US2256078A - Frequency modulation detector - Google Patents

Description

Sept. 16, 1941. M. G. CROSBY ET AL FREQUENCY MODULATION DETECTOR Filed July 5l, 1940 .Hc n

a w M w /Ww l e fm. ummkl. NQMOUMX w E n. NI 1D r rm f 5 @Wm a C ef n @im f r o Q C m nl F e Anw maf lmventors f [Wurf-1y G. yzwy and 8g Tama .attorney Patented Sept. 16, 1941 2,256,078 FREQUENCY MODULATION DETECTOR Murray G. Crosby,

Riverhead, and Harry Tuniek,

Rye, N. Y., assignors to Radio Corporation of America, a corporation of Delaware Application July 31, 194.0, Serial No. 348,850 4 Claims. (Cl. Z50-2.7.1)

Our present invention relates generically to detectors of angular velocity-modulated carrier waves, and more especially to such detectors employing full wave rectification.

One of the main objects of this invention is to utilize push-pull detectors of the oppositely mistuned input type for frequency modulated waves (FM) in place of single-sided detectors of the same type, whereby full wave rectification is had with consequent balancing advantages.

Another important object of our invention is to provide an FM detector network comprising full wave rectiers having outputs in phase opposition, while the inputs are oppositely mistuned with respect to the mean, or center, frequency of applied FM waves whereby less by-passing of carrier currents in the output circuit is required.

Still other objects of our invention are to improve the eiiiciency and reliability of FM detectors, and more particularly to provide such detectors in an economical manner.

The novel features which we believe tobe characteristic of our invention are set forth in particularity in the appended claims; the invention itself, however, as 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 we have indicated diagrammatically a circuit organization whereby our invention may be carried into effect.

In the drawing: Y

Fig. 1 shows one embodiment of the invention;

Fig. 2 illustrates a modification;

Fig. 3a illustrates the detector input circuit characteristic; and

Fig. 3b shows the detector output characteristic.

Referring now to the accompanying drawing, wherein like reference characters in the different gures designate similar circuit elements, there is shown in Fig. 1 the second detector of a superheterodyne receiver of FM waves. It is believed that those skilled in the art are so very well acquainted with FM receivers that it will be sufficient for the purposes of the application to disclose merely the detection network. It is to be clearly understood that phase modulated carrier waves may be received by the present detection system. The generic term angular velocitymodulated carrier waves is used to cover both types of modulation. Let it be assumed, then, that the resonant circuit l is the output network of the usual amplitude limiter of the receiver. Briey, the limiter is a stage of intermediate frequency (I. F.) amplification which follows one or more normal I. F. amplifiers, and acts to eliminate from the FM signals fed to the detector any amplitude variation. It may be assumed that the FM band is of the order of 42 to 50 megacycles` (ma), and that the rst detector would reduce the carrier frequency to an I. F. value of the order of 2.1 mc. The maximum frequency deviation of the carrier may be narrow or wide, and may be chosen from a range of 30 to 200 kilocycles (kc.).

It will be understood that the symbol fc designates the mean, or center, frequency of the FM signals. At I. F. the value of je would be 2.1 mc. Hence, circuit l is xedly tuned to the center frequency fc. The I. F. energy of variable frequency and constant amplitude atV network l is applied to a pair of full wave rectifiers having independent input circuits, and a common output circuit. The rectiers may comprise a pair of twin-triode tubes 2 and 3. vTube 2 has two pair of triodes in its envelope, the cathodes being connected in common to ground. The control grids 4 and 5 are connected to opposite ends of the input coil E, and shunt condenser tunes the coil to a frequency fc-l-a. The incremental frequency a may be any value, but will usually depend on the maximum permissible frequency swing of the carrier. The plates of the tube are connected in common, or cophasally, to one end of the primary winding 8 of the audio transformer 9.

The mid-point of coil 6 is connected to ground through the grid leak resistor lo. Hence, the tube 2 and its associated circuit functions as a full wave detector of the grid leak type. The usual I. F. by-pass condenser ill across the leak resistor l may be minimized since the input circuit E-l is connected to grids i` and 5 in pushpull arrangement, and, therefore, balances out the I. F. component. Similarly, tube 3 has its cathodes connected in common `to ground. The control grids H and I2 are connected to the opposite ends of input coil I3, and the latter is tuned to a frequency fc-a by condenser I4. The plates of tube 3 are cophasally -connected to the opposite end of winding 8. The mid-point of winding 8 is connected to a source of positive voltage. The grid leak resistor l5 is connected to ground from the mid-point of coil I3. Here, again, the tube 3 functions as a full wave detector of the grid leak type. The usual by-pass condenser l5 across resistor I5 may be minimized, in view of the balancing out of the I. F. component in the circuit l3--IG- This type of detection is especially advantageous when it is desired to use a low I. F.

. spectively. The

f rier frequency vis music or speech.

Y switches I6 could .be used Y Thus, the double diode 20,

lea'd24 and resistor element V2l Work I3-I4.

Yis fc+a is the audio network. 1

The primary circuit I is magnetically coupled, as at M and M1, to each of 6-1 and I3-I4, resecondary winding of the transformer 9 may be connected to any desired type of audio frequency network, assuming of course that the modulatingsignal used to swing the car- Where phase modulated carrier signals Vare received, a correction filter I6 should be connected to the winding 9. Such a lter wouldV have a characteristic, shown in the rectangle I6, which relates output and frequency in .an inverse sense. The to vcut "out filter I 3 when receiving FM waves.

The arrangement of Fig. 2 differs from that of 1 Fig. 1 in that full wave diode -re'ctiers are em- Y wave triode detectors. which may be of the 6I-I6 type, has its anodesconnected to theopposite ends'of inputcoil 6, while 'the cathodes are connected back to the'mid-point vof -coil E through In the same way, the-anodes'of double diode tube 2-2 are connected to the-opposite ends of coil I3,-while the cathodes of the 'tube 22 are connected'back to the midpoint of coil I3 through resistor 23 and lead 24. The cathode endof resistor 23 isat ground potential. Y Y

Each of resistors 2| and 23 acts as the output load of its respective full wave rectifier; that is, tubes and 22 respectively. Since the in,- put circuit of `each 'double diode is in push-pull relation to the diodes, the I. F. component across each load resistor is balanced out. Hence, Vthe ployed in place of full VusualAI. F. by-pass condensers across each load resistor -2I and 23 may readily be minimized. Here, as in Fig. l, the input network I is reactively'coupled to each of the circuits 6-1 and I3-I4. Across resistors 2| and -23 in series will be developed Iboth ydirect current andV audio frequency voltages which will correspond in amplitude to the frequency swings ofl the modulated carrier.

In Fig. 3a there isyshown the input circuit characteristic of the'detectors of both Figs. 1 and 2. It will be recognized that the curve whose midfrequency is fe-a is the resonance curve of net-` The curve Vwhose mid-frequency resonance curve of network 6 1.v

The mean, or center, frequency fc is located Vmidquency varies vfrom the value ,fe

.the output impedance of each of Y 2,256,078 Y which it is'dicult Yto lter out of the follo and 23 in Fig. 2. If the two rectified voltages produced by each full wave rectier are equaLthe net voltage is zero. Since the two tuned circuits 6 1 and |3-I4 are peaked to different frequencies, their response to the frequencies present in the primary circuit will be different. Fig. 3a graphically shows the response of Athe two input circuits in terms of frequency.. It isonly at the mean frequency (fc) that the response of both circuits isthe same. At all other frequencies the response of one input circuit is different from Ythat of the other input circuit with the result that the magnitude of signal voltage fed to .the two full wave rectiers for rectification depends upon the response of the input circuit to the frequency of the signal voltage. The typical SV characteristic shown in Fig. 3b is secured by simply combining the voltages developing across the full'wave rectiers. For example, at. the mean frequency (fc) the response of each of the input circuits is the same, and the voltages developed across the output load impedances are of equal magnitude but of opposite polarity, and hence the net output voltage is zero. Now, as the carrier fredue to the modulating signal, one or the other of the output impedances'will develop the greater voltage depend` ing upon the direction of the frequency deviation of the carrier frequency. In other words,Y

the arrangements shown in Figs. 1 and 2 provide rectifying means whereby it is possible to convert frequency variations into audio yvoltages of different amplitudes. In order to secure a linear output, the frequency excursions of the carrier portion of the dis-V should not exceed the linear criminator characteristic. y v

In Fig. 2 it has been indicated that the audio volt-agedeveloped across the output load V2te-3 is utilized `for further amplication in an audio frequency amplifier network. However, the direct-current' voltage developed between the upper end of 'resistor 2l and ground may be employedA for automatic frequency control (AFC), as is well known to those skilled in theart. That is to say,

the direct current voltage can be used to vary the usual frequency control tube provided in ase sociation with the'local oscillator tank Vcircuit so as to correct for frequency shift ofthe 4local oswayof `the upper Vand lower resonant frequencies. Y

In Fig. A3b there is shown the detector output characteristic of each of the detector arrangements-of Figs.' 1 and2. It will be observed that rectified voltage varies somewhat in an S shaped curve with respect to the center `frequency (je). In other words, the'detected'voltage developed across the winding 8 of transformer 9 in Fig. '1 varies y with frequency deviation as shown in Fig. 3b.

Again, in theV case of Fig. 2 the detected voltage across Yresistors 2l and 23 varies withV vfrequency y Vdeviation as shown -in Fig. 3b.

It will be seen that the -two pairof full wave rectiflers in each 'of Figs. 1 and 2 provide differential rectifier networks. Although each full cillator. This may be done because a shift 'in the local the I. F. value, and the latter shift may readily be 'translated into a, correction bias which will adjust the oscillator tank circuit frequencyin a sense to maintain the desired I. F. Value.

It will now be seen that there has'been provided by the present invention FM wave -detectors which generally-comprise a pair of full wave rectiiiers having their outputcircu'its arranged i wave rectifier has its own output load circuit, and the rectified voltage vdeveloped in each Vrectier is present across the respective loads, the two output voltages are of opposite polarities and' therefore tend'to zcancel each other. The nal,

or resultant, output voltage present across 'the output circuit depends upon the relative magni- Q tudes' of the mdividualjvoltagesacross -each half f of winding 8 jin Fig. v1, or across resistors -2I in common in phase opposition, whereas the input vcircuits of the full wave rectiers are oppositely vmistuned from the operating mean frequency of the FM signals Yby Aequal frequency The result of the full wave rectif Y cation in eachhal-'f Vof the Idetector circuit is to magnitudes.

improve balancing with respect to the I. F, component. An advantage of such improved `balancing resides in the fact that the usual I. F. bypass condensers across the output load resistors in Fig. `2, or across the grid leak resistors in'Fg. llfmaybe reduced in magnitude thereby providing distortion-free detection.

- Whilewe lhave vindicated and-described a system vfor 'carrying our invention into effect, it will be apparent fto one oscillator frequency results in a shif-tof skilled inthe art that our .in-'- vention 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 our invention, as set forth in the appended claims.

What is claimed is:

1. In a frequency modulated carrier wave detection system, a pair of full wave rectifiers, said rectifiers having independent wave input circuits, means for applying to said independent input circuits frequency modulated waves, said independent circuits being oppositely mistuned by equal frequency magnitudes with respect to the mean frequency of said waves, and means for connecting the output circuits of said full wave rectifiers in phase opposition to provide a modulation voltage output circuit.

2. In combination with a source of frequency modulated carrier waves, a pair of rectier devicesy a resonant input circuit coupled to said source and tuned to a frequency which differs by a predetermined frequency value from the center frequency of said frequency modulated waves, said rectier devices having input electrodes connected to said input circuit in phase opposing relation, a second pair of rectifier devices, a second input circuit coupled to said source and being tuned to a frequency which differs from said center frequency by said frequency magnitude but in the opposite sense, said second pair of devices having input electrodes connected in phase opposition to said second input circuit, and an output impedance connecting output electrodes of said two pairs of devices in phase opposition.

3. In an angularl velocity modulated carrier wave detection system, a pair of full wave rectiers, each of said rectiers having its own wave input circuit, means for applying to said input circuits angular velocity modulated waves, said input circuits being oppositely mistuned by equal frequency magnitudes with respect to the center frequency of said waves, and means for connecting the output circuits of said full wave rectiers in opposition to provide a modulation voltage output circuit.

4. In combination with a source of angular velocity modulated carrier waves, a pair of detectors, a resonant input circuit coupled to said source and tuned to a frequency which differs by a predetermined frequency value from the center frequency of said modulated waves, said detectors each having input electrodes connected to said input circuit in phase opposing relation, a second pair of detectors, a second input circuit coupled to said source and being tuned to a frequency which differs from said center frequency by said frequency magnitude but in the opposite sense, said second pair of detectors having input electrodes connected in phase opposition to said second input circuit, and an output impedance connecting output electrodes of said two pairs of detectors in phase opposition.

MURRAY G. CROSBY. HARRY TUNICK.

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