US2302834A - Discriminator-rectifier circuit - Google Patents

Description

Nov. 24, 1942. w. H. BLISS DISORIMINATOR-RECTIFIER CIRCUIT Filed Dec. 3, 1940 ATTORNEY Patented Nov. 24, 1942 DISCRIMINATOR-REGTIFIER CIRCUIT Warren H. 'Bliss, Orono, Maine, assignor to Radio Corporation of America, a corporation of Delaware Application December 3, 1940, Serial No. 368,286

3 Claims. (Cl. 250-20) My present invention relates to detection circuits for frequency modulated carrier waves, and more particularly to discriminator-rectifier networks for such waves.

One of the main objects of this invention is to provide a frequency discriminator of a simplified.

type for use where a comparatively wide band of frequency-deviated carrier waves is to be converted so as to have a wide band variation in amplitude; the discriminator essentially comprising parallel connected reactive paths, and each path including a reactance of opposite sign to the reactance of the other path.

Another important object of my invention is to provide a series resistance-capacitance circuit and a series resistance-inductance circuit in parallel therewith; frequency modulated carrier waves being applied to said circuits to develop alternating voltages across the resistances of the parallel circuits which differ in magnitude in dependence on the extent of frequency deviation of the carrier.

Another object of my invention is to provide a frequency modulated carrier Wave discriminator wherein no push-pull or reversed phase alternating current voltage .is needed for operation of the discriminator; the latter utilizing parallel paths consisting of series resistance and reactance elements, and the reactance elements being of opposite sign and of equal magnitude at the center frequency of applied modulated carrier waves.

Still other objects of my invention are to improve the simplicity and efiiciency of frequency modulation detectors, and more especially to provide a detector circuit for frequency modulated signals which is reliable and efficient in operation, and economical to manufacture and assemble.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims ;v 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 several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows one embodiment of the invention,

Fig. 2 shows a modified form of the invention.

Referring now to the accompanying drawing, wherein like reference characters in the two figures designate similar circuit elements, let it be assumed that the networks shown in Fig. 1 are inserted between the intermediate frequency amplifier and audio frequency amplifier of a free quency modulation receiver of the superheterodyne type. The presently assigned band for frequency modulation reception is the 43 to 50 megacycle range. Of course, the present invention could be used in receivers operating in other frequency, or phase, modulated carrier reception bands. An intermediate frequency of 4.3 megacycles is usually chosen to minimize image interference, but it will be understood that the intermediate frequency may be chosen from a range of 2 to 5 megacycles. While the carrier frequency deviation may be narrow or wide, present practice is to use a wide band frequency modulated carrier. For example, if the carrier has a maximum frequency deviation of kilocycles, then the overall band width is 200 kilocycles. The center, or carrier, frequency at intermediate frequency would be, of course, 4.3 megacycles. As is Well known, the stage prior to the second detector, or demodulator, is the limiter. This stage is desi nated in Fig. 1 by the numeral 4, and upon its input terminals I are impressed the amplified frequency modulated carrier waves at intermediate frequency.

"It is not believed necessary to describe the detailed process of deriving the intermediate frequency energy from the received carrier waves. The frequency modulated carrier waves acquire a certain amount of amplitude modulation due to passage through the cascaded resonant stages; noise impulses will produce amplitude variation effects; finally, fading will create such effects. The limiter stage 44 is constructed as an easily overloaded amplifier. Those skilled in the art are so fully acquainted with the manner of constructing amplitude limiters that it is believed a general reference to the functions thereof will suffice. The limiter acts to eliminate all amplitude variation from the modulated carrier thereby supplying to the following discriminator a purely frequency-variable carrier of constant amplitude. Of course, the limiter may possess gain, if desired.

The resistor 1 is connected from a positive point on the voltage supply source to the limiter tub-e plate. The discriminator network comprises a path consisting of coil 9 and resistor II in series between the high potential side of the system and ground. The direct current blocking condenser 8 is connected between the plate end of resistor I and the high potential end of coil 9. In shunt with path 9H is arranged a path consisting of condenser l0 and resistor 12. The detector tube ii may be of the double diode type, for example a 6H6 tube, and is provided with electrodes i3--l 3' and Hie-l4 to act as independent diodes. The

cathode I3 is connected to ground through load resistor I5, while anode I3 connects to the junction of coil 9 and resistor I I.

The cathode I4 is connected to the junction of condenser I and resistor I2, while the anode I4 is connected to the cathode end of load resistor I5. Audio voltages developed across resistor I5 are fed to the following audio frequency amplifier network through a filter network which comprises in series the coils I1 and I9 and in parallel the condensers I6, I 8 and 20. The terminating resistor 2| is connected across the filter output terminals to ground. One or more stages of audio amplification may be utilized and a reproducer will follow the final audio amplifier stage. In general the discriminator elements comprise a series resistance-capacitance path and a series resistance-inductance path. These paths are connected in parallel, and the voltages appearing across the resistance elements thereof are combined, after half wave rectification, to produce the demodulation volta e.

An advantage of this method is that no pushpull or reversed phase alternating current voltage is needed for operation of the discriminator. The magnitudes of coil 9 and condenser Ill must be such that their reactances are numerically equal at the mid-band, or center, frequency of the applied frequency modulated carrier waves. In other words, the inductive magnitude of coil 9 must be equal to the capacitative magnitude of condenser I0 at the operating intermediate frequency. Furthermore, these reactive magnitudes should be several times as great as the resistive values of resistors II entire high frequency band of operation. It is desirable to have resistors II and I2 of equal magnitude. The resistor I5 acts as a common load resistor for both diode rectifiers, and, as stated previously, the modulation voltage output is fed through a low pass filter prior to utilization by the following audio network.

In considering the operation of the discriminator-detector network it will be understood that the modulating signal appears in the modulated carrier wave as a carrier frequency deviation. That is, the amplitude of the modulation frequency components corresponds to the extent of the carrier frequency deviation, while the modulation frequencies themselves determine the rate of deviation of the carrier frequency. This theoretical aspect of the composition of a frequency, or phase, modulated carrier wave is too well known to those skilled in the art to require further amplification. The frequency modulated carrier waves to be detected are supplied to the discriminator after amplification by the preceding amplifier stages to the desired level. The alternating current voltage, which is the voltage of the frequency modulated carrier waves, is then applied to the two parallel discriminator paths. Since the magnitude of resistor I I is small compared to the reactance of coil 9, the potential developed across resistor II lags the applied voltage by nearly 90 degrees. Similarly,

and I2 for the diode I3--I3' rectifies the alternating current voltage developed across resistor II, while diode I4-I4' rectifies the alternating current voltage developed across resistor I2. The diodes are connected, as shown in Fig. l, in polarity opposition. Accordingly, the average voltage developed across the common load resistor I5 will be zero at the balanced condition. When the modulation present on the'incoming carrier wave causes the instantaneous frequency to be higher than that for the balanced condition, the voltage across resistor I2 will rise, while that developed across resistor II will fall because of the change in reactances of condenser II] and inductance 9. In such case the diode I4I4' will pass more current than diode I3-I3', and the resistor I5 will then have a negative voltage developed across it. On the other hand, when the instantaneous frequency of the carrier swings to a low value the voltage developed across resistor II will exceed that developed across resistor I2, and, hence, a positive voltage will appear across the load resistor I5.

By virtue of this action a potential is developed across the load resistor I5 which follows the frequency variations of the carrier of the collected frequency modulated carrier waves. If the carrier is being deviated in correspondence with an audio frequency wave, then the original audio voltage will be developed across the load resistor I5. Since the diodes of detector 6 together give only half wave rectification, and since they also pass some alternating current due to inter-electrode capacitance, a low pass filter is necessary to remove the high frequency components remaining in the output voltage.

In Fig. 2 there is shown a modified arrangement wherein the output alternating voltages developed across each of resistors II and I2 are subjected to full wave rectification. In this case diodes 21 and 28 are arranged to develop a negative voltage across the common load resistor 3|, while the diodes 29 and 30 are arranged to develop a positive voltage across the load resistor. The discriminator section is constructed exactly the same as in the case of Fig. 1, and the limited frequency modulated carrier waves, after acting upon the discriminator, may have the resultant voltages thereof which are developed across resistors II and I2 amplified in a twin triode tube 40. The signal grid M of one of the triode sections of this tube may be connected to the junction of condenser I0 and resistor I2, while the signal grid 42 of the other triode section may be connected to the junction of coil 9 and resistor II. The use of the triode amplifiers is rimarily for the purpose of eliminating any loading effect on the parallel discriminator elements. The grids M and 42 draw no current. The cathodes of both triode sections may be connected to ground through a self-biasing resistor 43 of proper magnitude, and the latter may be properly by-passed for intermediate frequency currents.

The diodes 21 and 28 may have their cathodes connected in common to ground, while their anodes are connected to the opposite ends of the secondary winding 28 of transformer 44. The primary winding 45 of transformer 44 may be connected between the plate 46 and a positive potential source. Similarly, the diodes 29 and 30 have their anodes connected to the opposite ends of the secondary winding 45 of primary 41. The primary winding 48 of the latter is con} nected between plate 49 and the positive potential source which supplies plate 46. The midpoint of winding 46 is at ground potential, while the cathodes of diodes 29 and 30 are connected in common to a lead which joins the mid-point of winding 28 to the high potential end of the common load resistor 31.

The audio voltage developed across resistor 3| is passed through a low pass filter network comprising the series inductance and the shunt condenser elements If and T2, the terminating resistor 13 being shunted across the output terminals of the filter network. As stated previously, the arrangement of Fig. 2 permits the utilization of a discriminator network which functions in the manner described in connection with Fig. 1, but wherein full wave rectification is secured with all its attendant advantages. Not only is the efiiciency of the detector circuit increased by the use of full wave rectification, but a low pass filter of simpler construction may be used for removing undesired high frequency components.

While I have indicated and described several systems for carrying my invention into efiect, 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 I claim is:

1. In a discriminator network for frequency modulated carrier waves, a pair of input terminals and a pair of output terminals, a pair of reactive paths connected in shunt across said input terminals, one of said paths including an inductive reactance and a resistive impedance arranged in series relation, the second path including a capacitative reactance and a second resistive impedance in series relation, and the reactive magnitudes of said reactances being numerically equal at the center frequency of frequencymodulated carrier waves applied to said input terminals and separate full wave rectifier means for rectifying alternating current voltage developed across each of said resistive impedances.

2. A discriminator-detector network comprising input terminals upon which is applied a frequency-deviated carrier wave, a discriminator including a pair of reactive impedance paths arranged in parallel across said terminals, each path consisting of a reactive element and a resistive element in series relation, the reactive elements of said paths being of opposite sign, a rectification circuit individual to each resistive element and coupled thereto to rectify the alternating voltage developed thereacross, and means for combining the rectified outputs of said rectification circuitsin polarity opposing relation, each of said rectification circuits being a full wave rectifier, and said combining means consisting of a single load resistor common to both rectifiers.

3. A discriminator-detector network comprising input terminals upon which is applied a frequency deviated carrier wave, a discriminator including a pair of reactive impedance paths arranged in parallel across said terminals, each path consisting of a reactive element and a resistive element in series relation, the reactive elements of said paths being of opposite sign, a rectification circuit individual to each resistive element and coupled thereto to rectify the alternating voltage developed thereacross, means for combining the rectified outputs of said rectification circuits in polarity opposing relation, an amplifier coupling each resistive element to its rectiflcation circuit, and each rectification circuit being of the full wave rectifier type.

WARREN H. BLISS.

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