US2053076A - Modulation analyzer - Google Patents

Description

Sept. 1, 1936. v D. GlMEsEq-,AL v2,053,076

MODULAT I ON ANALYLER Filed April 251932 Ili- INVENToRs /F-n I -DAVID GRIMES w1 ATroRNEY Patented Sept. 1, 1936 Y UNITED gsifli'rlas PATENT .OFFICE 2,053,076 l Y n l Island, N. Y., assign ors to Radio Corporation f of America, a corporation of Delaware Application April 26,

8 Claims.

Our present invention relates to methods of measuring percentage modulation, and more particularly to an improved method of measuring percentage modulation of modulated signal fi energy.

While it is often convenient to measure radio frequency or intermediate frequency gain with an unmodulated signal, the case of a modulated signal is important because of receiver sensitivity and over-all performance measurements. For this reason, and due largely to the considerable amount of interest which is frequently noted in the matter of signal structure, the present invention concerns itself with signal analysis in general, and with a method of measuring percentage modulation in the usual cases. As is well known, a standard modulated signal comprises a carrier and two side components whose properties can be determined by considering an ideal modulator. c

`In such a case, a pure carrier potential is* applied'in the control grid circuit of a single stage amplifier whose output circuit is tuned to the carrier frequency, and is sufficiently broad to present a tuned impedance which is substantially a pure resistance of constant value over a band of frequencies including the two side components. This ideal modulator is conceived to exhibit a strictly linear relation of radio frequency output to the low frequency control potential applied for example in the screen or plate circuit. 'Ihe modulating potential is superimposed on `the steady screen or .plate voltage.

It can, then, be theoretically and physically demonstrated that in this ideal case the gain of the stage is controlled at the modulatingrfre-r quency, modifying the output at any instant with an intended and systematical variation of relatively low periodicity, although this ratev of variation need not be restricted to a relatively slow time function. Mathematical analysis -of the aforementioned ideal case reveals that modulation has not altered the carrier, but has developed two side components which are equal. in magnitude, and bear frequency symmetry with respect tothe carrier.

This method of considering the effect of modulation on the carrier, with consequent development of two side components of equal magnitude, has limited utility, but is a convenience in some cases, especially where Vcertain complications arise. Occasionally, a vectorial conception of modulation is of great assistance. Thus, each of thecarriers, upper side component and lower side component may be represented by a vector 1932, seri-a1 No. 607,536

having a constant magnitude, and rotating with a constant angular velocity.

It is customary,A and proper, to employ the carrier vector as a reference line, consider it to be stationary, and regard the side component vectors tov rotate in opposite directions with 2m as their angular velocity; that is to say, these vectors may be considered as rotating at the audio. modulating frequency, instead of at their absolute radio frequencies. From this conception there can be derived the subsequent conclusion that the aforementionedl threevvectors vhave a resultant vector instantaneous magnitude.

Now, we have found that as far as observable results with any known device as concerned, the degree of modulation can be determined by measuring the side components and the carrier, indi` vidually. `Such av methodis employed in the signal analyzer of the presentinvention wherein a piezo-electric crystal is utilized as ameans'of suf? ciently high yselectivity to discriminate-,against the carrier and the lower side component, `Vfor example, whenthe upper'side component is being measured. vl g Briey, it may be stated that the present'signal analyzer embodies a device kfor adjusting a local oscillator frequencyV to beat with any desired? signal componentv at the-frequency of crystal res onance. Thus, the `output of a conventional first detector contains a particular component whichis selected by they crystal, amplified, and meas'- ured by a tube voltmeter. `This process is per--V Total side component magnitude y Caz-riet magnitude This fraction is readily evaluated from the data afforded by the signal analyzer, and, thus, the modulation indicating device Von a signal gener.

ator may be calibrated, vdue regard to certain fol-v lowing considerationsbeing paid. Thesel are interesting from the standpoint of signal structure in general, as well as from the standpoint of modulation measurement.

It iswell recognized that many signal genera` tors depart from ideal production of a modulated carrier wave. Hence, it is essential to consider the'eifects of these departures on the simple analyzer method of measuring percentage modulation, as disclosed in the present application. A signal generator may introduce frequency modulation, in conjunction with the legitimate amplitude modulation, and may introduce side component phase distortion due to resonance phenomena embodied in the design for good reasons. Harmonics of the modulating tone may be pres-l ent, at the modulator. Any, or all, of these eX- traneous effects are capable of causing the foregoing definition of M to be in error. f, f

Suppose a signal generatorto have side com-f ponent phase distortion as its only essential defect as far as modulation is concerned, and consider the modulated signal to exist in a selective circuit with the carrier on resonance. The side components are off resonance, and therefore suffer aphase vshift relative to themselves in a pure resistance device. However, it'can be readily demonstrated that the' matter of phase'distortion due to usual tuned circuits, which are purely resistive to the carrier and resistive and reactivel to the'upper and lower side components, is regarded as being of no consequence. vOi? course, side component attenuation, relative to the carrier in the resonator, decreases the' percentage modulation.

Since phase is ignorable, the analyzer reveals the actualV percentage modulation of delivered signal. At 400 cycles per second, side component Aattenuation due to resonance is probably unim portantv in most cases. In the oase of 5000 cycles per second modulation, as involved in receiver over-all fidelity testing, side component attenuationV due to resonance may decrease -the original percentage modulation by 25%. The present analyzer determinesthe actual percentage modulation of the radio frequency output, which is the desired information.

When harmonics of the modulating toneY are impressed, the system being otherwise ideal for example, each modulating component sets up a pair of rradio frequency side components independently of anyV other modulating" components present. It can be shown that in the'absence of frequency, or phase, modulation, the usual harmonic content'pres'ent in the modulating potential is not a source of material error, except when receiver over-all distortion measurements are to be taken. In this case the analyzer of the present invention maybe used to advantage as a of investigation. l n

Of course, pure monotone modulation is striven for. Frequency, or phase, modulation effects are commonly referred to inconnection with signal generators, particularly when the oscillator is' modulated directly. These phenomena aremost easily avoided by modulating a radiofrequency stage. The presence of frequencymodulation can be readily determined by the analyzer described in the'presentapplication, by virtue of unequal magnitudes of corresponding side components.

It remains onlyl to determine whether or not the foregoing definition of M is valid when they means Yly that of a standard signal as regards phase.

In the former case, lower than critical modulating frequency, one of the side components is reversed .in polarity due to over-neutralization.

The variable-modulating frequency procedure is, thus, seen to be a convenient and trustworthy method for arriving at the orientation which is in some cases necessary to rigorously arm signal structure, and evaluate M by means of the data afforded by the simple analyzerV of the present invention. c Y

Hence, it may no-w be stated that it is one of the prime objects of our present invention to provide a signal structure and percentage modulation analyzer which essentially consists of a sharply discriminating resonator disposed between a frequency changer device and a beat frequencyamplier, the resonator being utilized as a means of sufficiently high selectivity to discriminate against the carrier and one of the side components when the other side component, for example, is being measured.

Another important object of the present invention is to 4provide a signal wave analyzer, readily adapted for the determination of percentag-e modulation of a signal wave, the analyzer being constructed and arranged to measure the ratio of total side component magnitude of the Wave to the carrier magnitude of the wave, the said ratio comprising a measure of the percentage modulation of the wave, the analyzer consisting of a local oscillator electrically coupled with a frequency changer, an intermediate frequency amplifier, a subsequent tube voltmeter, a sharply selective resonator being utilized between the output of the frequency changer and the input ofthe intermediate frequency amplifier. Y

Another object of our present inventionv is to provide a signal wave analyzer which essentially comprises an electrical receiving circuit operating :on the heterodyne principle, land utilizing a piezo-electrical coupling device between the frequency changer circuit and the beat frequency utilizing circuit, a pair of coupled resonant circuits being employed subsequent to the piezoelectric cou'pling to eliminate inherent spurious resonance crystal frequencies separated from the desired resonant frequency of the piezo-electric coupling device by a relatively slight number of cycles.

Still other objects of the present invention are to improve generally,l the simplicity, accuracy,

and efficiency of signal wave analyzers, and to especially provide a signal wave percentage mod-v ulation analyzer which is not only durable and reliable in operation, but economically manufactured and assembled.

The novel features which we believe to be characteristic ofY our invention are set Yforth in particularit'y in the appended claims, the invention itself, however, as to both its organization and method of operation will best b-e understood by reference to the following description taken in connection with the drawing in which we have indicated diagrammatically one circuit arrange- `input to be measured.

ment whereby our invention may be carried into effect. Y

. Referring now to the accompanying drawing, which diagrammatically shows an analyzer capable of carrying out theaforementioned objects and purposes, there is showna pair of terminals I, 2 .adapted to be coupled to the radio frequency In other Words, the terminals I, 2 maybe connected to the output of a signal generator, or they may be even connected to an antenna circuit where the signal wave to be analyzed is radiated from the signal rWave source. An electron discharge tube 3, preferably of the screen grid type, has its control electrode, or grid, adjustably connected tothe terminal I, while the cathode is connected to ground through a path including a conductor 4, an inductance coil 5, and a condenser 6, the latter having a magnitude of about 0.1 mi., the con denser being shunted by a resistor 1 having a magnitud-e of about 30,000 ohms. They resistor I supplies bias for the grid of tube 3.

A grounded variable resistor 8, functioning as a gain control device, is shunted across the input electrodes of tube 3, the resistor having a magnitude of about 3 megohms, the grid of tube 3 being adjustably connected to a'poi'nt on resistor 3. In order to check the drift of the analyzer, a switch 9. is` connected in shunt with the adjustable resistor 8. The anode oftube 3 is connected to the. positive terminal of a source of potential (not shown) capable of supplying 135 volts to the anode.

The connection between the said source and the anode of tube 3 comprises a path consisting of the conductor I0, the resistor II and the conductor I2. The screen grid electrode of the tube 3 is connected, through a path which includes the conductor I3, to a point on the aforementioned source of potential which maintains the screen grid electrode at a4 potential of 67.5 volts. It will be noted that the negative terminal of the aforesaid potential source is noted by the designation B- and that it is connected to ground.

The tube 3, and its associated circuits, cornprises the frequency changer device of the analyzer, local oscillations being impressed upon the frequency changer from a local oscillation source which comprises an electron discharge tube I 4, preferably a 238 pentode tube having an indirectly heated cathode, it being pointed out that al1 the tubes shown in the present analyzer employ indirectly heated cathodes. The, pentode oscillator circuit described herein isrmore fully disclosed in a copending application Serial No. 592,461, of W. S. Barden, iiled February 12, 1932. 'I'he anode of tube I4 is connected to be operated at a potential of 135.Volts through a path which includes the conductor I 5',. the coil I1, the conductor I3 and the conductor I2. n

'Ihe control electrode of tube I4 is grounded through a path which includes the feed-back in- Vductance coil I6 and the lead I5", it being noted second' switch 22 is connected in series with the' Vernier condenser 20. The function of these switches, main and Vernier condensers will be described at a later point. Theuse'of the abovev named'pentode'` oscillator circuit results in substantial elimination .of undesired second harmonics and. zero grid current flow in theoscil-` lator.y

The cathode of the tube I4 includes in series therewith a grid biasing resistor'24, having a magnitude of about 850 ohms, the resistor being shunted by a fixed condensery 25 of a magnitude of about 0.2 mf. The screen electrode of tube I'4 is connected by a lead I2 to the 135 volt source, and the suppressor grid is connected to the cathode within the tube in the usual manner. Oscillations produced in the tuned circuit I'I, I8, are impressed on the coil 5, through coupling. M3; thus injecting the local oscillation frequency into input of tube 3. i

The output circuit of the tube 3 is coupled to the input circuit of the intermediate frequency amplifier tube 26 through a sharply selective resonant circuit. The sharply selective resonant circuit comprises a resistor I I, the high potential side of the resistor being connected to the conductor I Il, and the low potential side of the resistor being connected to the conductor I2. The control electrode of tube 26, and the indirectly heated cathode of tube 26, are coupled across resistor I I, by means of a piezo-electric crystal 29, preferably of quartz. The quartz crystal 29 is cut to be resonant to a frequency of kilocycles, and is disposed between four metallic electrodes, a pair 30 of the said electrodes being disposed adjacent one end of the crystal, while the remaining pair 30 are disposed at the opposite end of thecrystal.

The coupling arrangement of the crystal 29, and its location with respect to the four electrodes are well known to those'skilled in theart, and it is w`e11 recognized that such a coupling arrangement is sharply selective with respect. to the-frequency transmitted through the coupling. The pair of electrodes 30 are connected to opposite sides of the resistor II through a fixed direct current blocking condenser 28, each of the fixed condensers having a magnitude of 0.00025 mf. The remaining pair of electrodes 30 are connected to opposite sides of a resistor 3|, the latter being connected in shunt between the control grid andthe grounded side of the cathode of tube 26, the resistor 3| having a magnitude of about 3 megohms.

' The screen grid electrode of tube 26 is connected by means of a conductor 32 to the conductor I3, the latter being connected to the screen grid electrode of tube 3, and is thus arranged to have a positive potential of 67.5 volts applied thereto.

It will be noted that the anode of tube 26 Vhas a positive potential of 135 volts applied thereto through conductors I2 and v33, the primary coil 34 of the transformer coupling M2, and the con.- duct'or 35. Theindirectly heatedr cathode of tube 26 includes, in the grounded leg thereof, the usual' biasing resistor 36 having a magnitude of about 400 ohms, the resistor being shunted by a fixed condenser 3'I having a'magnitude of 0.1 mf. It will be noted that the anode supply potential lead lI2 and the screen grid potential supply lead I3 are shunted to ground by the fixed condensers' 38 having magnitudes of 0.2 inf. In order to preventl undesired radio frequency coupling between the Y output and input circuits of tubes 3 and 26, there isfdisposed a grounded metallic shield 39 between the electrodes 30 and 30.

The output circuit of tube 26 and the input circuit of the electron discharge tube 33, which isy preferably a' triode detectorV havingv an indirectly heated cathode, are coupled by a sharply resonant circuit comprising the primary coil 34 of transformer M2, shunted by an adjustable condenser 40, and the secondary coil 4I shunted by an' adjustable condenser 42. A source of negative grid bias voltage C is connected between the grounded side of the cathode of tube 39 andthe grid ofthe tube so that the tube 39 and associated circuits function as a second detector. The anode circuit of tube 39 includes the micro-ammeter 43 to indicate the carrier, or side, component magnitudes, the anode being connected by a conductor 44 to a point on the source o'f anode potential which will maintain the anode at volts. The meter 43 is shunted by a variable resistor 45 including in series therewith a source of current46.

Considering, now, the operation of the analyzer shown in the drawing and described in detail heretofore, Vthe local oscillator frequency is adjusted by means of the main condenser I8A and either of the Vernier condensers 20 or 'I9 to beat with any desiredV signal component Yimpressed across the input electrodes I, 2, at the frequency of crystal resonance. Thus, assume that a carrier is modulated by a monotone, and that it is desired toy measure the percentage modulation of the resulting modulated carrier. In the case of more than one modulating tone, additional side bands can be measured and total modulation .determined. The magnitude of the carrier would first be determined by adjusting the large condenser I8 to the setting which will beat with the carrier frequency tov produce an intermediate frequency of approximately 50 kilocycles, the frequency at which the Width of the crystal 29 is resonant. Any operation of the crystal at the thickness resonance Which would be likely to occur at some frequency of the oscillator is precluded by theV resonant shunt path l0 Il to ground. This path is resonant to the thickness frequency of the crystal. At this setting of the condenser I8, the reading on the meter 43 is taken, and this reading indicates the magnitude of the carrier.

It should be noted that each of the condensers 4E) and 42 are adjusted to resonate'their coupled resonant circuits to the desired intermediate frequency. The function of the coupled resonant circuits between tubes 26 and 39 is to remove any spurious responses which might be caused by multiple crystal resonant frequencies near the legitimate crystal resonance. These undesired resonant peaks result from slight irregularities in grinding of the crystal but are far enough off proper resonance to be effectively removed by the resonant coupled circuits.

The tube 39 and its associated circuits actually comprise a tube voltmeter, Vand the function of the Variable resistor 45 and potential source 46 is to Ybalance out of the micro-ammeter 43 all permanent direct current so that the meter Will be free to indicate slight changes in the plate current caused by the incoming carrier or side bands. Assume, now, that the side component magnitudes are to be measured. One of the switches 2l or 22 is closed and the associated vernier condenser is adjusted to the desired side component. At the desired side component frequency the reading .of meter 43 is taken. This reading gives the magnitude of said side component frequency without the value of the carrier, as the carrier frequency is off the crystal resonance frequency and cannot pass through the crystal.

In this case, the local oscillator has been adjusted to that frequency which will beat with the side component frequency to produce the 50 kilocycle intermediate frequency. It will be found in employing theanalyzer that one ofthe Vernier condensers can be employed for the lower frequency side bands, While the Vother one can be Yemployed forthe. higher frequency Aside bands.

After the meter reading of the carrier C has been obtained, and thereadingof the lower side component S1 alone and the .upper side component S2 have alone been secured, the percentage modulation is equal to: l Y

The square root of eachfdirect current reading is used in this formula because the tube 39 is Very nearly a true. square law detector throughout the intended range of applied signal intensities.

It Will thus be seen that the output of the first detector tube3 contains a component which is selected by the crystal, amplified, and measured by the tube voltmeter. This process is performed individually on the carrier and each side component, thus determining Vtheir relative magnitudes. The tube voltmeter may be accurately calibrated. If not, such a'device, when square law, as is usual, gives the squarerof the proper reading. Hence, the actual readings are obtained by a square root process.

While we have indicated and described one arrangement for carrying our invention into effect, it will be apparent to one skilled in the art that our 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 our invention as set forth in the appendedclaims.

What We claim is: f

1. A method of determining percentage modulation of amodulated carrier wave which includes beating the modulated Wave with local oscillations to produce a predetermined beatfrequency, transmitting the beat frequency energy through a sharply selective path, measuring the intensity of vthe beat frequency energy, and adjusting the frequency of theA local oscillations toselectively produce said beat frequency energy for the carrier frequency and theside band frequency in the absenceof the carrier frequency, consecutively. K

2. Amethod of 'analyzing a modulated carrier wave including heterodyning the Vcarrierof theV modulated 'carrier wavewith local oscillations of a frequency sufficient to produce a predetermined intermediate frequency, amplifying the latter, detecting the amplified intermediate frequency energy, indicating the intensity of the Vdetected energy, and laterproducing intermediate frequency energy by heterodyning, said local oscillations with either of the side componentsonly of said modulated Wave.

3. A signal `analyzer comprising a'screen grid tube having an input circuit and an output circuit, vmeans in `the input circuit of the tube adapted to be connected to a signal wave so as to be analyzed, a local oscillator comprising a tube having its output circuit coupled to the input circuit of said screen grid tube, an intermediate frequency amplifier Y comprising a `screen grid tube providedrwith an input circuit and an output circuit, a coupling network connected between the outputcircut of said first. screen grid tube'and saidsecond screen griditube and comprising a piezo-electric couplingdevice resonant to the desired intermediate frequency, a tube voltmeter circuit comprising an electron discharge tube provided With an input circuit coupled to the output circuit of said intermediate frequency amplifier tube, the output circuit of said electron discharge tube including a visual indicator,

4. A signal analyzer comprising a screen grid tube having anY input circuit and an output circuit, means in the input circuit of the tube adapted to be connected toa signal wave so as to be analyzed, a local oscillator comprising a tube having its output circuit coupled to the input circuit of said screen grid tube, an intermediate frequency amplier co-mprising a screen grid tube provided with an input circuit and an output circuit, a -coupling network connected between the output circuit of said rst screen grid tube and said second screen grid tube and comprising a piezo-electric coupling device resonant to the desired intermediate frequency, a tube voltmeter circuit comprising an electron discharge tube provided with an input circuit coupled to the output circuit of said intermediate frequency amplifier tube, the output circuit of said electron discharge tube including a visual indicator, and a tuned circuit in the output circuit of said intermediate frequency amplifier tube, said last mentioned tuned circuit and the tuned input circuit of said electron discharge tube being coupled and resonant to said desired intermediate frequency.

5. A signal analyzer comprising a screen grid tube having an input circuit and an output circuit, means in the input circuit of the tube adapted to be connected to a signal Wave so as to be analyzed, a local oscillator comprising a tube having its output circuit coupled to the input circuit of said screen grid tube, an intermediate frequency amplifier comprising a screen grid tube provided with an input circuit and an output circuit, a coupling network connected between the output circuit of said first screen grid tube and said second screen grid tube and comprising a piezo-electric coupling device resonant to the desired intermediate frequency, a tube voltmeter circuit comprising an electron discharge tube provided with an input circuit coupled to the output circuit of said intermediate frequency amplier tube, the output circuit of said electron discharge tube including a visual indicator and a tuned circuit, resonant to an undesired crystal thick-L ness frequency, connected in the output circuit of said first screen grid tube and arranged in shunt with said piezo-electric coupling device.

6. A signal analyzer comprising a screen grid tube having an input circuit and an output circuit, means in the input circuit o-f the tube adapted to be connected to a signal wave so as to be analyzed, a local oscillator comprising a tube having its output circuit coupled to the input circuit of said screen grid tube, an intermediate frequency amplifier comprising a screen grid tube provided with an input circuit and an output circuit, a coupling network connected between the output circuit of said first screen grid tube and said second screen grid tube and comprising a piezo-electric coupling device resonant to the desired intermediate frequency, a tube voltmeter circuit comprising an electron discharge tube provided with an input circuit coupled to the output circuit ofsaid intermediate frequency amplifier tube, the output circuit of said electron discharge tube including a visual indicator, and said local oscillator tube including in its input circuit a variable tuning condenser and one or more Vernier condensers.

7. In combination in a percentage modulation analyzer, a source of local oscillations comprise ing an electron discharge tube having an input circuit including a device for varying the frequency of oscillations produced, a frequency changer circuit including an electron discharge tube having its input circuit coupled to said oscillator output circut, a beat frequency amplifier, a piezo-electric element arranged to couple the output circuit of said frequency changer tube an-d the input of said amplifier, said piezo-electric device being sharply resonant to a desired beat frequency, a circuit resonant to a frequency different from said beat frequency coupled to said piezoelectric device and arranged to prevent it from resonating at a frequency other than said beat frequency, a detector circuit including an ammeter having a graduated scale in its output, and a pair of resonant circuits, tuned to said beat kfrequency, arranged to couple the output of said amplifier to the input of said detector circuit, and means connected in the output circuit of said detector for adjusting the operation of said ammeter.

8. A device for measuringthe percentage modulation of a modulated carrier wave by comparing the readings of a graduated ammeter comprising, the combination of a vacuum tube frequency changer having an input and an output circuit, a second detector having an input and an output circuit, means for impressing a carrier frequency modulated by a single audio frequency on the input circuit -of sai-d frequency changer, a source of local oscillations coupled to the input circuit of said frequency changer, a circuit network coupling the output circuit of said frequency changer to the input circuit of said second detector and including a piezo-electric crystal resonant at a relatively low frequency and whose selectivity is. such that it will transmit the beat frequency formed by the local oscillator and carrier frequencies but will block the beat frequency formed by the local oscillator and the sum frequency of the carrier and modulation frequencies,

and a graduated ammeter connected in the output circuit of the second detector.

DAVID GRIMES. WILLIAM S. BARDEN.

Images