fre-` quency swing of the received FM waves. It is thel usual practice to make the passband of eachofv the selector circuits of the receiving systemof an,

FM receiver substantially in excess of the maximum frequency swing, as taught in my aforesaid,

patent.- By way of example it is pointed out that the passband width of transformer 2 may be of the order of 200 kc. The, grid circuit of tube l` The I. F. value may be one best curves of each of coils Lrand L2. Itis seen that 4 includes the usual resistor-shunt condenser net- Work 5 to provide the limiting action. It is, also, desirable to adjust the operating direct current voltages of the screen and plate of tube l so that on the negative half--cycle of the waves applied to tube l the amplifier I will be readily cut oi thereby to augment the limiting action. Above thetube Il have depictedan idealized limiting characteristic .for Vthe limiter tube, an'd it will be seen that amplitude variations of the FM signals applied to the discriminator will be inhibited. Of course, my invention is in no way restricted to the tube l being ai limiter tube, since it may be desired todispense with limiting action and to have l"tube l function as an I. F. amplifier.

. `Regardless of the function of tube l, the resonant plate circuit 6 thereof will be tuned to the operating'. F.'va1ue. The plate of tube l is connected through the coil 'I of resonant output circuit 6 to a point of suitable positive potential. In accordance with one ofthe objects of my invention the tapping point of the signal voltage to be applied to the subsequent discriminator circuit is at a relatively low potential point of the coil 1. rIhe tapping point may bemade adjustable by providing a sliding contacter 8 which is connected to the junction-'of the condensers C1 and C2. The adjustable' 'condenser C3 connects the vcontactor 8 tolground, and adjustment of condenser C3 may be employed to'regulate the coupling impedance betweencircuit e vand the discriminator circuit. If desired, the condenser Ca may be chosen to have a'xed value. The condenser C1 has connected in series therewith a coil L1, while the condenser Cz has connected in series therewith a coil Lz. The junction rof coils L1 and L2 is effectively connected to ground for signal potentials through a lead 9 and the bypass 'condenser SQ across resistorRz, which will be further described hereinafter. c

. Condenser'C1 and coil L1 provide a first seriesresonant circuit 'tuned to a frequency located on one side of the operating I. F. value and spaced therefrom by a predetermined frequency value. CondenserCz and coil-Ilz provide a second seriesresonant circuit tuned .to a second frequency which is located on the opposite side of the operating I. F. value and spaced therefrom by the same predetermined frequency value.

In Fig. 3 I have' shown the frequency'response the responseacross each coil isa maximum at they resonant 'frequency F1 or F2 of its respective series-resonant circuit C1, Li or C2, L2, although the voltage drop across each entire series-resonant' circuit is, of course, substantially zero at its respective tuned frequency. Hence, the signal voltages developed across each of coils L1 and Lz are used for rectification by the opposed rectifiers if! and Il. The spacing betweenthe peak` frequencies F1 and F2 Ain Fig. 3 is about 200 kc., and thecrossover, center or balance frequency is designatedFc. shown located vbeyond the dotted vertical `lines representing the .maximum frequency swing.

end-of resistor R2 is grounded, the rectified volt--A ages across resistors Ri'and Ra are combined iny 'v lIhe differential resultantv voltage of the opposed rectiedvoltages is taker.

polarity opposition.

The peak frequencies F1 and :F2 are.

on through I. F. choke coil I2 from the cathode end of resistor R1.

It will be noted that the anode I3 of rectifier diode It is connected to the upper end of coil L1, while cathode I4 is connected to the lower end of coil L1 .through load resistor R1 and lead 9. In the same manner diode anode I5 is conne-cted to the junction of coil L2 and condenser C2, while cathode I6 is connected to the junction of coils L1 and L2 through resistor R2 and lead 9. Hence, each of the coils L1 and L2 acts as the signal input element of its respectiv-e diode I 0 or Il. At the instant when the I. F. energy is at a frequency Fc the voltages across each of coilsL1 and L2 are equal, and, therefore, the rectified signal voltages across each of resistors R1 and R2 will be equal. Being of opposite polarity the voltages across resistors R1 and R2 will cancel each other, and the output will be zero. However, frequency variations or deviations from Fc will result in relative variations of magnitude of the rectified voltages across coils L1 and L2 depending on the sense and amount of frequency deviation. The differential voltage taken off at the cathode end of resistor R1 will vary in correspondence with the aforesaid frequency variations in a manner known tothose skilled in the art.

Prior discriminators have usually required high impedance coupling means between circuit 6 and the discriminator circuits. Such high impedance coupling means were subject to stray pick-up of interfering potentials. In my present invention a low impedance coupling element is used, as for example the -condenser C3 tapped down (i. e., toward the low alternating current poten-tial end) on coil l. Such a low impedance coupling device is not easily affected by stray fields. There may be used an inductive form of low impedance coupling element in place of condenser C3. Thus, in Fig.2 the junction of condensers C1 and C2 is connected to the upper end of coupling coil I1 whose lower end is grounded. The coil I yof resonant circuit 6 and the coupling coil I1 are magnetically coupled. If desired, there may be employed adjustable magnetic coupling between the two coils. The diodes In 4and I I may be embodied in a common tube envelope, as in a 6H6 type tube. It is noted that each of the bypass condensers Ill' and 9' across resistors R1 and R2 will have la su'icien'tly large impedance value at all useful audio frequences so as not to interfere with the transmission of the highest modulation frequency.

The magnitude of the low impedance coupling in either of Figs. 1 and 2 is preferably maintained below 1000 ohms. When a coupling coil is used, as in Fig. 2, the coil I'I may be from one to several turns. In the cases where it is desirable fto employ the capacity coupling Cs, the Value of the capacitance C3 may be chosen to make its impedance `at the operating frequency, for example 4.3 of some relatively low value between 50 and 1000 ohms. It will be noted that the condenser Cs furnishes a circuit to ground for the input of the discriminator circuits. That is, oscillating currents in the discriminator series-tuned circuits iiow to ground through the bypass condenser 9 around resistor R2, and from ground back through capacitive impedance C3 to the series-tuned circuits again. The same would apply to an inductive impedance or a resistance (see Fig. 6) used in place of condenser C3. The input power is, also, coupled to the discriminator through the impedance of condenser C3. The advantage of a low impedance at condenser C3 is that it must be comparatively low in order for the series-tuned circuit scheme' to be eiective. Otherwise tod much radio frequencyvoltage would be used up in condenser Cs, andl not enough would be left for the series-tuned circuits. Further, :a low impedance at condenser C3 permits the use of an ordinary insulated wire for lead 8, such as 1a lowvolt-age, insulated switchboard wire laid close to the metal panel. This, permits easy shielding of the discriminator elements from other radio frequency sources. The use of the low impedance condenser C3 means that the tapping to a relatively low alternating current potential pointon coil I is necessary, which gets away from high potentials and high electric fields that may cause trouble due to stray capacity couplings, etc.

The modification shown in Fig. 4 is similar to the circuit of Fig. l, except for the manner of securing the response characteristics of Fig. 3. The condensers C1 and C2 are constructed in the form of a variable condenser having two groups of stator plates, each group having its corresponding group of rotor plates. A common rotor shaft 20 has all the rotor plates aiiixed thereto. Hence, adjustment of shaft 2li results in simultaneous tuning of the series-resonant -circuits C1, L1 and C2, L2 to a common desired frequency. The fr-equency split or displacement of the response characteristics of the pair of tuned circuits on opposite sides of the mean frequency Fc is provided by adjustable condenser C4. The latter is shown in shunt with condenser C1. Alternatively, it may be shunted across condenser C2, if desired.

Each ofcoils L1 and L2 is shunted by a respectivevresis-tor R3 or R4. The magnitude of each resistor R3 or R4 is selected to give the desired slope lor decrement to the tuning characteristic of the series-resonant circuits. In Fig. 5 I have shown a solid line frequency response curve 30 denoting the response across each of coils L1 and L2 ,for a selected setting of the rotor shaft 20. The

rotor shaft, with condenser C4 effectively out of the circuit, is adjusted to tune each of circuits C1, L1 and C2, L2 to the common off-center frequency F2; which is higher thanFc. The condenser C4 now yadjust-ed so as to tune circuits C1, L1 to the frequency F1 which is located on the opposite, lowfrequency side of Fc, andis spaced from the latter by a. frequency value equal to the frequency dfferencebetween Fc :and F2. The curves in Fig.v5, of course, are the response curves across each fof coils L1 and Lz.

The advantage of using an auxiliary condenser C4 as in Fig. 4 is that it Apermits the use of a standard split-stator, or balanced, condenser construction for condensers C1 and C2. In this case the two stators and the two rotors are alike, and the rotors are fastened rigidly on the same shaft. This permits easy tuning of the two series circuits simultaneously. Also, the condenser C4 can be adjusted separately to provide the desired phasesplit or shift in the tuning response of the two series tuned circuits. In wide band FM reception the phase split in the discriminator is made wide, but for narrow band FM, or for PM reception, the phase split is made comparatively small.

Instead of using a reactance as the input impedance to the frequency discriminator circuit, I may use a resistor element. Thus, in Fig. 6 I show resistor R5 as the input impedance element. Further, a long concentric cable is used between the limiter output circuit G and the junction of condensers C1 and C2. The resistor R5 is located at the output end of the cable, between the conductor 50 and ground. The input end of con-V the cable to circuit B in place of the blocking, This modification is shown in.

condenser 40. Fig. 7.

The resistance input impedance for coupling to the discriminator is preferable when it is desired to operate the discriminator at the end of a cable, in which case the resistance is givenv a Value such as to match the cable impedance. The advantage of this is especially evident when the discriminator is used at the end of a long cable. For example, an unmatchedy cable will` discriminate against some frequencies so that some side frequencies may not be transmitted in proper phase and amplitude. This would result in distortion of the signal.

Inductive coupling of the discriminator to the signal source, as shown by coil 60 in Fig. 7, has

some advantages over capacitive coupling as con` stituted by condenser 40 in Fig. 6. The latter is used when the coupling is to be conductive. coupling by coil 60 can be made variable by moving it mechanically into and out of the magnetic field of coil '1. Coil B0 is convenient to use on a monitor discriminator to pick up modulated radio frequency energy from any desirable location in a transmitter. Sometimes it is moreconvenient mechanically, as in the case of a plug-in coil,vv

to wind the coil 6U on the same form as vcoil 'l sothat the coupling is of the desired amount for each plug-in coil.

It is to be understood that I may wish to use the discriminator circuit alone as a monitor re-y While I have indicated and described several systems for carrying my invention into effect, it

TheI

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- What I claim is:

In combination with a source of angle modulated carrier'waves, a resonant circuittuned to a predetermined reference frequency coupled to said source, a pair of series-resonant circuits.

tuned to respective peak frequencies equidistantly located on opposite sides of said reference frequency, said pair of series resonant circuits being connected in parallel relation, a relatively low impedance coupling connection from arelatively low potential point on said resonant circuit to a common connection point of said pair of circuits, a pair of opposed rectiers, each of said rectiers comprising an anode and a cathode, each series resonant circuit consisting of a condenser and an inductance, said common connection point being .the common connection between the condensers of said series resonant circuits, said low impedance coupling connection consisting of a reactive impedance connected betweenv said common'connection point and thecathodes of said rectiers, a connection from the anode of each respectiveV rectifier to the junction of the condenser and inductance of a respective one of said series resonant circuits, and a common connection from' thecathodes of said rectiers to the common connection of said inductances whereby there'is applied to each respectiveone of said rectiersfthe voltages developed across the respective inductances of said series-resonant circuits.

' GEORGE L. USSELMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS

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