US2503968A - Frequency multiplier - Google Patents

Description

April' 11, 1950 J. J. ROOT FREQUENCY MULTIPLIER Filed March 15, 1946 70171465 FREQUENCY #1504 #708 201/515)? I 1 Z0 Z/ 007/ 07 C /RC (1/ Z' NTOR ATTORN EY Patented Apr. 11, 1950 John J. Root, New York, N.

- Application March 15 1946, Serial No. 654,783

This invention relates to frequency multipliers.

Ingeneral, thermionic tube frequency multipliers operate by-selecting from th output, a harmonic of the signal applied to the input. The output is generally provided with a tuned or resonant circuit which selectsthe desired har' monic to, the exclusion of the fundamental frequency or other harmonics which may be present. It is apparent that such a multiplier will operate for only one frequency, namely the frequency of the resonant circuit.

Bearing the foregoing in mind, it is an object of the present invention to produce a frequency doubler which does not require a tuned output and which is operative therefore over an extremely wide range of frequencies.

Another object of this invention is to produce a frequency doubler of pronounced simplicity and economy of manufacture. Allied with this ob ject is the provision of such a doubler which requires no special tubes nor the conversion of the signal voltage into a pair of out-of-phase voltages to be applied to separate input grids. Thus, an ordinary resistance type input into the control grid is made possible and the use of an input transformer is unnecessary.

Other objects of my invention will be apparent from the following description, it being understood that the above general statements of the objects of my invention are intended to describe a and not to limit it in any manner.

Referring to the drawings:

'Fig. 1 is a circuit diagram of a frequency doubler arranged according to the present invention.

Fig. 2 is a block diagram illustrating the sequence of the components of the system.

. Referring to Fig.1, a thermionic tube I is indicated as being of the pentode type having an anode II, a suppressor grid II, a screen l3, a control grid I4 anda cathode IS. The anode II is connected through its load resistor Hi to battery ll which serves as the screen grid and anode potential source. The screen grid I3 is connected to the same potential source through its resistor l8. Control grid I4 is provided with the usual grid leak resistor l9 which may have a value in the neighborhood of one-quarter megohm.

The circuit above described which, as will be noted-includes no bias between the cathode and control grid, is suitable'for a thermionic tubesuch astype 6AG7. 'The heater thereof is connected 7 Claims. (Cl. 250-36) to a suitable source of energizing current, not

Resistors i6 and II are each 75,000 ohms. When the tube is operating, the anode draws more current than the screen grid so that the voltage on the screen grid is higher than that on the plate. Thus, during the operation of the tube as will be hereinafter described, a voltage reading at the plate terminal indicated a potential of 8 volts,

, input grid. The output showed substantially no gain, but the frequency was doubled. By virtue of the distortion resulting from the drawing of grid current when the wave applied thereto becomes positive, the electron stream is diverted from the anode circuit so as to produce less current in the anode circuit in spite of the fact that the grid becomes more positive. As a result, the anode voltage rises as the input grid voltage rises sufficiently. Since the anode voltage also rises when the grid becomes negative, it will be recognized that the frequency applied to the grid is doubled at the anode. It is necessary, however, that the grid voltage be so adjusted that the additional cycle produced at the anode through grid current distortion is equal in amplitude to the anode voltage produced during the negative swing of the control grid II. If the grid voltage is too low, the additional cycle will appear as a second harmonic distortion of less amplitude than the fundamental. If the input voltage is too high, the distortion developed cycle will predominate. When the input voltage however, of such an amplitude that the grid current distortion produces a cycle equal to the anode output from the negative swing of the grid, it is found that the fundamental is substantially eliminated and only the double frequency is apparent. It is a simple matter to determine the correct grid input voltage with an oscillograph applied across the anode output. The input voltage may then be adjusted until the oscillograph exhibits a double frequency. It will be found that the output goes through the three conditions above mentioned wherein are included the subordination and dominance respectively of the distortion developed cycle as well as the production of the su tantially pure double frequency when the grid voltage is correct. In the described embodiment, onehaif volt at the grid produced the desired results.

In some thermionic tubes it may be necessary to include a slight bias in order that the negative transconductance portion of the characteristic curve produced through grid distortion equals the results achieved from normal operation or, when the grid is suitably negative. Thus, in tube type 6SG7, one-half volt fixed bias was included between the cathode and control grid with the remaining conditions identical. The output thereof exhibited the proper double frequency. In tube type 6SK7, it is found that the desirable negative transconductance is achieved by utilizing a bias of 9 volts. By connecting the oscillograph to the anode circuit, it can be seen that the input potential should be 3.4 volts to double the input frequency.

In utilizing the above described frequency doubler in a complete circuit, it will be desirable to include a voltage regulator prior to the doubling stage in order to limit the input voltage to the correct amplitude. Thus, referring to Fig. 2, the voltage regulator 20 feeds the frequency doubler stage 2| represented by the tube l and the elements connected thereto. Preferably, the signal supplied by the voltage regulator is of sine wave form. Thus, the frequency will be a substantially pure double only when the voltage achieves a predetermined amplitude and will depart from such purity when that amplitude is exceeded. When audio frequencies are used and earphones are connected across the anode output, the fundamental frequency is heard to disappear in favor of the double frequency when the correct grid voltage is applied.

The construction of the doubler is extremely simple, it being observed that only three resistors are required apart from the power sources. It is unnecessary and, in fact, undesirable to provide a resonant circuit in the output, and variably phased inputs to separate control grids are not necessary for producing the desired variations in the anode current. During the operation of the circuit above described, the measured anode potential was 8 volts while the screen potential was 25 volts. Whether the device operates solely by virtue of grid current distortion or whether a negative transconductance effect is achieved by virtue of the higher screen grid operating voltage, the desired result depends upon the rise in anode voltage which accompanies the rise of the control grid voltage during a positive portion of the input wave. When the term voltage is used in connection with alternating current, the R. M. S. value of such voltage is referred to.

I have shown a preferred embodiment of my invention, but it is obvious that numerous changes and omissions may be made therefrom without departing from its spirit.

I claim:

l. A frequency doubler comprising a thermionic tube having at least a control grid, an anode and cathode electrodes, means for feeding an input voltage to said grid so as to operate, said tube at a negative transconductance portionof its characteristic curve during a portion of one-half cycle of said input voltage whereby a cycle of output anode voltage is developed during.

said portion of one-half cycle so as to produce twice the frequency of said input voltage at said anode, said input voltage being regulated in amplitude so that said developed cycle is of substantially the same amplitude as the remaining varying anode voltages developed during the other one-half cycles of said input voltage.

2. A frequency doubler comprising a thermionic tube having at least a control grid, an anode and cathode electrodes, means to introduce an alternating voltage signal to said grid so that it draws grid current during a positive one-half cycle of said signal whereby the output anode voltage exhibits a negative transconductance characteristic during the time the grid draws current, said negative transconductance characteristic being operative to produce an increase in plate voltage with an increase in signal voltage applied to said grid, and means to regulate the positive signal voltage on said grid so that said positive voltage developed at said anode by virtue of its negative transconductance characteristic is substantially equal in amplitude to the anode positive voltage developed during the time of said alternating voltage signal when said grid is negative.

3. A frequency doubler comprising a thermionic tube having at least a control grid, a screen grid, an anode and cathode electrode, means to maintain said screen grid at a higher potential than said anode during the operation of the tube, means to introduce an alternating voltage signal on said control grid, the amplitude of said signal being such as to cause said control grid to draw current during a positive portion thereof whereby sald anode exhibits a negative transconductance characteristic when said control grid is drawing current, said amplitude being controlled soas to regulate the extent to which said control grid becomes positive, said regulation being 'when said alternating voltage signal applied to said control grid is negative.

4. A frequency doubler comprising a thermionic tube having a control grid, 9. screen grid, a suppressor grid, an anode and cathode electrodes,

-' means to maintain said screen grid at a higher wave signal voltage on said control grid, the amplitude of said signal voltage being such as to cause said control grid to draw current during a positive portion thereof whereby said anode exhibits a negative transconductance characteristic when said grid is drawing current, and means to operate the grid at a fixed alternating current potential wherein the positive voltage developed at the anode during said grid positive portion is substantially equal to the positive voltage developed at the anode during the time when said sine wave voltage applied to said control grid is negative.

5. A frequency doubler comprising a thermionic tube having at least a control grid, 9. screen grid, an anode and cathode electrodes, means to maintain said screen grid at a higher potential than said anode by virtue of the drawing of more. anode current than screen grid current in saidv tube during the operation thereof, said cathode being grounded so that no bias is developed between the cathode and the ground during the operation of the tube, means tointroduce a sub--.

stantially sine wave signal voltage on said control 5 during a positive portion thereof whereby said anode exhibits a negative transconductance characteristic when said grid is drawing current, and means to regulate the extent to which the grid becomes positive, the alternating potential thereof being fixed in amplitude so that the positive voltage developed therefrom at the anode is substantially equal to the positive voltage developed at the anode during the time when said sine wave voltage applied to said control grid is negative.

6. A frequency doubler comprising a thermionic tube having a control grid, a screen grid, 8. suppressor grid, an anode and cathode, electrodes, said screen grid and anode being connected to a common source of direct current and having equal load resistors, means to maintain the screen.

age output exhibits a negative transconductance characteristic when said control grid is drawing current, said signal voltage being limited in amplitude so as to limit the extent to which said control grid becomes positive, the alternating current of the si nal voltage being hired in amplitime so that the positive voltage developed there- 6 from'at the anode is substantially equal to the positive voltage developed at the-anode during the time when said sine wave voltage applied to said control grid is negative.

'7. A method of doubling frequency which comprises operating a vacuum tube so that at a predetermined grid voltage the plate current reverses in magnitude, and feeding a sine wave signal to said grid of an amplitude such that the grid voltage during the positive portion of said sine wave exceeds said predetermined grid voltage so as to produce one cycle of plate current through a half cycle of grid current distortion and another cycle during feeding of the negative portion of said sine wave, said signal being regulated in amplitude so that produced cycles of plate current are substantially equal.

' JOHN J. ROOT.

REFERENCES crrnn The following references are of record in the file ofthis patent: I

UNITED STATES PATENTS Number Name Date 2,139,366 Herold Dec; 6, 1938 2,141,673 Thompson Dec. 27, 1938 2,230,243 Haifcke Feb. 4, 1941 2,261,430 Andrews Nov. 4, 1941 2,306,456 Mayne Dec. 29, 1942 2,806,457 Mayne Dec. 29, 1942 3.42 .24

Koch May 6, 1947

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