US2420268A - Frequency modulation detector - Google Patents

Description

May 6, 1947. c. G. SONTHEIMER 2,420,268

FREQUENCY MODULATION DETECTOR Filed Feb. 9, 1945 T111 .Llh d ZZ 70 j 47,62.

Eaientecl May 6, i947 FREQUENCY MODULATION DETECTOR Carl G. Sontheimer, Stamford, Conn., assignor to Radio Corporation of America, a corporation of Delaware Application February 9, 1945, Serial No. 577,002

(Cl. Z50- 27) 8 Claims.

My present invention relates generally to detectors of angle modulated carrier waves, and more particularly to novel and improved frequency modulated (FM) carrier wave detectors.

Special problems arise in the detection of FM signals transmitted at the ultra-high frequencies over wide band widths. Moreover, it may, for example, be required in certain ultra-high frequency transmission that the FM detector be capable of responding to total frequency changes covering a `band as wide as 12 megacycles (mc.)

or more.

It is an important object of my present invention to provide a detector of angle modulated carrier waves which is capable of providing modulation signals of high gain, possesses substantially no interaction between its high frequency discriminator network and output connections, and is capable of responding with substantial linearity to relatively wide angle variations of the carrier wave. By angle modulation is generically meant FM, or phase modulation (PM), or hybrid modulations possessing characteristics common to both FM and PM.

Another important object of my invention is to provide an FM detector using a double triode tube as a high input impedance detector.

Another object of my invention is to provide in an FM detector of the type whose discriminator is a pair of sloping filters, an improvement wherein the filters feed a pair of triodes provided with a common cathode circuit; one of the triodes acting as an innite impedance detector, while the second triode functions as an anode bend detector.

A more specic object of my invention is to provide a detector of frequency modulated carrier waves, the carrier frequency of which is in the ultra-high frequency range and is varied over a wide `band by video signals,

A still more speciiic object of my invention is to provide a reliable, efficient and economical form of detector for FM television signals.

Other features of my invention will best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated -diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows diagrammatically an illustrative embodiment of the invention;

Fig. 2 illustrates the response characteristic of the present detector; and

Fig. 3 shows a modied form of the invention.

Referring now to the accompanying drawing, wherein like reference characters in the several figures designate similar elements, the detector circuit of Fig. l is explained in connection with a system for receiving FM television signals.

However, I wish it clearly understood that my invention is entirely generic so far as frequency ranges are concerned. Further, there is no restriction on the nature of the modulation, since the invention may, for example, also be used for angle modulated waves (as FM waves) in the present 42 to 150 mc. band assigned to FM transmission. In the illustrative case given herein, the carrier may be chosen from a range up to 100 mc. or more. The video or picture signals may vary or deviate the carrier frequency to a maximum of 6 mc., on either side of the carrier or center frequency of the radiated wave. The total frequency change may exceed, or be less than, 12 mc., if desired.

Those skilled in the art of radio communication are fully aware of the Various methods for collecting the radiated FM waves, selectively amplifying the collected waves at carrier frequency, reducing the mean frequency of the selectively amplified FM waves to a suitable lower frequency value. Let it be assumed, for example, that the FM wave energy has been reduced to a center or mean frequency of mc..prior to detection, Va1- though no limitation is imposed on my invention b-y virtue of this illustrative assumption. It may be further .assumed that it is desired to derive from the 70 mc. wave energy video signals which correspond with frequency variations of the '70 mc. wave over a band of 12 mc.

As is Well known to those skilled in the art of FM communication, at the transmitter the modulation signals cause the carrier frequency to vary or Vchange in accordance with the amplitude of the modulation, while modulation frequencies per se determine the rate of the frequency change. Fig. 2 shows, in ideal form, the nature of the response desired at the FM detector. Curve a depicts the FM signal at the high frequency discriminator input circuit of the FM detector The wave varies at maximum deviation from amean or center frequency Fc of '70 mc.V to frequencies of 64 mc. and 76 mc. on either side thereof. Curve b shows the response characteristic of the FM detector. Frequency changes are plotted against rectified voltage to secure curve b. As the curve a varies between 64 and '76 mc., the rectified voltage output of the detector varies from maximum positive voltage to maximum negative voltage. At 70 mc. the rectified voltage output is zero, and between the peaks of curve b it is desirable to have the curve as linear as possible. The rectified voltage variations correspond to the modulating signal variations at the transmitter. It will be understood that the peaks of curve b could actually beat vsubstantially'less than 64 mc., and at greater than 76 mc. to provide a pass band at the detector input circuit in excess of 12 mc. v

The characteristic shown ideally as curve b in Fig. 2 is provided, in accordance with my invention, by using a circuit as shown in Fig. 1 preferably employing a tube I of the twin triode type. The tube may be of any well known and suitable design such as 6J6. Separate tubes may be used if desirable. The triode comprising cathode 2, control grid 3 and anode or plate 4 may for convenience be called the input triode, and the triode comprising cathode 5, control grid 6 and anode or plate 'I may be termed the output triode. Cathodes 2 and 5 are connected in common to ground through the resistor S. The resistor 8, which may have a magnitude of between 500 to 1000 ohms and up to 3000 ohms if desired, is bypassed for carrier frequency currents, but not for currents of modulation frequencies. Numeral 9 indicates the bypass condenser, represented by dash lines, as distributed capacity of the leads. A physical condenser could be used at 9, but at high frequencies of the order of 70 mc. and above the distributed capacity will usually sulce.

Plate 4 is connected to a suitable positive potential point (+B) of a direct current source. A filter resistor I is inserted in the plate supply lead. The plate 4 is bypassed to ground by condenser II for the radio, i. e., carrier frequency, currents as well as video, modulation frequency components. Plate 'I is connected to the +B terminal through the output resistor I2, plate I being bypassed to ground by condenser I3 for radio frequency currents only. Resistor 8 and plate I cannot be bypassed for video frequencies without losing the desired video signal. An infinite impedance detector works best, in general, with its plate vbypassed to radio frequency and video currents. Hence, the plate 4 is completely bypassed. The load resistor I2 could have a magnitude of from 1000 to 2000 ohms for a video band of 6 mc.

The control grids 3 and 6 are connected to suitable points of any desired type of discriminator input circuit. My invention is not limited to the specific discriminator circuit shown in Fig. 1, which is of the type disclosed in U. S. Patent No. 2,057,640 to Conrad. This input circuit comprises a tuned primary circuit I4 resonated to Fc (70 mc.). The circuit I4 may be located in the plate circuit of a prior 70 mc amplifier tube, or it may be the output circuit of an amplitude limiter tube. The shunt resistor I5 provides damping for suitably broadening the response curve of the discriminator network.

Secondary circuit I6, magnetically coupled as at Mi to primary circuit I4, is tuned to a frequency greater than Fc. For example, circuit I6 could be tuned to a frequency somewhat greater than 76 mc. Damping resistor I'I shunts circuit I6 to broaden the response thereof. Secondary circuit I8 is magnetically coupled to primary circuit I4, as at M2, and is tuned to a frequency less than Fc. The frequency of circuit I8 could be as much below 64 mc., as the frequency of circuit IS is above '76 mc. Damping resistor I9 shunts tuned circuit I8.

The high alternating potential side of circuit I6 is connected by lead 20 to grid B, while the high potential side of circuit I8 is connected to grid 3. The low potential sides of both secondary circuits I6 and I8 are grounded. Operating bias is provided for each of grids 3 and 6 by the voltage drop across resistor 8. The operating bias in this circuit may approach the cut-off bias for the tube, and is in any event greater than the the circuit I4 by equal frequency values.

bias that would be used with the same tube as I an amplifier.

The radio frequency voltages at points c and d will vary in polarity and magnitude relative to ground. The variation will depend on the sense and amount respectively of frequency deviation of the applied FM signal energy relative to Fc. The action of the discriminator circuit is well known. At the frequency Fc of the FM wave energy the voltage at point c is equal to the voltage at point d, because the circuits I6 and I8 are mistuned from In response to a deviation of the signal energy to its maximum frequency approaching the frequency of circuit I6 the signal voltage at point c will be a maximum, while the voltage at point d will be a minimum. The reverse situation takes place in response to a frequency deviation to minimum.

frequency approaching the frequency of circuit I8.

To explain the functioning of the triode sections of tube I, let it be assumed that the frequency of the signal energy at primary circuit I4 at 76 mc. In that case the signal voltage applied to grid 6 is a maximum. This will cause a large flow of rectified current through the resistor 8 due to maximum plate current flow in the output triode 5, 6, 'I of tube I. It will, also, be noted that the signal voltage at point d is a minimum, and the rectified plate current flow due to the input triode 2, 3, 4 will be small. Thus the output triode acts as an anode bend detector, or plate rectification detector, and the anode end of resistor I2 will have minimum positive polarity (or be relatively most negative) as indicated by the lower peak of curve b in Fig. 2.

If, now, the signal frequency at circuit I4 deviates to 64 inc., the signal voltage at point d is a maximum. This will increase the plate current flow of the input triode to a maximum, and cause the cathode end of resistor 8 to become highly positive relative to ground. With this applied frequency, only a relatively small radio frequency voltage appears between the grid 6 and ground; rectified current in the output triode due to voltage on grid 6 is, therefore, small. As a consequence, the anode end of resistor I2 rises to a maximum positive potential, as indicated by the upper peak of curve b in Fig. 2.

The negative bias on grids 3 and 6 is such that neither of these grids is ever driven positive by the signal energy. Hence, the input triode acts in the manner of an infinite impedance detector driving the output triode through the cathode, as in a grounded grid amplifier. Since cathode 5 is bypassed to ground for all radio frequency current components, no radio frequency voltage appears across this element. The combination of the input triode section acting as an infinite impedance detector, and the output triode as a grounded grid amplifier, causes an increase in radio frequency voltage at point d to produce a rise in potential at the output plate. There is only second order interaction between the discriminator input network and the video output circuit. The latter comprises the video output connection 2i and coupling condenser 22. Relatively high gain is provided due to the amplification provided by the output triode of tube I.

It is pointed out that with my present invention less interaction between the high frequency discriminator circuit and the video output circuit is secured than would be the case with known types of FM detector circuits. Both triode detectors in this circuit present an infinite grid impedance to the applied signal in that the radio frequency voltage draws no current to the electrode where it is applied. This should be contrasted to the action of an FM detector employing diodes. I-Ience, the tube impedance is much higher than the impedance of the damping resisters il and i9 used to broaden the discriminator input circuits. The value of these damping resistors il and I9 is thus not appreciably affected by the parameters chosen for the video circuits, e. the output resistor i2. This would not be the case of an FM detector employing diodes in a wide band system. Improved linearity may be achieved since resistor i2 and the damping resistors il and i9 may be chosen independently for the best performance of their individual functions.

At frequency Fc the grids 3 and will provide plate currents through resistor 8 such that the voltage of the anode end of resistor l2 is intermediate between the peak values of curve b of Fig. 2. Hence, the output voltage at resistor i2 is proportional to the difference in excitation between the pair of resonant circuits EE and i8. The system has a response characteristic balanced at Fc, and providing a single-ended output. The portion of the characteristic b between the peaks may be kept highly linear by proper radio frequency circuit design.

In Fig. 3 I have shown a modication of the FM detector circuit, wherein the discriminator input network is of the type disclosed and claimed by S. W". Seeley in his U. S. Patent No. 2,121,103, granted Llune 2l, i938. While the construction and functioning of this type of discriminator circuit are substantially different from those of the Conrad discriminator circuit shown in Fig. l, yet the resultant signal voltages produced at points c and d are respectively the same as at the respective points in Fig. l. In the case of Fig. 3 it is assumed that the primary resonant circuit id is located in the plate circuit of the last intermediate frequency (I. F.) amplifier of a superheterodyne receiver suitably constructed to receive the ultra-high frequency signals. Circuit ifi is, of course, tuned to the operating I. F. value, say, for example, 70 rnc. The low alternating potential side of circuit id is established at ground potential by means of condenser ifi for signal frequencies.

rEhe secondary circuit 3l is tuned to the operating I. value and the midpoint of its coil 3i is connected by direct current blocking condenser 3@ to the high potential side of circuit id. Magnetic coupling M3 provides sufficient coupling between the circuits i4 and 3i to provide a band pass characteristic such as efliciently to pass a band of frequencies in excess of l2 mc. IThe shunt damping resistors i5 and 32 aid in broadening the pass band of these circuits.

The opposite ends of coil 3l are connected to ground by means of grid leak resistors 33 and 34, these ends being designated as points d and c respectively. As in the case of Fig. 1, control grids t and 6 of the twin triodes are connected respectively to points d and c. The circuits associated with tube i are generally the same as described in connection with Fig. l, with the exception of the common cathode circuit of the tube. It will be noted that the cathodes f2 and 5 are connected together by means of a radio frequency choke coil 35. The purpose of this choke is to prevent coupling at radio frequency between the two sections of the tube, and here serves to augment the effect of bypassing the cathodes to radio frequency. The resistor'is connected from cathode 5 to ground, and the resistor is shunted by the condenser 9 which bypasses the radio frequency currents only. Condenser 3B, also, bypasses radio frequency currents only, and is connected from cathode 2 to ground.

The circuit of Fig. 3 functions in the same manner as the circuit of Fig. l. In general, it can be stated (more specific reference being made to the Seeley patent) that the resultant signal voltages at points c and d are produced by virtue of signal voltages induced in the secondary circuit 3i through the magnetic coupling M3 and the direct connection which includes condenser 30. That is to say, the signal voltage at primary circuit I4 is directly applied at the midpoint of coil Si', and this signal voltage is applied in parallel relationship to grids 3 and 6. On the other hand, the signal voltage induced in circuit 3l through the magnetic coupling is applied in push-pull relation to grids 3 and 6. Hence, the resultant signal voltage applied t0 each of grids 3 and 6 is the vector sum of a pair of voltages in normal phase quadrature relation at the operating I. F. value. For frequencies other than mean or center frequency (the I. F. value) the vector voltages at the grids will be unequal, because of the fact that the phase shift introduced for such off-frequencies by the magnetic coupling M3 differ from the normal shift. It will, therefore, be seen that at center frequency the signal voltages at points c and d are equal, Whereas for frequencies different from center frequency the polarity and magnitude relation of the voltages will depend upon the direction and degree respectively of frequency deviation of the signal energy at circuit Ul with respect to Fc. This, of course, is precisely the voltage relations existing at points c and d in Fig. 1.

The discriminator input circuit may be so arranged as to provide for an increase in signal voltage at point c when the signal energy deviates to frequencies in excess of Fc, While the signal voltage at point d becomes relatively greater when the frequency deviation is less than Fc. Here, again, the relations would be the same as in the case of Fig. 1.

Summarizing the features of my present invention, there is secured a high radio frequency impedance thereby permitting a choice of damping resistors at the discriminator input circuit without reference to video requirements; video voltage obtained from plate 'I to ground is developed across a single load resistor I2 With a relatively low shunting capacity (as small as 1.3 mmf. (micro-micro-farads) for a twin triode tube of the 6J6 type); much higher gain is secured than in the case Where diodes are employed; greater linearity is provided because the output resistor l2 may be specifically chosen` for that purpose and is not influenced by required` damping at the input circuit; moreover, higher linearity can be obtained in the present circuitv with a lower value of resistor l2 than would be the case if diodes were employed, it being pointed out that a low-magnitude resistor is importantV in a wide band video circuit.

While I have indicated and described a system 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 de. parting from the scope of my invention.

What I claim is:

1. In a detector of angle modulated carrier Waves, a pair of electron discharge devices each including at least a cathode, control electrode and anode, a resistive impedance connecting the cathodes of said devices to ground, means applying a positive direct current voltage to the` anodes of both devices, an output resistor of relatively ioiv magnitude in circuit with only one of the anodes,.a modulation signal voltage output connection from the anode end of saidoutput resistor, and meansfor varying the control electrodes of saiddevices with respective signal voltages derived from said Waves and'varying in relative polarity and magnitude in accordance With the angle modulation of said Waves.

2; In a detector of angle modulated carrier waves,4 a pair of electron discharge deviceseach including at least a cathode, controll electrode andanode, a resistive impedance connecting the cathodes of saiddevices to ground, means applying apositive direct current voltage to the anodes ofv both devices, an output resistor in circuit With only one of the anodes, means bypassing from the second. anode currents of carrier and modulation signal frequencies, a modulation voltage output connection from the anode end of said output' resistor, and means for varying the control electrodes of said devices Withrespective signalvoltages derived from saidwaves and varying in relative polarity and magnitude in accordance with the angle modulation of said Waves.

3. In a detector of angle modulated carrier waves, a pair of electron discharge devices each including at least. a cathode, control electrode and anode, a resistive impedance connecting the cathodes of said devices to ground, means applying a positive direct current voltage to the anodes of both devices, an output resistor in circuit with only one of the anodes, a modulation signal voltages output connection from the anode end of said output resistor, means for varying the control electrodes of said devices with respective signal voltages derived from said Waves in accordance with the angle modulation of said Waves, and meansbypassing each of said'resistive impedance and one anode for currents of carrier frequency.

4. InY combination with a source of a pair of high frequency voltages whose relative phases andimagnitudes vary from a predetermined relation, means for providing a rectified voltage responsive to said variation, said means comprising a pair of electron discharge devices each having at least a cathode, control grid'and plate, a common resistive impedance for both cathodes adapted to be traversed by the space currents of both devices, circuit elements operatively associated with the electrodes of one devce to render it operative as an infinite impedance detector, meansV applying one of said voltages to the control grid .of said one device, circuitelements operativelyk associated with the second device torender it operative as a plate rectification device, means applying the second voltage to the control grid of the second device, and means for, utilizing said rectied voltage. developed in the plate circuit of the second device.

5. In combination with a source of a pair of high frequency voltages Whose relative phases and magnitudes vary from a predetermined relation, means for providing a recti'ed voltage responsive to said variation, said means comprising a pair of electron discharge devices each havingY at least a; cathode, control grid and plate, a

common resistive impedance for both cathodes adapted to be traversed by the space currents of both devices, circuit elements operatively associated with the electrodes of one device to render it operative as an infinite impedance detector, means applying one of said voltages to the control gridof said one device, circuit elements operatively associated with the second device to render it operative as a plate rectcation device, means applying the second. voltage to the control grid of the second device, and means for utilizing said rectified voltage developed in the plate circuit of the second device, said source including damping resistive impedance of high value, and said plate circuit of said second device including resistive impedance of relatively low magnitude.

6. In a detector of frequency modulated carrier Waves, a pair of electron discharge devices each including at least a cathode, control electrode and anode, a resistor connecting the cathodes of said devices to ground, a source of positive direct current voltage, a rst resistor in circuit between said source and the anode of one device, an output resistor in circuit between said source and the second one of lche anodes, a modulation signal voltage output connection from the anode end of said output resistor, means for varying the control electrodes of said devices with respective signal voltages derived from said waves, means bypassing the said anode of said one device for carrier and modulation frequencies, and means bypassing said cathode resistor and second anode for carrier frequencies only.

7. In a detector of frequency modulated signals, a pair of electron discharge devices each including at least a cathode, control electrode and anode, a resistive impedance connecting the cathodes of said devices to ground, means applying a positive direct current voltage to the anodes of botlidevices, an output resistor in circuit with only one of the anodes, means bypassing the anode for currents of carrier and modulation frequencies, a modulation voltage output connection from the anode end of said output resistor, a signal input circuit constructed as a discriminator for varying the control electrodes of said devices with respective signal voltages derived from said signals, means highly damping the discriminator circuit to give it a relatively wide pass band, and said output resistor having a relameans bypassing the second anode for high frequency and modulation currents, a modulation voltage output connection from the anode end of said output resistor, a discriminator input circuit of relatively high damping, said discriminator circuit providing for the control electrodes of said devices respective signal voltages derived from said waves related in polarity and magnitude in accordance with the angle modulation of said Waves, means bypassing said one anode for current of highl frequency only, and said output resistor being of a relatively low resistive magnitude.

CARL G. SONTHEIMER;

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