US2351212A - Convertible demodulator circuit - Google Patents

Description

June 1944. w. D. HOUGHTON I ,2

CONVERTIBLE DEMODULATOR CIRCUIT Filed Feb. 10,1942

I PUSH PULL AUD/O C/flCU/T INVENTOR ML L IA M D Houay TON ATIORNEY Patented June 13, 1944 CONVERTIBLE DEMODULATOR CIRCUIT William D. Houghtomsetauket, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application February 10, 1942, Serial No. 430,171

16 Claims.

My present invention relates to convertible demodulation circuits, and more particularly to a detector circuit capable of receiving either amplitude modulated carrier waves (AM) or frequency modulated carrier waves (FM).

In application Serial No. 423,881, filed Dec. 22, 1941, C. W. Hansell has disclosed and claimed demodulator circuits adapted to detect angular velocity-modulated carrier waves; the demodulator simultaneously functioning to provide deemphasis and removal of the effects of amplitude modulation.

As is well known at the present time, reception of AM waves and FM waves is desirable in a common receiver without the employment of excessive networks. It has been customary to utilize special FM networks for receiving FM waves, particularly since the FM and AM detectors operate diiferently.

It is one of the main objects of my present invention to provide a common demodulator network capable of being employed for detection of either AM or FM waves, and the demodulator being of the type disclosed in the aforesaid Hansell application.

Another important object of my invention is to provide a detector circuit capable of detecting AM waves in one circuit arrangement thereof; and by virtue of a relatively simple rearrangement is adapted to detect angular velocitymodulated carrier waves with concurrent elimination of undesired amplitude effects.

Another object of my invention is to provide an FM receiver which uses no limiter prior to the detector, and common plate and cathode resistors being utilized in the detector circuit to receive AM waves.

Still other objects of my invention are to provide improved AM-FM demodulators, and more especially to provide such a demodulator in a reliable, efiicient and economical manner.

The novel features which I believe to be characteristic of my 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 I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring, now, to the drawing, there is shown only so much of a radio receiver as is essential to a proper understanding of my invention. Let it be assumed that the receiver is of the superheterodyne type, although the system may be of any other well known form. Further, suppose the FM waves to be received are of the presentlyassigned FM band of 42 to megacycles (mc.). Of course, the AM waves may be in the ultra high frequency range. The FM waves may have channels which are of any desired width. The carrier frequency deviation may be small or large in the case of the employed FM band. Finally, the reception of FM waves is purely illustrative, as phase modulated carrier waves (PM) could be received. The term angular velocity-modulated carrier waves is to be understood as generically covering FM or PM waves or even hybrids thereof.

Assume, then, that the received modulated Waves have been collected, amplified at radio frequency, heterodyned in the usual manner to the intermediate frequency (I. F.), and amplified at I. F. Where the I. F. energy is frequency modulated, it is necessary to translate the FM waves into AM waves. Hence, a discriminator network is used for this purpose. The discriminator shown herein has been disclosed and claimed by S. W. Seeley in his U. S. Patent No. 2,121,103 granted June 21, 1938. It is to be understood that any other well known form of discriminator may be used in place of that shown. For example, in the aforesaid Hansell application there are shown various types of discriminators that may be used to derive a wave of constant carrier frequency and variable amplitude from one of constant amplitude and variable frequency.

The I. F. transformer has its primary winding 2 tuned by shunt condenser I to the operating I. F. value. Thi may be chosen from a range of 2 to 15 mc., say, for example, 4 me. The secondary coil 5 is tuned by condenser 4 to the I. F. value. The coupling condenserS connects from the high side of coil 2 to the midpoint of coil 5 thereby dividing coil 5 into two sections. The radio frequency voltage across the upper half is designated Ea, while the lower section voltage is Eb. To accommodate the FM swing, the pass band of the network l,2, 5-4 should be wider than the most extreme frequency deviation of the applied waves. This will, of course, be wider than the AM band width.

The tubes l0 and l I provide the rectification of thederived AM waves;. assuming FM reception is being considered. These may be separate triodes; or they may be any other types of tubes. For example, tubes of the remote cut-off type are advantageous. The common cathode connection is returned to ground through resistor 18. The grids I0 and H are connected to the upper and lower ends respectively of coil 5 by grid condensers 6 and 1 respectively. Grid leak resistors 8 and 9 return grids l and II respectively to The plates of tubes Hi and H are connected to the positive terminal +B of the direct current power supply source. connected to +13 through the path consisting of series resistors l2 and [6. The opposite plate is connected to +3 through the resistor l3 in series with same resistor It. The resistive path i2--l3 provides the output load of the demodulator. I. F. bypass condensers M and connect each plate lead to the cathode end of resistor IS. The bypass condenser I! is connected from the junction of load resistors I2-l 2 to ground. The midpoint of coil 5 is returned to ground by resistor 29.

The condenser l9 grounds the junction of grid leaks 8-9 and the cathodes for I. F. currents. The lower end of primary coil 2 is established at ground potential for I. F. currents by condenser 2 i. To convert from demodulation of AM to FM there are provided contacts designated FM connected to the opposite'ends of resistors l2-l 3 while the AM contacts are connected to the upper ends of resistors I8 and I3 as shown. The adjustable taps 22 and 23, actuated by a common control device 24, are employed to switch over from the FM contacts to the AM contacts. In receiving FM waves the taps 22 and 23 are connected with the lower FM contacts. When receiving AM waves the taps 22 and 23 are adjusted to the upper AM contacts. The audio coupling condensers 25 and 26 areadapted to connect taps 22 and 23 respectively to the common input circuit of a following push-pull audio circuit. The audio circuit may include, as many amplifiers as is desired, terminated by any desired type of reproducer.

The operation of the demodulation circuit will now be explained in. detail. At the I. F. value the modulated wave voltage applied to the grid it is the vector sum of the voltage across resistor 28 and one-half the voltage across 5-4. That is, the voltage applied to grid [0' is Ea (which is in phase quadrature with the voltage across l--2) plus E20. The I. F. voltage applied to grid l l. is of equal magnitude, but out of phase. At the I. F. value, therefore, the rectified voltage across resistor 8 is equal to the voltage across resistor 9. The magnitudes of E8 and E9 will be proportional to the input magnitude. If the value of. the applied frequency is instantaneously deviated to one side of the mid-band frequency (that is, the operating I. F. value) the quadrature relation between E20 and Ea. (and E20 and Eb) no longer exists. Hence, the vector addition of E20 and Ea will not equal the vector sum of E20 and Eb. Therefore, the I. F. voltage on the grid l9 will not equal that on grid H, and rectified voltage Es will be unequal to rectified volt-age E9.

The rate of change of the bias voltages E8 and E9 is determined by the rate of frequency swing in the applied I; F. energy, and the magnitude depends upon the frequency deviation fromthe mid-band value. Hence, the grids of IE] and II act in the manner of diode detectors supplying a bias for the triode action of tubes l0 and H. The plate current of tube lil flows from -}-B through resistors l6, l2, tube In and resistor l8 to ground. The current of tube H flows from -|-B through path l6l3-l |.l8 to ground.

For FM detection the current through resistors l8 and. It remains constant, while the current the common cathode connection of tubes IQ and p The plate of tube [ii is through resistors i2l3 varies. Hence, a varying voltage will appear across resistors l2-l3. This voltage is applied to the following push-pull audio circuit.

The eiTect of amplitude modulation is removed by virtue of the variable mu effect of the grid leak biased detectors. This concurrent removal of amplitude modulation is provided for eliminating any voltage variations across I2-l3 due to amplitude variations in the applied I. F. carrier. Such undesired amplitude effects are caused by noise pulses, resonant circuits, fading and other factors. The removal of amplitude modulation is automatically secured in the demodulator, assuming that taps 22-23 are contacting the FM contacts, for these following reasons.

As stated previously, at the instantaneous midband frequency value the biases on the grids l0 and. H. will be equal, and will increase as the input voltage magnitude is increased. This will cause the detector tube currents to decrease equally until the tubes practically cut off. Hence, the voltage across resistors l2 and I3 will be equal regardless of the amplitude of the I. F. voltage applied to the primary circuit |--2. Considering the situation next when the frequency is instantaneously shifted to one side of the mid-band frequency value, the bias on each tube will increase unequally until both tubes are practically cutofi.

t zero input voltage the voltage across resistor 2 will be equal to the voltage across resistor 13, and when both tubes are practically cut off the voltages across resistors l2 and I3 will be equal.

For both cases the voltage across the push-pull grids will be zero, but at values of input voltage between zero and cut-off bias the voltages across resistors l2 and I3 will be unequal. Hence, the voltage across the push-pull grids will rise from zero, go through a maximum and-then return to zero. For frequency modulation detection the operation will be along the maximum portion of the curve. The maximum portion is made fiat by using large values for the grid leak resistors 8 and 9. Condensers 6 and 1 are so chosen as to give the proper time constant to secure deemphasis according to frequency modulation standards. The magnitudes and relations have all been explained in the aforesaid Hansell application.

For amplitude modulation detection the applied I. F. energy is, of course, of constant mid-band frequency value, but the I. F. carrier amplitude is variable in accordance with the signal modulation applied to the carrier. The I. F. voltage in that case applied to the grid of tube ID will be the vector sum of E20 and Ea, and the I. F. voltage applied to the grid of tube II will be the vector sum of E20 and Eb. These two I. F. voltages will be equal. Hence, the bias voltages developed across each of grid leak resistors 52 and 9 will be equal, and will increase equally as the input is increased as described for. FM detection. The current for tube Iii will flow from +B through the path comprising Hi, l2, l6 and [8.

The current for tube II will flow from +B through the path comprising 16-, 13, H and I8. As the input voltage changes, both tube currents will change equally. Hence, a varying voltage will appear across resistors l8 and I6, and also across resistors l2 and I3. The rectified voltage across resistor I8 is degrees out of phase with that across resistor IS. The voltages across resistors l8 and [5' are fed to the push-pull audio circuit, and are the audio modulation voltages of theamplitudemodulated carrier-wave which has been detected. This latter explanation with respect to amplitude modulation detection is based, of course, upon the assumption that tubes 22 and 23 have been adjusted to make electrical connection with contacts AM.

It will now be seen that I have provided a simple and compact demodulator circuit which is capable of detecting amplitude modulated carrier waves or frequency modulated carrier waves. Further, in the case of frequency modulation detection the demodulator concurrently acts to limit and to produce deemphasis. Furthermore, the network is quickly converted from a frequency modulation detector to an amplitude modulation detector by a simple switching manipulation which does not in any way interfere with the natureof the audio output load fed from the demodulator.

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 organization 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 I claim is:

1. In combination with a pair of grid leak detector tubes having a common signal input circuit upon which may be impressed either frequency modulated carrier waves or amplitude modulated carrier waves, a common output load impedance connected to said detector tubes, means for deriving modulation voltage from across said output impedance upon the application of frequency modulated carrier waves to said common inputcircuit, a resistive impedance connected to a point on said output impedance such as to divide the latter into sections of substantially equal magnitude, a second resistive impedance connected between the cathodes of said detector tubes and a point of invariable potential, and means for deriving modulation voltages across said two resistive impedances upon the application of amplitude modulated carrier waves to said common input circuit.

2. In combination with a pair of detector tubes of the grid rectification type having a common signal input circuit upon which may be impressed either frequency modulated carrier waves or amplitude modulated carrier waves, a common output load impedance connected to said detector tubes, means for deriving modulation voltage from across said output impedance upon the application of frequency modulated carrier waves to said common input circuit, a first resistive impedance connected to a point on said output impedance such as to divide the latter into sections of substantially equal magnitude, a second resistive impedance connected between the cathodes of said detector tubes and a point of invariable potential, means for deriving modulation voltages across said two resistive impedances upon the application of amplitude modulated carrier waves to said common input circuit, a modulation voltage utilization network, and means for selectively connecting said utilization network either across said output impedance or to said tworesistive impedances.

3. In combination with a pair of grid leak detector tubes having a common signal input circuit upon which may be impressed either frequency modulated carrier waves or amplitude modulated carrier waves, a common output load impedance connected to said detector tubes,

means for deriving modulation voltage from across said output impedance upon the application of frequency modulated carrier waves to said common input circuit, a first resistor connected to a point on said output impedance such as to divide the latter into sections of substantially equal magnitude, a second resistor connected between the cathodes of said detector tubes and ground, means for deriving modulation voltages across said two resistors upon the application of amplitude modulated carrier waves to said common input circuit, said common input circuit being constructed and arranged to convert a frequency modulated carrier wave into a corresponding amplitude modulated carrier wave.

4. In combination, in a radio receiver, a demodulator network including a pair of separate grid leak detector tubes, an input network for said detector tubes, means for combining the output current of said detector tubes in phase opposition, said grid leak detector tubes each including in its respective input network a grid leak resistor and grid condenser, the resistance values of said leak resistors being sufficiently high to cause substantial suppression of amplitude modulation components of applied waves, and additional means, common to the space current paths of said detector tubes, for providing modulation voltage upon application to said input cir cuit of amplitude modulated carrier waves.

5. In combination, in a demodulator network for detecting either amplitude modulated carrier waves or angular velocity-modulated carrier waves, at least two electron discharge devices, each device including an electron emission element, an output electrode and an electron control electrode, a common input circuit connected to the control electrodes of said devices in pushpull relation, said common input circuit being constructed and arranged to convert angular velocity-modulated carrier waves to amplitude modulated carrier waves, a resistive output impedance connected between the output electrodes of said devices and arranged to be traversed by the space currents of said devices, means for deriving from said resistive impedance modulation voltage corresponding to the modulation on the angular velocity-modulated carrier wave, a second resistive impedance connected to an intermediate point on said first impedance and arranged to be traversed by the space currents of said devices, a third resistive impedance connected to the electron emission electrodes of said devices and arranged to be traversed by the said space currents, and means for deriving modulation voltage from solely across said second and third impedances upon the application of amplitude modulated carrier waves to said common input circuit.

6. In combination with a pair of detector tubes having a common signal input circuit upon which may be impressed either frequency modulated carrier waves or amplitude modulated carrier waves, a common output load impedance connected to said detector tubes, means for deriving modulation voltage from across said output impedance upon the application of frequency modulated carrier waves to said common input circuit, a first resistor connected to an intermediate point on said output impedance, a second resistor connected between the cathodes of said detector tubes and a point of invariable potential, and means for deriving modulation voltages across 'isaid'two resistors'upon the application of amplitude modulated carrier waves. to. said. common input circuit.v

7. In combination, in a radio receiver, a demodulator network including a pair of separate grid leak detector tubes, an input network. for said detector tubes, means for combining the output current of said detector tubes in phase opposition, said grid leak detector tubes each including a grid leak resistor and grid condenser, the resistance values of said leak resistors being sufiiciently high to cause substantial suppression of amplitude modulation components of applied frequency modulation waves, and common plate and cathode resistors connected in the space current paths of said detector tubes for providing modulation voltage upon applicationto said input circuit of amplitude modulated carrier waves.

8. In combination, in a demodulator network for detecting either amplitude modulated carrier waves or angular velocity-modulated. carrier Waves, at least two electron discharge tubes, each tube including an electron emission element, an output electrode and an electron control electrode, a common input circuit. connected to the control electrodes of said tubes in push-pull relation, said common input circuit being constructed and arranged to convert said angular velocitymodulated carrier waves to amplitude modulated carrier waves, a resistive output impedance connected between the output electrodes of said tubes and arranged to be traversed by the space currents thereof, means for deriving from said resistive impedance modulation voltage corresponding to the modulation on the angular velocitymodulated carrier wave, a resistor connected to an intermediate point on said output impedance and arranged to be traversed by the space currents of said tubes, a second resistor connected to the electron emission electrodes of said tubes and arranged to be traversed by the said space currents, and means for deriving modulation voltage from solely across said two resistors upon the application of amplitude modulated carrier waves to said common input circuit.

9. In combination with a pair of grid leak detector tubes having a common signal input circuit upon which may be impressed either frequency modulated carrier waves or amplitude modulated carrier waves, said tubes each being of the triode type, said input circuit being constructed and arranged to transform frequency modulated carrier waves into amplitude modulated carrier waves, a common output load impedance connected between the plates of said detector tubes, means for deriving modulation voltage from across said output impedance upon the application of frequency modulated carrier waves to said common input circuit, a common plate resistor connected to a point on said. output impedance, a common cathode resistor connected between the cathodes of said detector'tubes and a point of invariable potential, and means for deriving modulation voltages across said two common resistors upon the application of amplitude modulated carrier waves to said common input circuit.

10. In combination, in a demodulator network for detecting either amplitude modulated carrier waves or angular velocity-modulated carrier waves, at least two electron discharge devices, I

each device including an electron emission element, an output electrode and an electron control constructed and arranged to transform angular velocity-modulated carrier waves to amplitude modulated carrier waves, a resistive output impedance connected between the output electrodes of said devices and arranged to be traversed by the space currents of said devices, means for deriving from said resistive impedance modulation voltage corresponding to the modulation on the angular velocity-modulated carrier wave, a common output electrode resistor connected to an intermediate point on said first impedance and arranged to be traversed by the space currents of said devices, a second common resistor connected to the electron emission electrodes of said devices and arranged to be traversed by the said space currents, and means for deriving modulation voltage from solely across said two common resistors upon the application of amplitude modulated carrier waves to said common input circuit.

11. In combination with a pair of detector tubes having a common signal input circuit upon which may be selectively impressed frequeneoy modulated carrier waves or amplitude modulated carrier waves, a common output load resistor connected to said detector tubes, means for deriving modulation voltage from across said output impedance upon the application of frequency modulated carrier waves to said common input circuit, a resistive impedance connected to a point on said output resistor such as to divide the latter into two sections, a second resistive impedance connected from the cathodes of said detector tubes to a point of invariable potential, and means for taking on modulation voltage from across said two resistive impedances upon the application of amplitude modulated carrier waves to said common input circuit.

12. In combination, in a radio receiver, a demodulator network including a pair of separate grid leak detector tubes, a common input network for said detector tubes, means for combining the output current of said detector tubes in phase opposition when frequency modulated waves are applied to said input network, each grid leak detector tube including in its respective input network a grid leak resistor and grid condenser, the resistance values of said leak resistors being sufliciently high to cause substantial suppression of amplitude modulation components of said frequency modulated waves, and additional resistive means, common to the space current paths of said detector tubes, for providing modulation voltage thereacross upon application to said common input circuit of amplitude modulated carrier waves.

13. In combination with a pair of grid rectification tubes having a common signal input circuit upon which may be impressed either frequency modulated carrier waves or amplitude modulated carrier waves, a common output load resistor connected to said tubes, means for deriving modulation voltage, from across said output, resistor upon the application of frequency modulated carrier waves to said common input circuit, a first resistor connected to an intermediate point on said output resistor, a second resistor connected from a point common to the space current paths of said. tubes toa point of invariable potential, and means for deriving modulation voltage from across said first and sec.- ond resistors upan the application of amplitude modulated carrier waves to said common input circuit.

14. In combination, in a frequency modulation radio receiver, a demodulator network including a pair of grid leak detector tubes, an input network common to said detector tubes, means for combining the output current of said detector tubes in phase opposition, said grid leak detector tubes each including a grid leak resistor and grid condenser, the resistance values of said leak resistors being sufiiciently high to cause substantial suppression of amplitude modulation components of applied frequency modulation waves,

and separate plate and cathode resistors, connected in the space current paths of said detector tubes, for providing modulation voltage thereacross upon application to said input circuit of amplitude modulated carrier waves.

15. In combination with a pair of grid leak detector tubes having a common signal input circuit upon which may be impressed either frequency modulated carrier waves or amplitude modulated carrier waves, said input circuit being constructed and arranged to transform frequency modulated carrier Waves into amplitude modulated carrier waves, a common output load impedance connected between the plates of said detector tubes, means for deriving modulation voltage from across said output impedance upon the application of frequency modulated carrier waves to said common input circuit, a common plate resistor connected to a point on said output impedance, a common cathode resistor connected from the cathodes of said detector tubes to a point of invariable potential, means for deriving modulation voltages across said two common resistors upon the application of amplitude modulated carrier waves to said common input Gil circuit, and means for selectively connecting a modulation utilization circuit to either of said modulation deriving means.

16. In combination, in a demodulator network for detecting either amplitude modulated carrier waves or angular velocity-modulated carrier waves, at least two electron discharge devices, each device including an electron emission element, an output electrode and an electron control electrode, a common input circuit connected to the control electrodes of said devices in push-pull relation, said common input circuit being constructed and arranged to transform angular velocity-modulated carrier waves to amplitude modulated carrier waves, a resistive output impedance connected between the output electrodes of said devices and arranged to be traversed by the space currents of said devices, means for deriving from across said resistive impedance modulation voltage corresponding to the modulation on the angular velocity-modulated carrier wave, a common output electrode resistor connected to an intermediate point on said first impedance and arranged to be traversed by the space currents of said devices, a second common resistor connected to the electron emission electrodes of said devices and arranged to be traversed by the said space currents, means for deriving modulation voltage from across said two common resistors upon the application of amplitude modulated carrier waves -to said common input circuit, and a switching means for selec-

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