US2041040A - Carrier augmentation circuit - Google Patents

Description

May 19, 1936f w. s. BARDEN CARRIER AUGMENTATION CIRCUIT Filed NOV. 26, 1954 All S S 1NVN`TOR WILUAM S. BARDENv BY iff? wv-IA., ATTORNEY @55k b l m ...I

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Patented May 19, 1936 UNITED STATES.v

2,041,040 CARRIER AUGMENTATION CIRCUIT William Stoddard Barden,

Grasmere, Staten Island, N. Y., assgnor to Radio Corporation of America, a corporation of Delaware Application November 26, 1934, Serial No. 754,713

9 Claims.

My present invention relates to signal carrier selection networks, and more particularly to an improved method of, and means for, selecting l side band modulated carrier energy with carrier augmentation.

Intelligence modulated radio frequency signals transmitted to a receiver by radiation through space are subject to different types of fading phenomena. To compensate for amplitude fading, there has been provided in the receiver an automatic volume, or gain, control network which functions to maintain the signal input to the demodulator of the receiver substantially constant over a predetermined range` of collected lmodulated signal intensities. However, it has been recognized for some time that an additional type of fading occurs during the reception of'radiated signals; and this type of fading is particularly troublesome in the reception of signals radiated from points located a relatively great distance from the location of the receiver.

This latter type of fading, often called selective fading will occur regardless of the degree of selectivity of the selector network, and in spite of the use of an automatic gain control network. It has been found that when a transmitter radiates a signal comprising a carrier and both side bands, and the distance between the transmitter and receiver is relatively great, various effects in space may bring about relative phase shifts among the radio frequency components, or selective attenuation among the radio frequency components, of the radiated signal may occur.

The desired carrier may fall to zero, or to a small value closerto zero; on the contrary, it may increase, and it has beenrfound that both these effects can occur at the receiving antenna. Furthermore, it has been found that the carrier may undergo a 90" phase shift with respect to its proper self, and any side band component may do likewise. As a consequence it has been found that the loud speaker output in typical highly selective receivers may become badly distorted, or greatly diminished from time to time during the reception of side band modulated 'carrier signals from transmitters located a relatively great dis tance from a given receiver.

Many arrangements have been proposed in the past to minimize the effect of `selective fading. Many of these proposed circuits and arrange- 'ments of the prior art have proven to be unconomical, in many cases they have been found to be complicated in operation and construction.

(Cl. Z50-20) Accordingly, it may be stated that itis one of the main objects of my present invention to pro' vide an improved and simplified circuit arrangement for substantially minimizing the effects of selective fading during the reception of side band modulated carrier energy, the arrangement em-f ployed essentially comprising a sharply selectiveV circuit, sharply tuned to the frequency of Vthe de sired carrier, arrangedv in series with an imped` ance, and thecombined-series arrangement be, ing disposed in the signal transmission networkk of the receiver in such a manner that carrier augmentation, or exaltation, is obtained whereby selective attenuation of the side bands of the car- .rier is compensated for by the carrier' augmenta,.-l tion effect. l Another important object of the invention is' to provide in a radio receiver successive devices for minimizing selective fading effects at the receiver due to selective attenuation and relative phase shifts among the radio frequency components of the received signals, one of the devices used in the receiver functioning to augment the carrier with respect to its side bands suiciently to compensate for the selective attenuation effect, and the other of the devices functioning to substantially reject one of the side bands withl the result that the relative phase shift effect is substantially compensated for. v

Still another object of the invention is to provide in a superheterodyne receiver, adapted to receive side band modulated carrier signals from relatively remote points, simple and effective devices for materially decreasing the effects of radio frequency distortion in space upon the desired signals, which effects may be due to relative' phase shifts and selective attenuation among the radio frequency components of the desired signals, the superheterodyne receiver including be-Y tween the first and second detectors, devices f or substantially rejecting one of the side bands of. the received signals and increasing the carrier strength relative to the strength of the remain# ing side band. A And still other objects of the invention are to improve generally the eiciency and simplicity ofradio receivers utilized for receiving modulated signals from distant stations, and more especially to provide such receivers with devices to mini` mize the effects of selective fading, and which devices are reliable in operation, and economical.` ly constructed and assembled in a radio receiver. The novel features which I believe to be char-' acteristic of my invention vare set forth in particularity in the appended claims, the inventionl itself, however, as to both Vits organization l reference.l to the following description taken inY Y superheterodyne type. Athat this type of receiver is chosen to embody the and method ofY operation will'best be understood by connection with the drawing in which I'have indicated diagrammatically a circuit organization whereby myinvention may be carried into effect.

Fig. Y1 schematically shows a. radior'rreceiverr` embodying the present "invention,

Fig. 2 graphically shows the operation of the invention. f

, Referring now to the accompanying drawing, itrwill be observed that thercircuit'diagram in Fig; 1 schematically designates a receiver of the It is lto be understood invention because the carrier augmentation network employs a piezo-electric crystal, as an element'thereof, and since the intermediate frequency amplifier network of a superheterodyne receiverk remains ixedlyV tuned to the operating intermediate frequency, 4the piezo-electric crystalV Yelement may be'empl'oyed therein to advantage.

' ThesuperheterOdyne receiver shown Yin `Elig. 1,

inrgeneral, comprises theconventional and wellknown networks. VThe signal collector' may consist of the usual grounded antennaY circuit; one or more tunable stages ofradio frequency ampli'- fication,;designated at |,omay be coupled to the signal collector A.Y vThe frequency'changerfnet- 'Y work comprises Ya, first detector 2 and a local oscillator 3,'and the output of Vthe rstfdetector -is impressed `upon an-intermediate frequency amplifier 4.A Y Y The tuner device of the. receiver is conven- Y Ytionally represented by the variable condenser representations 5 and 6. Itis to be understood vthat vthe Arepresentation 5 designates the variable tuning condensers usually employed to tune the input circuits of the radio'frequencyamplifier, or ampliers, fand the rst detector; therepresentation EiV designates the variable tuning con- Y denser commonly utilized to'tune the local oscillator'net'work. The various variable condensers are, of course, uni-controlled, and any'well known device is utilized in the local oscillator network to maintain the local oscillation frequency at a value which differs fromY the desired signal lfrequency by the voperating intermediate frequency. Such devices for maintaining a substantially constant intermediate frequency difference be- Ytween the signal circuits of the local oscillator circuit Vare well known to those skilled in the art, and `need not be explained any further;` Furthermore, it is Yto be clearly understood that the networks 2 and ,3 may be combined in a. single cir- Vcuit arrangement; that is to say, a single tube may be employed to perform the functions of generation of' local oscillations and production of the intermediate frequency. Y Y

The-intermediate frequency energy amplified Y in the network 4 is transmited to the second de- Ytector, or demodulator, 1 through Ya transmission network embodying the present invention. The Y demodulated signalenergy is then fed toV an audio frequency network,rand the lattermay comprise one or more stagesof audio lfrequency `amplifica-v tion followed by a reproducer; Thetransmission network between the amplifier 4 and the second detector 'I comprisesra pair ofiintermediate Vfre- Y quency amplifers'arranged in` cascade, there be Y ving disposed between the' amplifier 8 and the amplifier 9 anetwork for producingcarrier augo, mentation with respect'toi the side bands, while.

between the amplier's :and the second detector 1, there is utilized a, Vnetwork for substantially rejecting one of the side bands Vof the signal energy. transmitted from the amplifier/Sito the Y demodulator 1.

The amplifier 8 may be ofV any Ywell Yknown type, and is shown by way of exampleas a screen grid tube. The input electrodes of amplifier 8 are coupled to the anode circuit of lamplifier 4 through a coupling net work M1 which may comprise a pair of coupled circuits tuned to the oper-V ating intermediate frequency, and having the Vconstants Ythereof chosen to provide a band pass characteristic. Thus, there is impressed uponV amplifier 8 the desired signal carrier, now at intermediate frequency, and its associated modulation side bands. As pointed out heretofore,v when the receiver is utilized to receive side band modulated signals from a relatively remote point various causes in space may bring about relative phase shifts among the radio frequency components, .or selective'attenuation among the components. YThe carrier amplitude may fall sube stantially to zero with respect to the'modulation side bands; or the carrier may undergo a phase shiftwith respect to its proper self.V 'Ihis @mayroccur whether the'received signalsare in the broadcast band, 500 to 1500.ki`locycles,1or

whether the received Ysignals arein the Vshort-V wave band, 8 to 23 megacycles.'Y The selectivefading phenomena referred to willV resultVv in the loud speaker output becoming badly distorted, 'or Vgreatly diminished, from vtime to time.

Additionally, the eiect of selective fading willV occur eventhough the selective network preceding the secondjV detector is sharp. The use of an automatic vvolume control network in the, receiver is ofV substantially no` aid in overcoming the effect .of selective fading. In other words, the receiver shown in Fig. 1 is to be understood as including an automatic volume controlv network, usually embodying a rectifier supplied with intermediate frequency energy from a point preceding the second detector 'l and feeding this rectified direct current energy'as bias to the networks I, V2 and 4. Such a network is well known to those skilled in the art, and isnot shown on the drawing in order to preserve simplicity of disclosure.

The important fact is that such an automatic gain control arrangement is of little use in overcoming the effects of selective fading at the receiver, since the Vconventional automatic gain control arrangement does not operate to discriminate between attenuation of the carrier, or phase shift thereof, with respect to the associated side' bands. y Y Accordingly, there Yis disposed in the anode Ycircuit of amplifier '8 a simple, but highly effective, network for overcoming the effect of the attenuation of thencarrier with respect to its side bands. The anodeof tube 8 is connected to the high alternating potential side of the tuned.

circuit L-C through a condenser I0. The anode is further connectedrtofa source of energizing direct current voltage B through a resistor Il which may have amagnitu'de of about 5000 ohms. A resistor 1: is'arrang'ed inseries with the tuned circuit L-C and this resistor may have a mag- Y Vanimee ofV about 30,000 ohms; the 10W potential sides of resistors l l and r being connected by a condenser I2. The coil L may have -a magnitude Y of about 20 millihenries, and the capacity of condenser CY is chosen tohave' a value such that with thefvalue of the capacity existing between the plates of thecrystal. holder, it tunes theI coil Lto the operating intermediate frequency.

for present receivers to utilize an intermediate frequency of 450 kilocycles. The invention does not depend in any way upon the value of the intermediate frequency chosen, nor upon the specific short-wave frequency which it is desired to be received.

In shunt with the coil L, and with the condenser C, there is disposed a piezo-electric crystal device I3, and those skilled in the art will readily Y appreciate that such a device customarily comprises a quartz crystal disposed between metallic holder plates, and the crystal being designed to be resonant to the operating intermediate frequency. The input electrodes of amplifier 9 are connected across the resistor r, while the output of amplifier 9 is transmitted to the demodulator 1 through a band pass filter which is designed to substantially reject one of the side bands of the intermediate frequency energy transmitted to the demodulator.

In explaining the operation of the network included between amplifiers 8 and 9, reference is made to the solid line curve shown in Fig. 2 which represents the resonance curve characteristic of the network. It will be observed from this solid line curve that the effect of the network is to greatly attenuate `the side bands of the intermediate frequency energy with respect to the carrier. Such a characteristic is secured by virtue of the fact that the coil L neutralizes, or tunes with, the crystal holder capacity so that at crystal resonance, the operating intermediate frequency, there appears across points a, b, an equivalent series resistance of the crystal I3. For a crystal actually utilized, this resistance was approximately 10,000 ohms. Thus, at crystal resonance the high resistance of the tuned circuit L-C, which was about one megohm, was nearly shorted by the 10,000 ohms resistance of the crystal device I3. As a consequence nearly all of the signal voltage was applied across the resistor r connected in series with the crystal device I3 and having a magnitude of 30,000 ohms.

However, o crystal resonance, the 10,000 ohms is removed by virtue of the crystal impedance rising greatly. Hence, 01T crystal resonance the circuit L-C is a simple wave trap, and prevents substantially 95% of the side band voltage from being applied across resistor 1'. As a result, across the resistor 1' all of the carrier energy is developed, and only about 5% of the associated side bands of the intermediate frequency energy. In other words, the action of the crystal device I3 is to automatically reduce the signal energy voltage developed across resistor r as the frequency of the signal energy departs from the operating intermediate frequency, which is also crystal frequency. The solid line characteristic in Fig. 2 demonstrates, furthermore, that there has been a relative carrier augmentation of the intermediate frequency energy. That is to say, the amplitude of the carrier has been exalted with respect to the amplitude of the side bands.

As a consequence of the network inserted between amplifiers 8 and 9 the effect of selective fading, wherein the amplitude of the carrier is greatly diminished with respect to the side bands, on all signals collected at antenna A is substantially minimized. The network disposed between amplifiers 8 and 9 functions to produce the reverse effect of the selective fading, and therefore there is transmittedfrom the amplifier 9`to the demodulator 'l signal energy wherein the amplitude of the carrier with respect to the side bands is'- substantially unaltered as compared to the same relation at the transmitter.

When, however, there also occurs in the radiated signals a phase shift among the radio frequency components, I have found that the utilization of a band pass filter, conventionally designated by the reference numeral 20, designed to substantially reject one of the side bands, greatly minimizes the phase shift effect. The band pass filter 20 is designed to change the solid line characteristic of Fig. 2 to the dotted line characteristic. The latter shows a substantial rejection of one o-f the side bands.

Without entering into any theoretical considerations, it can be demonstrated that the effects of carrier phase shift becomes of almost no consequence, and a carrier fade of ten to one becomes of no practical consequence when the demodulator 'I is a linear detector. Those skilled in the art will readily appreciate the manner of designing the band pass filter 20 so as to reject one of the side bands of the intermediate frequency energy. Merely by way of illustration, it is pointed out that the design of such sharply tuned band pass iilters is described by G. A. Campbell in U. S. Patent 1,227,113 of May 22nd, 1917. For the purpose of enhancing the dotted line characteristic shown in Fig. 2, it may be preferable to employ an operating intermediate frequency of 50 or 75 kilocycles.

The demodulator 1, when employing linear detection, and preceded by the compensating devices described, materially decreases the eiects of radio frequency distortion in space on desired signals. Any desired type of linear detector may be utilized, and by way of illustration it is pointed out that a diode rectifier circuit can be employed because of the considerable intermediate frequency amplification preceding the demodulator network. It will therefore be seen that there has been provided between the demodulator and the converter network an intermediate frequency amplifier which includes devices for materially decreasing the effects of radiov frequency distortion, or selective fading, in space upon thel desired signal energy by rejecting one of the side bands of the intermediate frequency energy and increasing the carrier strength relative to the strength of the remaining side band.

It is to be clearly understood that the side band rejection step may precedev the carrier augmentation step, and furthermore that the carrier augmentation network may be disposed at ,any point between the converter network and the demodulator 1. When utilizing the receiver for reception of signals ordinarily not subject to selective fading effects, as in local reception of broadcast signals, a switch 2| may be closed to short-circuit the crystal device I3 and remove it from the 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 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. A method of operating a radio receiver of the super-heterodyne type which includes collecting side band modulated carrier energy radiated from a transmitter relatively remote from the receiver, converting the collected energy to in- VVtermediate frequency energy, augmenting` Ythe r'amplitude of the carrier frequency of the intermediate frequency energy with respect to its ,asso-- ciatedside bands, substantially'rejecting one of the side bands, and subjecting the resulting energy to Vlinearrdetection. Y

' Y V2. A method of operatingV a radio` receiver of the superheterodyne type which includes collecting side band modulated carrier energy radiated from a transmitter relatively remote from the receiver, converting: the collected energy 'to intermediate Vfrequency energy, augmenting the amplitude 20Y ,ceiver to receive side band modulated carrier enof thev carrier frequency of Ythe intermediateY frequency energy with respect to its associated v side bands, in such a manner that the ratio of the carrier amplitude to the side band amplitude is of the order of nine to one, substantially rejecting one of the side bands, and subjecting the resulting energy to linear detection. t Y

3. A method of operating Ya superheterodyne reergyffrom a transmitter relatively distant from the receiver in'such'a manner asY to substantially minimize the effects ofY selective fading on the signals radiated. from the transmitter, which consists in reducing the carrier frequency of the collected modulated carrier energy to aidesired inf termediate frequency of tl1e-order of 50 toV 450 kilocycle's, rejecting cnenof the side bands of Vthej intermediate frequency energy and increasing the carrier amplitude relative to the strength of the remaining side band, and finally subjecting the resultant energy to linear detection.V

4; In a superheterodyne receiver of the type including a Vconverter network having an output circuit tuned to an operating intermediate frequency and a demodulator of the linear detection type, an impedance of relatively low resistance arranged in series with the said tuned output circuit, means for coupling the demodulator input `across the said impedance, and a Vpiezoelectric device, tuned to the intermediate frequency, connected across the tuned output circuit in such a manner that signal energyfoi"Y a. fre-VA quency diiTerent from the intermediate `frequency is greatly attenuated andprevented from appear'-Y ing across said impedance.V Y Y Y 5. In a system as dened in claim 4, said piezoelectric device having a resistive impedance substantially lower than the resistance of said series impedance. v Y Y 6. In a system as described in claim 4, said coupling means between said series impedance and said demodulator including a network constructedV to substantially Vreject one of the Vside bands of Y the intermediate frequency energy.

7. In a superlieterodyne receiver of the type provided with an `intermeradi'ate frequency ampli-- fier followed by a demodulator operating by virtue of linear detection, aV resonant circuit including a coil and condenser arranged in parallel in the anode circuit of the amplifier, said resonant circuit being tuned Vto the intermediate frequency, a resistor having :a substantiallyrlow resistance arranged in series with the resonant circuit,l an

intermediate frequency coupling network con- Ynesting the series resistance to the said demodulater so as. to impress -upon the'dem'o'dulator in-V 25 Ytermediate frequency energy developed across the said series resistor, andaV piezo-electric crystalconnected-in shunt with'sai'd4 coil andcon'densen said crystal beingl sharply tunedto the intermediatev frequency and having its constants chosen to substantially inhibit the development across said series resistor of the side band frequencies of the intermediate frequency energy. 8. In a system as defined in claim '7, means for rendering said crystal inoperative.

9. In a system as defined in claim 7, a band pass filter disposed in said intermediate frequency cou plingnetwork and having its constants chosen whereby one of the side bands of the intermediate frequency energy transmitted to said demodulator is substantially rejected. Y

WILLIAM sToDDARD BARDEN.

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