US2421724A - Method and means for measuring power output of high-frequency signal generators - Google Patents

Description

June 3, 1947. v. A. STEWART OF HIGH FREQUENCY SIGNAL GENERATORS METHOD AND ME NS FOR MEASURING POWER OUTPUT Filed Oct. 13, 1944 IN V EN TOR.

BY EJ222204?! MAL m Patented June-3, 1947 POWER OUTPUT OF HIGH-FREQUENCY SIGNAL GENERATORS William A. Stewart, Philadelphia, Pa., assignor.

by memo assignments, to Philco Corporation, a

corporation of Pennsylvania Application October 13, 1944, Serial No. 558,493

12 Claims.

This invention relates to an electrical apparatus and more particularly to a system and method of measuring power and power output of an unknown signal generator. In many instances, it is essential to measure the amount of power at a 'pre-determined frequency in a circuit or being supplied by a generator. It is difficult to supply energy at a fixed'critical frequency in high frequency operation due to vagaries of the system and instability of the oscillator. This is particularly true if the system includes sharply resonant high Q" circuits and operates at low energy levels. Rather than attempt to supply energy at one desired frequency, this invention provides means whereby the frequency is varied over a pre-determined range which range includes the desired or critical frequency at which power is to bemeasured. Inasmuch as power is a function of energy and time, it is clear'that the rate of sweep over the frequency range will be an important factor.

Furthermore, an oscillator having a variable frequency may have the frequency deviation and deviation rate controlled to distribute its energy over a predetermined frequency spectrum. Thus, any desired fraction of oscillatorpower output may be disposed in any desired part of the frequency spectrum. After calibration with respect to some particular deviation range and deviation rate, the determination of unknown oscillator output powers is a mere matter of arithmetically proportloning the deviation rate and range,

In addition to power measurement, it is frequently desirable to calibrate the power output of a frequency generator. The invention hereinafter described permits this to be done in a simple and effective manner.

The invention in general provides an oscillator whose frequency may be varied over a Dre-determined range at a pre-determined rate. The output may be fed either in its original form or in the form of a derived frequency to a sharply critical resonant circuit and the selected frequency may then be applied to suitable power measuring means. The oscillator whose power isto be calibrated may then be substituted for the first-named oscillator, and either this substituted oscillator or the remainder of the system is so adjusted that the original or derived frequency of the substituted oscillator will pass through the resonant circuit. The calibration may be carried out in a simple manner in terms of the known characteristics of the sharply resonant circuit, the range of frequency variation or iii deviation rate. It is also possible to measure the insertion loss of a passive networkby taking measurements with the passive network included in the energy path between the oscillator and sharply resonant circuit. The insertion loss may .then be calibrated in terms of frequency devlation and deviation rate.

For a further description of the invention, reference will be made to the drawings wherein Figure 1 is a circuit diagram embodying the invention and Figure 2 shows some curves illustrating the operation of the system.

An oscillator generally designated as I is adapted to have its frequency varied by means generally indicated as H. As illustrated in the drawing, oscillator I0 is of the electron. coupled type and the frequency thereof may be varied by a reactance tube system. Oscillator II! as illustrated here comprises multi-electrode vacuum tube I! having control grid l3 connected through blocking condenser II to tank circuit lb. Tank circuit [5 comprises inductance l6 and variable condenser ll connected across the inductance.

deviation and the rate of frequency variation or 88 Tank circuit l5 has lower terminal l8 grounded. From intermediate point IQ of inductance i6 line 20 goes to cathode 2| of vacuum tube l2. Cathode 2! may be energized by suitable heater 23 fed by any source of current. Both terminals of the heater supply are preferably by-passed by grounded condensers 24 and 25. Accelerating electrode 26 in tube i2 is maintained at a. suitable positive potential by connection through line 21 to junction point 28 between a series of resistances 29 and .30 and connected between B plus and ground. Vacuum tube :IZ has anode 32 energized through radio frequency choke 33 from B plus. Bypass condensers 34 and 35 are provided for grounding B plus and accelerating electrode 2-5 respectively for radio frequency.

In order to vary the resonant frequency of tank circuit l5, means are provided for introducing a controllable phase angle betweenvoltage and current. This means generally indicated by rectangle Ii is coupled to tank circuit I6 through line 31, the return circuit being through ground. Line 31 has resistance connected between it and junction point 3-5. From junction point 39, small condenser 40 is connected to ground- Resistance 38 is large compared to the reactance of condenser 40 for the radio frequency in tank circuit l5. Thus the radio frequency current through resistance 38 and condenser ID will be substantially in phase with the. radio frequency voltage across tan-k circuit IS. The voltage across condenser 40 will be substantially 90 degrees be hind the current.-

Line 3| is also connected through coupling condenser 4| to anode 42 of reactance tube 43. Reactance tube 43 is preferably of the multi-grid type and as shown here has cathode 45 connected to ground through suitable bias resistance 46 bypassed by condenser 41. Control grid 48 is connected back to junction point 39 and also is connected to ground through grid resistor 49. Screen grids 50 and 5|, these being grids number 2 and 4 respectively, are connected together to junction point 52, which point is bypassed by grounded' condenser '53 and these grids are maintained at asuitable positive potential through dropping resistor '55 connected to junction point 56 which is supplied with suitable 13 plus potential. Junction point 56 is connected to anode 42 of the vacuum tube through radio frequency choke '51. Suppressor grid '58, this being grid number 5, is connected in the usual fashion to cathode 45.

Modulating grid 60, this being grid number 3, in the tube is maintainedat suitable direct potential by means not shown and is modulated in a desired manner by audio frequency oscillator gen- .tion filter, or if the frequency is high enough, of

a cavity resonator. Short sections of transmission line for high frequencies may also be used. In the event that cavity resonators and transmission lines are used, ultra-high frequency oscillators may be necessary. In any event, filtercircuit 8| has a high Q and has an extremely sharp resonant characteristic. The output of filter 8| is fed to line 84 and thence to grid 85 of cathode follower tube 86. Grid '85 is provided with usual grounded grid resistance 81. Tube ,86

erally designated as 6|. Audio frequency oscilla- I tor 6| may have manual frequency control 82 for varying the frequency. Thus if the output is sinusoidal, oscillator 8| may be of so-called Wien bridge type, an example of which is shown on page 505 of Radio Engineers Handbook (1943 edition) by Terman.

The audio frequency output from 6| thus results in a variation of phase angle in line 31 at an audio frequency rate. This effectively varies the resonant frequency of tank circuit |5 which resonant frequency is preferably in the radio frequency range and may even be in the ultra-high frequency range.

For calibrating purposes, it may be desirable to provide means for indicating a particular frequency within the'range of frequencies existing within tank circuit l5. s may be done by loosely coupling to inductance I 6 pick-up coil 55 feeding wave meter =66. Wave meter =|i6is preferably of the type capable of substantial accuracy and may include a suitably calibrated crystal as part of the meter system. An example of a frequency meter which may be used is given on page 395 of 1944 edition of the Radio Amateurs Hand book. Other types of wave meters may be used. The modulated radio frequency of vacuum tube I2 is fed from anode 32 by line 68 through coupling condenser 68 and switch Hi to converter or mixer II. This may be any one of a number of well known circuits. Thus a pentagrid converter as shown on page 570 of Termans book may be used. Mixer ii is adapted to combine the output of, oscillator tube l2 wtih fixed frequency local oscillator I2. Oscillator 12 is of any suitable type such as shown in dotted rectangle In with the exception that the frequency is fixed at a predetermined value. Oscillator 12 may have its frequency either above or below the range fed from tube l2 in a manner well known in the superheterodyne art. The output of mixer 1| may be fed to control grid 14 of cathode follower 15. Grid 14 is provided with the usual grid resistance 16 to ground while cathode 11 of the vacuum tube is grounded through suitable load resistance I8. Vacuum tube 15 has anode 19 connected to a suitable source of B plus potential.

The output of cathode follower I5 is taken across load resistance '18 to line 80 and supplied of variable value.

has anode 88 connected to a suitable source of B plus potential while cathode 89 is connected to ground through load resistance 90. The output of cathode follower 86 is fed through switch 9|. In one position of switch 9|, terminal 92 is connected to anode 83 of diode -94. Diode 84 has cathode 95 connected through condenser 95 and limiting resistance 91 to ground. Volt meter 98 is connected across condenser 96,to ground to indlcate the potential existing across the condenser terminals. It is understood that the Various cathodes are suitably energized by heaters. Switch 9| in its other position is adapted to feed terminal I00 and thence grounded volt meter Hi Referring back to the mixer input, switch 10 is adapted to cut-01f oscillator I2 and supply the mixer with output from calibrated signal generator I02. Signal generator I02 may be any one of a number of radio frequency oscillators having desired stability and adapted to be manually adjusted for generating any one of a number of frequencies.

During operation, oscillator ID will generate an output whose frequency will vary over a pre-determined range at a pre-determined rate. It is understood that the actual frequency output of oscillator |0 may be varied in a manner different from that shown and for example may be varied by mere manual control of tuning condenser Thus a simple motor drive may be provided for oscillating this condenser at a predetermined rate, the rate being adjustable by some manual control over motor speed. This variable output will beat against the fixed output of oscillator 12 to provide a beat frequency This variable beat frequency is fed through isolating cathode follower l5 and then applied to high Q filter system -8|.

Referring to Figure 2, the dotted line curve shows the variable frequency output due to oscillator Ill. The actual value of the frequency may either be the output of oscillator In or the beat frequency output from mixer II. In any event, it is clear that the band covers a substantial range of frequencies. Within this range is a narrow band of frequency indicated by the full resonance curve. This narrow band indicates the frequencies at which filter circuit 8| is responsive. Thus as the variable frequency oscillates between the two extreme values, it passes through the resonant frequency of filter circuit 8|.

It is important that the speed with which the frequency output from oscillator I0 is varied should be low enough so that circuit 8| has suflicient time within which-oscillations may build up to full value. The higher the Q of circuit 8|, the greater the number of cycles will be required within which to build up maximum amplitude therein. In practice, the modulating frequency for oscillator -I may be anythingwithin the audio frequency range since at radio or ultra-high frequencies, only a few microseconds are required for the build-up process.

I The filtered output from 8| is applied through isolating cathode follower 86 to an integrating It is quite important that impedance matching be present between the mixer and the various components feeding into the mixer. Thus maximum energ will be delivered tocathode follower under all operating conditions.

In order to calibrate the entire system, it is necessary to determine the frequency range over which oscillator I0 operates. This is done by means of wave meter 66. The relative output of oscillator I0 may be plotted for-various'frequencies to obtain a curve such as shown in dotted outlines in Figure 2. It is also necessary to know the rate of deviation or modulation of output from oscillator l0. This may be done by measuring the frequency in line 31 from the reactance tube. The resonance curve of filter circuit 8I must also be known and this may easily be determined by adjusting oscillator I0 and reading indications either in meters IM or 88, whichever one happens to be on.

For calibrating in terms of absolute power, it will be necessary to provide some device which puts out a measured amount of power at a definite frequency. Thus a signal generator I02 which has been previously calibrated may be used as the standard. This signal generator feeds mixer II, it being understood that switch 10 is operated to cut-off oscillator I0. The frequency of the signal generator is such that the beat frequency supplied to filter BI is the resonant frequency of this filter. The energy then goes on through cathode follower 86 and is switched to meter IOI which may be of the hot wire type. The meter indication is noted. After noting the meterreading due to signal generator I02, oscillator I0 is adjusted manually to the same frequency as signalgenerator I02 and the output is fed to mixer II. It is understood that signal generator I02 is cut off.

Through wave meter 68, theequality of frequency output of oscillator I0 and signal generator I02 is assured. The output is fed as before and the indication in meter IN is again noted. The ratios of power from generator I02 and oscillator I0 will be in direct proportionto the meter readings. This calibratesthe power output from oscillator I0 at a pre-determined frequency as far as meter I0I is concerned.

In the event that oscillator I0 puts out more power than the calibrated signal generator and if it is desired to obtain the same meter reading, then the following procedure may be provided. Oscillator I0 may have. its frequency varied at a known rate and over a known frequency range.

The deviation or deviation rate or both may be adjusted so that the reading in meter IN is the same as that dueto calibrated signal generator 102. Then a simple arithmetical proportion involving the deviation and deviation rate will give in the following manner.

the proportion of power transmitted through. the sharply resonant circuit to the total power put out by oscillator I0. The total power put out by oscillator l0 may then be computed in terms of the power of calibrated generatorji02.

In order to calibrate for meter 88, it is desirable to modulate oscillator l0 at a known rate and deviation and observe meter reading IOI. Since the modulation rate and deviation is known, the integrated power in. meter IN is readily determinable. Furthermore by modulating oscillator I0 at a very low frequency such as l0 cycles-per second, the reading of hot wire meter IOI will give the true amount of powerput out by oscillator I0 at a particular frequency. Switch 8| may :now be operated to out out meter IM and connect the integrating meter system coupled to terminal 82. The indication on meter 88 may now be compared to the indication on meter I'0I for the slow modulating frequency and the scale 'marked accordingly. Thereafter the modulating frequency for oscillator l0 may he stepped up to any desired value consistent with proper operation of filter circuit 8|. Meter 98 indicates a voltage which is either the peak voltage impressed. on the integrating system or a definite fraction of the peak, depending upon the type of instrument. The voltage indicated by meter 98 will also be proportional to the deviation rate, the higher the deviation rate, the greater the voltage reading. It

second.

It is only necessary to calibrate for one frequency and thereafter power output at other frequencies may be easily determined. Thusfilter circuit 8l may be varied or oscillator I2 may be varied so that the frequency passed through filter 8| may be moved over a range of frequencies. In the event that filter circuit 8| is varied, it will be necessary to calibrate the resonance curve for this circuit.

It is also possible to calibrate power readings Thus assume that a steady frequency at a known power level is applied. This should result in a resonant beat frequency for filter 8|. Reading on meter IN is noted. Then modulation of the frequency is effected at a known rate and deviation and the proportion of power at the one frequency handled by filter 8| may be calculated. Th reading of rriieter 98 should then show that power propor- Once the entire system has been calibrated, it is possible to measure attenuation characteristic of networks. Thus there may be inserted at any suitable place in the system such as for example between oscillator I0 and mixer II an attenuating pad or network I05. By notingthe power or voltage in meter 88 before'and after inserting attenuating network I05, it is possible to calibrate the attenuation constant of network I05. This may be expressed in decibels, nepers. or any other standard desired.

By virtue of the modulation of the high frequency output from oscillator I0 along a frequency range, as shown in Figure 2, it is possible to use extremely delicate and sensitive instruments for measurement. Thus as an example, let it be assumed that oscillator I0 has a frequency range of between one and two megacycles per second. Also let it be assumed that this variation occurs at the rate of 100 cycles per second.

This means that an intermediate frequency will be reached twice per cycle or once per 5000 microseconds. Thus the rate of frequency variation will be 200 cycles per microsecond. For resonant circuits, the band width between half power points on the resonance curve divided by resonant frequency is either one or two over the circuit Q, depending upon the type of circuit. Let itbe assumed as two over Q. Then if Q is 10,000 as one'example, and the resonant frequency is 1.5 megacycles, the band width comes out as 300 cycles. Since the frequency variation is 200 cycles per microsecond, it is clear that filter circuit 8| will be effectively energized for 1.5 microseconds. During this time, therewvill be about 1,000 cycles so that oscillations in filter circuit ill will build up to maximum value with little difficulty.

Actually, the Q of circuit 8| could be much higher.

By using a heterodyne principle, it is possible to have oscillator ill go up to any high frequency desired. The intermediate frequency may be kept at a desirable value so that proper operation is assured.

It is possible to calibrate the output of various generators by inserting them in place of I02 and reading the calibrated meters. Since power; is independent of frequency, calibration over great ranges is possible. What is claimed is:

1. A method of calibrating an oscillator which comprises feeding a known amountof power at a definite frequency, Passing said frequency through a circuit sharply resonant thereo, obtaining a meter indication at the output of said resonant circuit as a standard reading, supplying an un- 8 an oscillator, means for cyclically varying said oscillator over a frequency range at a definite rate, manually adjustable means for varying said frequency range and rate, means including a sharply resonant circuit for receiving said oscillator output to obtain periodic pulses of energy transmitted through saidresonant circuit, said resonant circuit havingknown resonance characteristics and forming part of the permanent test system, said frequency variation and rate of variation and Q'of said resonant circuit being so related that oscillations in said resonant circuit will build up to maximum value at each pulse, means for integrating said pulses, and indicating means responsive to the energy level in said integrating means, said indicating means being calibrated to show the power passed by said resonant circuit whereby said manually adjustable means may be operated to give an indication equal to the indication given by an unknown oscillator substituted for said first-named oscillator.

connected across said condenser and resistance.

6. In a test system of the character described,

I the combination of an oscillator adapted to operknown amount of power at a variable frequency to said resonant circuit with the variable frequency including as a part thereof the frequency to which said circuit is resonant, determining the portion of unit time during which said unknown power is delivered at the frequency to which said circuit is resonant, measuring the power passed by said sharply critical circuit, and including the proportionality factor to determine the true power generated by the unknown source.

2. A method of calibration which comprises feeding power at a fixed frequency to a circuit sharply resonant thereto and impressing said power on a meter, feeding power at a variable frequency to said sharply resonant circuit, said variable frequency including said fixed-frequency, impressing said power on a meter, and calibrating one reading in terms of the other reading.

3. In a test system of the character described, an oscillator, means for cyclically varying said oscillator over a frequency range at a, definite .rate, manually adjustable means for varying the frequency range and rate, means including a wherebysaid system may be used to calibrate the power output of an unknown oscillator when substituted for said first-named oscillator.

4. In a test system of the character described,

ate over a range of frequencies in a radio frequency band, means for cyclically varying said oscillator frequency output over said range, manually adjustable means for varying said range of frequency and the rate, at which said frequency varies, means for beating said variable frequency with a fixed frequency to generate a difference frequency varying over a different predetermined range, a filter circuit fed by said difference frequency and responsive to a frequency within the range of variation of said difierence frequencies, said filter circuit having known filter characteristics and forming part of the permanent test system, and indicating means responsive to the energy level in said pulses at the output of said filter, said indicating means being calibrated to show the power at the output of,said filter.

7. In a test system of the character described, the combination of an oscillator adapted to supply power at low level, means including a circuit having a sharp frequency response characteristic fed by said oscillator, said resonant circuit having known resonance characteristics and forming part of the permanent test system, manually adjustable means operating on said oscillator for supplying energy to said sharply resonantcircuit in the form of pulses at a desired pulse repetition lator is provided for beating said first oscillator output to provide a diiference frequency and wherein said difference frequency is fed to said Q circuit having known resonance characteristics and forming part of the permanent test system band so that, when said oscillator is frequency modulated, energy in the form of pulses is passed through said high Q-circuit at a definite rate, said rate being sufliciently slow so that the potential may build up to a maximum value in said high Q circuit during each pulse, a vacuumtube amplifier stage fed by said high Q circuit, said stage having an output circuit, a rectifier, condenser and resistance in series in said output circuit, said output circuit being adapted to function as an integrator of said energy pulses, and means for indicating the potential across the condenser and resistance, said potential indicating means being calibrated to indicate power with reference to the energy output of said oscillator, said oscillator being adapted to be replaced by an oscillator of unknown power whereby the relative power output of said unknown oscillator may be compared to said first-named oscillator.

10. The system of claim 9 wherein a watt meter is provided and wherein switching means are enough rate so that the resonant frequency may build up to a maximum, integrating the pulses of energy passed by said sharply resonant circuit, measuring the energy level of said integrated pulses, then inserting said passive network so that the variable frequency is impressed on said network and thence on said sharply resonant circuit and adjusting the frequency variation so that the energy level is the same whereby the insertion loss may be computed. v i

12. A method of calibration which comprises feeding, alternating current power to a circuit sharply resonant thereto and impressing the power output of said resonant circuit on a meter, feeding alternating current power in the format pulses to said resonant circuit and impressing said pulse power output of said resonant circuit on a meter system, and calibrating one reading in terms of the other reading.

Y WILLIAM A. STEWART.

REFERENCES CITED The following references are of record in the file of this patent:

Jacob Jan. 31, 1939

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