US1998859A - Means for and method of detection - Google Patents

Description

p i 23, 1935- s. BALLANTINE 1,

- MEANS- FOR ANb IMETHODOF DETECTIQN" Orig inal Filed April 10, 1929 Patented Apr. 23, 1935 UNITED sTATss MEANS FOR AND Ms'rnon F DETECTION: I

Stuart'Ballantine, ItIountain Lakes, N .1, assigner, by mesne assignments; to Radio Corporation of'America, New Yo'rk;N:Y.; acorporagtion of Delaware Original applicatioriApril-10,1929, Serial Nd.

354,022. Divided and this application Decem-- ber 20, 1929, Serial No. 415,571;

6 Claims. (erase-27) 7 called heater? type. Aradio frequency, by'pass Thisinventionrelates to, an electrical circuit for thereception of carrierswave signals, and

more particularly to new and improved. forms of detector circuits and methods of detection.

Objects of the present invention are to pro vide a detector and methodof detection in which the response in the form of audio frequency. output voltage bears an approximately linear relation to; the input in the form of radio frequency carrier wave voltage. I

Further objects are to provide a vacuum tube detector" of the type having a limited range oi linear response and carrier wave amplifiers capable of adjustment to bringthe amplified signal voltage within, or to maintain the (same within the limited range of linear response.

These and other objects of the invention; will be apparent. from the following specification when taken ,withflthe accompanying drawing :in which Fig. 1 is a circuit diagramof a receiver embodying the invention, Fig. 2 is a curvesheet showing the variation, with carrier voltage, of the fundamental audio frequency response, the harmonic frequencies, and-the distortion, respectively, and Fig. 3 is a circuit diagram of acornplete receiver which includes another embodi-a ment of the invention. g

In the drawing, the numerals l, 2, indicate the terminals across which the signal energy is impressed, being transferred by the tuned radio frequency transformer 3 to the amplifier tube t. Although but one, amplifier tube is illustrated, it will be understood that any'appropriate number of amplifiers will beemployed to raise the received signal voltageito the desired order ofmagy nitude. The amplified 'voltage'is transferred by the tuned transformer 5 to the detector tube 6, the transformersecondary being shunted by, a voltage divider l for adjustmentof the detector input voltage E0, as will be hereinafter described.

' The audio frequency voltage Ea is developed acrossan appropriate'plate impedance, such as the primary of an'audio frequency transformer 8, and the secondary'is preferably tapped, as shown at 9, or otherwise provided with adjustable means for'regulatingthe audio frequency energy impressed upon the single amplifier tube I!) which works into the loud speaker or other reproducer II. Y

As shown, the detector 6 is of thethree electrode type provided with acontrol grid G, anode or plate P and electron emittingcathode E which may, as is now wellunderstood, be of either the direct current or alternating current filamentary typ or may e. 0i the indirec l hea edprsw condenser C is connected between plate and fila-,

P TENT Y OFFICE??? I ment, and appropriate sources of grid bias and 7 plate potentials E0 and Eb, respectively, are'provided to secure plate circuit rectification;

In such anarrangement-if arnodulate'd radio frequency signal of the carrier-wave symmetrical side-band type, which maybe represented by:.

V e' E b(l+m sin at) sin wt, where 'Eo=impressed radio frequency carrier'volt'ag'e m=coefiicient of modulation a=frequency of audio modulation =angular velocity of the radio frequency current is impressed upo'ri the detector input terminals as indicated, and the output 'audio frequency voltage across the plate impedance is observed and plotted as ordinates against the radio fre quency carrier Voltage E0 as abscissae, a curve similar to that marked F in Figure 2 is obtained. The particular curve F shown inFigure 2 was plotted from actual experimental data obtained with a vacuum tube of radio [frequency "carrier voltage E0 is small the detector, 'operateslon the curved portionyof the plate current cliaracteristic,v and the response; as determined by audio frequency output voltage,

is proportional to the square ofthe impressed carrier voltage." Thus, the slope of curve 3', up

scale. The relationshipbetween signal voltage the 20l-A type," using logarithmic coordinates; 'When the impressed andjharmonic'sf is represented graphically by curve H of" Fig. 2;

In the prior artthe detector has ben operated at low signal voltages. In the conventional broadcast, receiver comprising a grid-rectifying detector succeeded by a two stageaudio amplifier, the normal output, as defined by the Standardization Committee of the Institute offRadio Engineers, "is 20 v olts across a resistance'of 5000 ohms in the plate circuit of the output tube. For? a mu of 5' and internal resistance of 5000 ohms in this tube this would correspond to 8 volts on the grid, and for an amplification of about persta'ge this wouldcorrespond ,to 8/625 or 0.013 volt vaudio on the detectorgrid. If the average modulation is 25% the corresponding radiofrequency carrier-vo1tage would be about so,- I

023- volt. Thisis Well within the small signal I region in which the detector" responds according to the square-law. It is also to be noted that in a conventional receiver of this type it will not be possible to get out of the square-law region because with this amount of audio amplification the output tube will overload before the carrier voltage on the detector grid has increased suificiently to do so. In order to get out of this region it is generally necessary to reduce the audio amplification, for example, by removing one audio stage, as hereinafter described.

The disadvantages of such square-law detection have been discussed in United States Patent No. 1,698,668, issued January 8, 1929,"to Ballantine and Hull. Briefly with a modulated carrierwave signal of the type e Eo(l+F(t) )sin wt the output of such a detector contains not only a term proportional to F(t) as it should, but also a second order F 6) term which is superfluous and represents distortion. If F(t) is of the form m sin at where m is the degree of modulation (0 to 1.0, or 0 to 100%), the distortion will be represented by a second harmonic, or double frequency term. If we define the distortion as the ratio: I

i L M where En represents the amplitude of the nth harmonic and E1 is the amplitude of the fundamental the distortion in the case of square-law detection with the above modulation is equal to In order to keep down this distortion the degree of modulation m must be limited, that is m should be kept small compared with unity. This is uneconomical and wasteful of power in the carrier, and limits the service area of a given broadcast transmitter for fixed interference production by the carrier. In'the early days'of broadcast transmission the transmitters were seldom capable of greater than 50% modulation. In English speech the ratio of the peak voltage to the average voltage is about 5, hence if the peaks are limited to 50% the average modulation will be about 10%. With such low modulation the distortion might. not be noticeable, and this would account for the complacent attitude with which the process of square-law detection has been viewed'by experts in the prior art. With 100% modulation, however, these detectors are not satisfactory because the distortion is too great. I have therefore sought a' detector which will respond, not as the square but asthe first power of the applied voltage, in order to provide satisfactory reception with complete (100%) modulation. v

In the previous UnitedStates patent to Ballantine and Hull, cited above, it is shown how with a rectifier which has an EI characteristic comprising two slightly curved branches meeting in a section of greater curvature, an approximation to linear detection can be achieved by increasing the signal voltage applied to the device. In the present case of a thermionic device'we do not have two nearly straight branches, but we dohave at least one perfectly straight branch, i. e., the part of the curve of zero current along the axis for retarding fields. I have discovered that with such a device, having at least one straight branch the response approaches linearity as the signal voltage is increased and more of the straight portion is utilized during the cycle.

The experimental proof of this is contained in Fig. 2, in which the slope of the response curve at points A and B is shown by broken lines a, b, respectively. As the applied carrier voltage is increased beyond the region in which the detection is according to the square-law, that is beyond the point A, the slope of response curves gradually decreases until at the point B the slope of the curve is unity, indicating linear response. I propose to take advantage of this fact by amplifying and adjusting the signal'voltage so that operation takes place at this point, thereby obtaining minimum distortion.

In making the foregoing curves a modulation of 20% was employed, and it will be seen that for small signal voltages the distortion approaches as shown at the left-hand end of the distortioninput voltage curve D. As E0 increases, however, the distortion diminishes when the exponent in the law of response stated above begins todecrease from 2 to 1. Thus at the voltage E0 shown at B, at which point the response is linear, the distortion is a minimum, and is seen to have decreased from 5% to 57%. For higher values of impressed carrier voltage than those indicated at B, i. e., in the regionof overloading, the distortion increases rapidly, and attains values'which are in fact much higher than the distortion for" small signals, the maximum value of the distortion curve in Fig. 2 being 50%. Y 7

I have found that there is no practical difiiculty in adjusting the signal voltage E0 to the optimum point B by manual control such as the voltage divider 7; nevertheless it is desirable in apparatus designed for commercial use to make the adjustment for linearity as easy as possible. In my copending application, Ser. No. 354,022, filed April '10, 1929, of which the present application is a division, I have described and claimed a method of extending the range of carrier voltage over which the response of the detector is approximately linearr I In Fig. 3, I have shown a receiver system in which the carrier voltage may be adjusted and maintained at its optimum value'by an automatic volume control such as described in my co pending application, Ser. No. 231,273, filed Nov. 5, 1927. As in the receiver shown in Fig. 1, there is no audio amplifier stage between the detector and the power tube since relatively high voltages are to be applied to the detector and the power tube would be greatly overloaded if the conventional audio amplifier system were employed.

The radio frequency amplifier is represented by a single triode stage; this is symbolic only. L1 L2 represent the input transformer tuned by the condenser C1; '1: is the detector tube arranged for plate rectification and is here shown as a triode; C4 is a by-pass condenser for radio-frequency currents; T is an audio frequency transformer; T3 is a power tube which supplies. power to the electrophone LS.

The automatic volume control which, in accordance with this invention, is to be adjusted to effect operation of the detector T2 within the range of substantially linear response, comprises a triode rectifier tube T4 which is actuated, through the isolation condenser C3, by the signal impressed upon the detector. The plate circuit of the control tube T4 includes a resistance R2 and high tension battery E1, the direct current potential drop across resistance R: being impressed as a grid bias upon the radio frequency amplifier tube T1 while the modulation frequencies are by-passed by condenser C5. With this arrangement, the bias voltage upon the radio frequency amplifier will vary automatically with the strength of the received signal and, by a suitable choice of the circuit constants, the grid bias and thereby the amplification of tube T1 may be so controlled that the amplified signal voltage impressed upon the detector may be maintained substantially constant. By means of the adjustable bias voltage E2 and grid leak R1, the operation of control tube T4 may be adjusted to any carrier voltage level, and, in accordance with this invention, the adjustment is set at the carrier wave voltage corresponding to the linear response range of the detector.

The output from the reproducer LS is adjusted, after setting the automaticrvolume control for linear detection, by adjustable control means S1.

The circuit arrangements herein described show certain embodiments of my invention and certain methods of operation for the purpose of explaining its principle and showing its application, but it is obvious to those skilled in the art that there is a great variety of arrangements which may be employed for producing the desired result and I aim therefore to cover all such modifications and variations. I

I claim:

1. In the operation of a detector of the type having a restricted range of linear response lying outside of the range of square-law response and immediately short of the point of overloading, the method of securing linear response which comprises amplifying received signal energy, impressing the amplified energy upon the detector, and adjusting the amplification rate to bring said amplified energy Within the restricted range of linear response. V

2. The invention as set forth in claim 1 wherein the rate of amplification is automatically controlled by the magnitude ofzthe signal energy to maintain the amplified" signal energy impressed upon said detector within the limited range of linear response. r Y

1 3. In the reception of modulated carrier wave signals with amplifiers and a detector of the vacuum tube type, said detector having a restricted range of linear response lying beyond an extended range of square law response and immediately short of the point of overloading, the method of reducing distortion which comprises amplifying received signal voltages to values in excess of those corresponding to square-law response of the vacuum tube detector, impressing said'amplified signal voltages upon the detector, and impressing upon the last amplifier stage of the receiver the unamplified audio frequency voltages developed in the detector stage.

4. In a carrier wave receiving system, the combination with a detector and associated elements effective to secure, over an approximately point range of impressed carrier voltages, a linearrelation between output current and carrier voltage, of means for amplifying received signal voltages to values within and in excess of said range,

andmeans for adjusting the amplified signal voltage impressed upon said detector to fall within said range.

5. In a radio receiving system, the combination with a detector stage comprising a vacuum tube, of the type having a restricted range of linear response lying beyond an extended range of square law response and immediately short of the point of overloading, circuit elements and means for applying to said tube energizing potentials effective to secure a linear response over a range'of carrier voltages, of an amplifier for amplifying received signal energy, and means automatically controlling the amplification of saidamplifier to maintain thevalue of the amplified signal voltage impressed upon said-vacuum tube detector within the said range of linear response.

' 6. In a radio receiving system, the combination of a vacuum tube and circuit elements-associated therewith to form a detector stage, said tube having a limited range of linear response lying between an extended range of square law response and the point of overloading, an output tube coupled to saiddetector and actuated by the unamplified audio frequency response developed in said detector stage, of a vacuum tube amplifier having a maximum gain adapted to amplify received signal voltages to values in excess of those corresponding to square-law response of said detector, and means automatically adjusting the gain of the vacuum tube amplifier as a function of received signal energy to maintain the detector input within that voltage range for which said detector has a linear response.

STUART BALLANTINE.

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