US2411166A - Frequency multiplier - Google Patents

Description

Nov. 19, 1946. l QLSON 2,411,166

FREQUENCY MULTIPLIER Filed Oct. 2, 1942 2 Sheets- -Sheet l U U U Nov. 19, 1946. CLSON 2,411,166

FREQUENCY MULTIPLIE'R Filed Oct. 2(1942 2 Sheets-Sheet 2 '0 I J7Zv67z757".

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Patented Nov. 19, 1946 Lilhltt UNETED STATES PATENT OFPEQE FREQUENCY MULTIPLIER Roy H. Olson, Marion, Iowa, assignor to Collins Radio Company, a corporation of Iowa Application October 2, 1942, Serial No. 460,560

10 Claims. 1

This invention relates to a frequency mu1tiplier, and more particularly to means for minimizing undesired voltage variations at fundamental frequency.

One feature of this invention is that it provides an improved output for driving a class C amplifier from a frequency multiplier, particularly where the frequency multiplier is operated at a higher order of multiplication as, for example, quadrupling; another feature of this invention is that it minimizes or eliminates undesired frequency components or variations in the desired multiple frequency component in the output of the class C amplifier operating from a frequency multiplier under conditions where such variations are transferred by the circuit tuned at multiple frequency; still another feature of this invention is that it enables the use of the same variable inductance with different fixed condensers for different orders of multiplication from an oscillator of limited frequency range without the output of a class C amplifier driven from the frequency multiplier containing undesired frequency components normally present due to the lower Q values in the frequency multiplier tank circuit at the higher multiplications such as quadrupling; and other features and advantages will be apparent from the following specification and the drawings, in which:

Figure l is a circuit diagram of one embodiment of my invention; Figure 2 is a schematic representation of decaying voltage oscillations; Figure 3 is a schematic representation of plate current pulses in a multiplier stage; Figure 4 is a schematic representation of the fourth harmonic output of a multiplier stage with the peak Values Varying periodically at fundamental frequency; Figure 5 is a schematic representation of plate current variations at fundamental frequency, and of voltage variations in the output circuit normally associated therewith; Figure 6 is a schematic representation of the undesired accentuation of the variations of the positive peaks; and Figure 7 is a schematic representation of the output from a quadrupling stage embodying my invention, showing elimination of variation in the crest values of the positive peaks.

The output of higher order multiplication stages, particularly quadrupling stages where higher values of effective reactance to resistance ratio is used in the tank circuit or Where the decay of the circuit is high, has heretofore contained periodic variations at fundamental frequency which could not be minimized or eliminated. I have found that these periodic undesired variations can be completely eliminated or minimized to a negligible value by combining with the harmonic output another voltage varying at fundamental frequency and being substantially equal in amplitude and opposite in phase to the undesired variations in the harmonic output. This is accomplished by causing the output circuit as viewed from the tube to present considerable capacitative reactance at the fundamental frequency.

In the particular embodiment of my invention illustrated in Figure 1, a tube lEl andxits associated circuits provide oscillations at a desired fundamental frequency. The tube ii and its associated circuits comprise the multiplier stage. The oscillator output is coupled to the grid circuit of the multiplier stage and causes periodic pulses of plate current in the multiplier stage, being sufficiently biased to work as a class B or class C amplifier, in accordance with conventional multiplier practice. The output circuit of the tube E 1, includes a tank circuit comprising the inductance I2 and the condenser i3 in parallel, this being tuned to any desired harmonic, as the fourth. This circuit, then, presents inductive reactance to a lower frequency, as the fundamental, and to overcome this and cause the output circuit to present capacitative reactance at the fundametal frequency, a, condenser M is connected in series with the tank circuit. This condenser should have a capacity somewhat less than that which would be required for series resonance at the fundamental frequency, preferably having a capacity such that its reactance exceeds that of the tank circuit at fundamental frequency by a fraction of one over the order of multiplication, but is less than the order of multiplication at which the stage is operating times such tank circuit reactance, less than four times in the quadrupler here being described. I have found in a particular case that best results are secured, in a quadrupler, with a condenser having a capacity of approximately one-third of that which would be required for series resonance with the tank circuit at fundamental frequency.

Where it is desired to tune the tank circuit of the multiplier stage, as is usually the case, best results are secured by tuning by inductance variation where the condenser M is a fixed condenser, as this tends to best keep the proper ratio of values between the circuit elements over a reasonable band of frequencies. The output of the multiplier stage is coupled to the input circuit of a succeeding tube 15, and since it is the crest values of the positive peaks of the harmonic oscillations which are leveled 01f, this succeeding tube must be sufficiently biased so that its operation is not affected by the negative harmonic peaks. That is, there is still variation in the negative peaks of the harmonic output, as will be more fully described later, but operating the succeeding stage with the proper cutoff value eliminates any difficulty with these variations.

In the particular circuit shown, the inductance l6 could have a value variable between about 40 and 60 microhenries, and condensers I! and I3 values of about 800 mmf., providing for some variation of the oscillator frequency in the neighborhood of one megacycle. Conventional values would be used for the coupling condensers I9 and 20, the grid leak 2|, and the radio frequency choke 22 connected'to the plate voltage supply. The tube It is illustrated as a triode, as type 801 having cathode, grid and plate elements Illa, lilb and I00.

The output of the oscillator is coupled through a condenser 23, which may have a value of 500 mmf., to the input circuit of the tube II, here illustrated as a type 802 pentode having an indirectly heated cathode element screen, and suppressor grid elements Ilb, H0, and Ild, and a plate element He. The grid leak 24 should have a value considerably higher than that usually associated with such a tube, preferably between 50,000 and 200,000 ohms. The bypass condenser 25 and screen grid resistor 26 may have conventional values, as .002 mi. and 20,000 ohms.

Where the multiplier stage is to operate as a quadrupler, the inductance I2 may have a value variable between about 9 and 14 microhenries, and the condenser I3 a value of about 110 mmf. for this inductance. These values may, of course, be varied with relation to each other; but the tank circuit must be tuneable through a band of frequencies four times that of the oscillator, the band having a corresponding range. It will be understood that, in accordance with conventional practice where the transmitter is intended to operate in a number of different hands, a plurality of condensers I3 of different values may be provided and selectively connected into this circuit. These may provide different multiples of the same frequency, or the same multiple of widely different fundamental frequencies. Condenser M,

in. connection with a tank circuit having a Q of about 50 and the values stated, would have a capacity of about 830 mmf. This condenser value is not sharply critical, although the closer it is held to the calculated value, the more completely undesired positive peak variations are eliminated. A radio frequency choke 27 of appropriate value may be used to connect the plate to a plate voltage source 28.

The output of the multiplier tube i I is conneoted through a coupling condenser 29 to the grid circuit of the tube I5, here schematically illustrated as a triode, although in commercial practic this would normally be a multi-element tube, the present representation being intended as a generic one. This tube might comprise part of another multiplying stage, again multiplying the output of the tube I I or it might com,- prise an intermediate stage between the multiplier and the power amplifier of a transmitter. If operating as another multiplier stage, another type 802 might be employed with circuit connections similar to those of the tube I I; and if as an intermediate amplifier, a beam power tube such as a type 813 tube might be used. In either case, it is essential that this stage be operated as a class C amplifier, that is, that the control grid be provided with sufiicient negative bias that the negative grid voltage peaks are all below the cutoff level.

It will be understood that the preceding detailed description of the circuit of Figure 1 is intended as illustrative only, since this invention may be embodied in any number of different circuits. Different type tubes might be used, a different I Ia, control,

oscillator arrangement employed, or the oscillator and multiplier stage combined in a single tube. Moreover, as the word plate is used in this specification and the accompanying claims, it is intended in a broad sense covering any anode element as a screen grid operating as an anode.

Figures 2 to 7 are intended as illustrative of the theory of operation of the present invention, and this will be described in connection with a quadrupler. When the periodic energy pulses at oscillator fundamental frequency energize the tank circuit in the multiplier output, fourth harmonic oscillations are induced, and the peak values of these oscillations decrease, as is illustrated in Figure 2. This figure is not intended as an accurate representation, since the peak variations follow an exponential decay curve; but it does illustrate what happens when a very brief energy pulse is admitted periodically to a tank circuit of reasonably good Q tuned to four times the energy frequency. The decay per oscillation is given by the formula D=e where R is the resistance and L the inductance of the circuit. Since t=1/F, and since substituting gives D=e Q Using this formula, the decay for a given number of cycles would be K times the power in this formula, so that where the circuit is tuned to the fourth harmonic, the amount of decay in the crest values of the positive peaks, before another energy pulse is received, is given by the formula 40 t 41 D=e Q In a quadrupler where the tank circuit has a Q of at the harmonic frequency, it will be seen that the decay for four cycles, in accordance with the above formula, is from whatever the initial crest value was to .777 of such value. This is a substantial change, amounting to about 20% modulation of the harmonic oscillations at fun- 50 damental frequency; and this modulation is an undesired-variation which goes through with the harmonic oscillations and is produced rather than eliminated by the tuned tank circuit.

Where the multiplier stage is being operated with an angle of flow of about 180 degrees, the plate current pulses are as illustrated in Figure 3. If a situation is conceived where these plate current pulses delivered energy to the tank circuit without any variation in the plate voltage at fundamental frequency, that is, where the tank circuit presents zero impedance at fundamental frequency, there would be a situation like that illustrated in Figure 4, with the harmonic oscillations 30 swinging about a fixed voltage base line here indicated as 3|, with upper and lower envelope curves 32 and 33 resulting from the decay modulation effect. Because of the decay action, the bottom or minimum point of the modulation curve 32 occurs approximately at the point of zero plate current flow, and the positive peak of this modulation envelope occurs approximately at the point of zero plate current flow 90 degrees in phase after maximum current, Figures 3 and 4 showing the relationships in this regard over two cycles.

The plate voltage, however, does vary at the fundamental frequency, this being illustrated in Figure 5 where the line 34 indicates current flow in the output circuit at fundamental frequency, the amplitude shown in Figure 5 being approximately in correct relationship to the harmonic oscillations 36 shown in Figure 4. Under output circuit conditions heretofore employed, the output circuit presents inductive reactance to the fundamental frequency, and therefore plate voltage variations, indicated by the line 35, are approximately 90 degrees ahead of plate current flow. It should be noted that, even though a bypass condenser has heretofore been frequently used in the output of a multiplier stage, this condenser has always had such a large capacity (in order to provide low impedance to ground) that the reactance of the output circuit has always been inductive, even when such a condenser was employed.

The result of combining the crest voltage variations illustrated in Figure 4 with the plate voltage variations at fundamental frequencies has always heretofore resulted in accentuating the positive peak variations of the harmonic oscillations, as illustrated in Figure 6, giving an undesirable amount of fundamental frequency in the output of the stage. By making the total reactance of the tuned output circuit capacitative, however, the phase of the voltage variation 35 at fundamental frequency is shifted approximately 180 degrees, so that it is out of Phase rather than in phase with the crest or positive peak value variations of the harmonic oscillations 30. This tends to reduce rather than to increase such undesired positive peak variations caused by the decay modulation effect, and if the amplitude of the voltage variation 35 is properly adjusted and the phase relation is exactly correct, variation in the positive peaks will be completely eliminated and the positive crest values of the harmonic oscillations 30 will all be equal, as shown in Figure 7. In practice, the phase relations will not be exactly correct, since the crest value of the envelope 32 and 33 will not necessarily occur 90 degrees out of phase with the crest value of the plate current pulse 34. The theoretically perfect result illustrated in Figure '7 can be very closely approached in practice, however, and positive peak variations of the harmonic oscillations reduced to a negligible amount. It will be noted that there is considerable variation in the negative peaks of the harmonic oscillations illustrated in Figure 7, but these are completely wiped out by biasing the grid of the succeeding tube to such a point that the cutoff level is above all of these negative peaks, as for example, on line 36 in Figure '7.

The amplitude of the voltage variations at fundamental frequency (illustrated as 35) can be adjusted to equal the positive peak voltage variations of the harmonic oscillations by varying the value of the condenser l4, and thus the amount of capacitative reactance, since the voltage developed in the output circuit is a function of the current and the reactance through which it flows. While I have determined that the capacity of the condenser should preferably be such as will provide a reactance somewhat greater but less than the multiplication factor times the inductive reactance of the tank circuit at fundamental frequency, therequired capacitative reactance in any given case can be mathematically determined. 4

In a pentode vacuum tube, when the plate current is substantially independent of plate voltage, and the control grid current is negligible, the plate current isgiven by the formula where Eg is the control grid voltage, Esg the screen grid voltage, [.Lsg the voltage amplification factor of the screen grid, k the tube constant, and a, a constant between 1 and 2 and normally about 1. An explanation of the theory behind this formula, and of the relationship between current harmonics at various frequencies, is contained in an article by Terman and Ferns in the March, 1934, Proceedings of the Institute of Radio Engineers, volume 22, No. 3, and will not be gone into here.

Where the voltage applied to the grid of the tube consists of a fixed negative bias Ecand an alternating potential of crest amplitude Es and angular velocity w, as is the cas in a frequency multiplier stage actuated by an oscillator, this formula becomes By Fourier analysis, the components of the plate current at various frequencies can be determined for various angles of current flow. Where a. is 1 and the angle of flow is about degrees, the ratio of the fundamental crest value to that of the fourth harmonic will be found to be about 12.5. Whether this ratio is derived theoretically by this formula, or by direct measurement and analysis of the plate current, this relationship and the circuit Q can be used, in connection with the particular decay being encountered, to provide a formula for the amount of capacitative reactance necessary. The voltage developed at a given harmonic K is, of course, a function of the current flow at that harmonic times the impedance of the tuned circuit at that harmohic, so that where Q is the reactance-resistance ratio of the tank circuit at the harmonic frequency, and XL is the reactance of the inductance at the fundamental frequency.

The voltage at the fundamental frequency is also equal to the current times the impedance, and is given by the formula IG-F where X0 is the reactance of the condenser [4. Since the peak value of the variation of this fundamental voltage above and below its center line should be equal to one-half the total decay of the harmonic oscillation peaks,

Equating the two functions which are equal to Er, with a negative sign before one to indicate proper phase relation, solving for Xe, we have the capacity of th condenser It should be substantially less than that which would provide series resonance at the fundamental frequency,

.preferably such that the capacitative reactance provided by the condenser l 4 is about three times the inductive reactance of the tank circuit at the fundamental frequency.

It is believed that the foregoing is a correct explanation of the theory and a correct mathematical determination of the Various factors considered. Whatever the theory and mathematics may be, however, there is no question that undesired variations, at fundamental frequency, in the positive peaks of the harmonic oscillations are minimized to the point of substantial elimination by the use of a condenser in series with the tank circuit and with a capacity low enough to provide a reactance substantially exceeding that of the tank circuit at the fundamental frequency.

While I have shown and described certain embodiments of my invention, it, is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as disclosed in the appended claims.

I claim:

1. A frequency multiplier of the character described, including: a tube having at least cathode, grid, and plate elements; means for causing periodic pulses of plate current at a certain frequency; and an output circuit connected to said plate, said circuit including a tank circuit tuned to a multiple of said frequency and a condenser in series with said tank circuit, said condenser having a capacity such that its reactance at said frequency is at least one divided by said multiple greater, but less than said multiple times, that which would provide series resonance with the tank circuit at saidfrequency.

2. A frequency multiplier of the character described, including: a tube having at least cathode, grid, and plate elements; means for causing periodic pulses of plate current at a certain frequency; and an output circuit connected to said plate, said circuit including a tank circuit tuned to four times said frequency and a condenser in series with said tank circuit, said condenser having a capacity such that its reactance at said frequency is substantially three times that which would provide series resonance with the tank circuit at said frequency.

3. Apparatus of the character claimed in claim 1, further including: a second tube having at ,least cathode, grid, and plate elements; a grid where Q is the reactance-resistance ratio of the tank circuit at the multiple frequency, I4 divided by I is the ratio of the crest values of the multiple and fundamental frequency current components,

and X1. is the reactance of the tank circuit inductance at said fundamental frequency.

5. Apparatus of the character claimed in claim 1, including means for varying the period of said pulses, and wherein said tank circuit includes a variable inductance.

6. The method of minimizing undesired periodic variations, at fundamental frequency, in the positive voltage peaks of the harmonic output of a frequency multiplier, comprising combining therewith an alternating voltage of said fundamental frequency substantially equal in amplitude but opposite in phase to said periodic variations.

7. The method of minimizing undesired periodic variations, at fundamental frequency, in the positive voltage peaks of the harmonic output of a frequency multiplier, comprising combining therewith an alternating voltage of said fundamental frequency substantially equal in amplitude but opposite in phase to said periodic variations, and eliminating the negative peaks of the combined voltages.

8. The method of minimizing undesired periodic variations, at fundamental frequency, in the positive voltage peaks of the harmonic output of a frequency multiplier, comprising using the current at said fundamental frequency to create an alternating voltage substantially equal in amplitude but opposite in phase to said periodic variations, combining said alternating voltage with the harmonic voltage, and eliminating the negative peaks of the combined voltages.

9. A frequency multiplier of the character described, including: a tube having at least cathode, grid, and plate elements; means for causing periodic pulses of plate current at a certain frequency; an output circuit connected to said plate, said circuit including a tank circuit tuned to a multiple of said frequency and a condenser in series with said tank circuit, said condenser hav ing a capacity such that its reactance at said frequency is greater than that which would be required to provide series resonance at said frequency; a second tube having at least cathode, grid and plate elements; a grid circuit for said second tube coupled to said output circuit; and means providing a negative bias on the grid of the second tube such that the negative peaks of the grid voltage variations are all below cut-oif level.

10. A frequency multiplier of the character described, including: a tube having at least cathode, grid, and plate elements; means for causing periodic pulses of plate current at a certain frequency; and an output circuit connected to said plate, said circuit including a tank circuit tuned to a multiple of said frequency and a condenser in series with said tank circuit, said condenser having a capacity such that its reactance Xe at said frequency is substantially equal to that determined by the formula where K is said multiple, Q is the reactanceresistance ratio of the tank circuit at the multiple frequency, Ik divided by I is the ratio of the crest values of the multiple and fundamental frequency current components, and X1. is the reactance of the tank circuit inductance at said fundamental frequency.

ROY H. OLSON.

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