US2311163A - Oscillator circuit - Google Patents

Description

ll""e b.'164 1943. fR. B. EDWARDS OSCILLATOR CIRCUIT Fild oct). 11. 1941 I A .X20 I INVENTOR ROBERT E. E DWQRDS y ATTORNEY.

Patented Feb. 16, 1943 UNITED sTATEsPATENT OFFICE oscnaA'roR ontoUrr Robert n. Edwards, Baltimore, Md. Application october .11, 1941, serial No. 414,683

8 Claims.

This invention relates to high frequency oscillators and more particularly to a high frequency oscillator which may be optionally piezoelectric crystal controlled or continuously variable in output frequency.

Piezo-electric crystal controlled vacuum tube oscillators are well known for their frequency stability and their wide employment in the communications field is a direct result of this characteristic. There are times, however, particularly in military maneuvers, when it becomes desirable, or even necessary, to employ transmission frequencies which could not be anticipated at the time of the design or the construction of the transmitting equipment. Only an oscillator continuously variable in output frequency can meet such requirements for exibility, but unfortunately this type of oscillator cannot be designed to meet the stability requirements ofthe normal fixed channel operation. For the most part, equipment which has been constructed in the past has been a compromise, having the frequency either crystal controlled or continuously variable. Equipment incorporating both features has been unduly heavy and bulky.

Attempts have been made to provide an oscillator circuit which, by a simple switching operation, may be adapted for operation at a fixed frequency under crystal control, or for operation over a band as a source of continuously variable frequency. The most notable of the attempts made so far has failed of wide application because best crystal performance and life require that the crystal itself oscillate at a subharmonic of the desired carrier frequency for transmission in the portion of the ether spectrum assigned to mobile craft, and, in the system referred to, the tuned output circuit must be tuned to, or very near, the natural frequency of the crystal element. The principal customers for transmission equipment offering such savings in space and Weight as the systems mentioned are the owners of mobile craft and, since the above system did not meet their requirements, the market for equipment incorporating this system was correspondingly limited.

One of the principal objects of this invention is to provide a radio transmitter having optionally crystal controlled fixed output frequency or continuously variableoutput frequency and which is lighter, smaller, and cheaper than such previous transmitters.

Another object of this invention is to provide an oscillator optionally delivering a fixed output A frequency under the influence of a highly stable frequency determining network or a continuously variable output frequency.

Still another object of this invention is to provide an oscillator optionally delivering a xed output frequency under crystal control which is a harmonic of the natural frequency of the crystal control element or a continuously variable output frequency.

Other objects and advantages will in part be disclosed and in part be obvious when the following specification is read in conjunction with the drawing, in which:

Figure 1 is a schematic diagram showing one form of the invention.

Figure 2 is a schematic diagram showing a modified form of the invention.

Figure 3 is a schematic diagram showing a simplified form of the invention.

It is to be understood that the drawing is intended to illustrate a preferred form of the invention, and is not to comprise a limitation on the content or scope of the invention. y

In the drawing, like parts are designated by like reference characters.

Following the mention of a circuit element there is shown in parenthesis a value which has proven satisfactory in the practice of the invention over the frequency range of two megacycles to seven megacycles.

Referring in greater detail to Figure 1, a vacuum tube I may be employed having, a cathode 2 which-is raised to a temperature suiiicient for thermal emission of electrons by the heater 3, a control grid 4, a space charge grid 5 and an anode 0. The commercial type 6V6 has been successfully utilized in the practice of my invention. A choke 'I (2.5 mh.), whose resistance provides control grid bias for the Vacuum tube I is connected between the cathode terminal 2 and a ground 8, and a capacitor 9 (10 mmfd.) is connected from the cathode 2 to the control grid 4. The resistor In (25,000 ohms) provides a direct current path from the control grid 4 to the ground 3 While the blocking capacitor I I (.01 mfd.) prevents any direct current voltage developed across the resistor I0 from being short circuited to ground upon closure of the single-pole, single-throw switch I3 which is connected, in parallel with the piezoelectric crystal I2, between the blocking capacitor II and the lground 8.

The parallel resonant ycircuit comprising the tapped inductance I4, bypass capacitor I5 (.01 mid.)` and a variable tuning capacitor I6 (100 mmfd.) is effectively connected in series with the anode circuit of the vacuum tube I, one terminal the parallel resonant circuit through the output coupling capacitor 23 (40 Inmfd.)

A voltage dropping resistor 20 (20,000 ohms) is connected from the positive terminal of the source I1 to the space charge grid 5, and a voltage stabilizing resistor 2l (50,000 ohms) is connected between the space charge grid 5 and the ground 8, the combination providing the desired direct current potential on the space charge grid 5. The capacitor 22 (.003 mfd.) serves to maintain the cathode 2 and the space charge grid 5 at substantially the same alternating current potential.

Cperation of the oscillator with the switch I3 closed, thus effectively grounding the control grid 2 for high frequency currents, takes place in the manner of the well known Colpitts Oscillator, for the cathode 2,is tied to a tap on a part of the tuning capacitance of a resonant circuit. The frequency of oscillation may now be varied by adjustment of the inductance I4 and the tuning capacitor I 6.

With the switch I3 open, the mode of oscillation is considerably different. It is Well known in the art that the control grid input impedance of a vacuum tube amplifier having a high capacitive reactance in the cathode lead has a considerable negative real component because of the capacitance existingbetween the cathode and the control grid. This negative resistance appearing across the piezo-electric element I2 causes the element to oscillate, -generating piezo-electric voltages at the natural frequency of the piezoelectric element. It is important to note that with' practical values of circuit constants the cathode circuit impedance will always be capacitive in nature and the crystal I2 will continue to oscillate whatever the frequency to which the parallel resonant circuit is tuned. In fact, the parallel resonant circuit may be entirely removed and the crystal will still oscillate. For this reason, the parallel Vresonant circuit including inductance I4 and the capacitances I5 and I6, which might also ibe termed the output circuit, may be tuned to any desired harmonic of the crystal frequency, so that the oscillator stage also serves as a frequency multiplier, eliminating the extra tube formerly required for this function.

Referring now to the modified form of the invention shown in Figure 2, several circuit elements have been rearranged in a manner not affecting their operation, and a few changes are shown which assist in the manufacture of this type of equipment using commercially available parts.

The capacitor 9 is here connected from the space charge grid 5 to the control grid 4, and the capacitor I8 is bridged from anode 6 to the space charge grid 5. Since capacitor 22 is of sufficiently low impedance to maintain the space charge grid 5 and the cathode 2 at substantially identical 'alternating current potentials this part of the circuit operates in the fashion previously described.

It frequently happens that the direct current resistance of commercially available chokes is not of the value giving the desired bias on the control grid 4. In this case the direct -current return for the cathode current may be provided by a circuit of the type shown, incorporating the choke 24 (2.5 mh.) and the resistances 25 (1500 ohms) and 25 (400 ohms). The sum of the direct current resistances of 25 and 26 is selected to .provide the desired phase angle of the cathode load circuit and the choke 24 is bridged across a suiiicient portion of this total resistance to secure the proper operating bias for the control grid 4.

The control grid 4 is connected to the movable member of a single-pole, double-throw switch 21, the fixed contacts of this switch being connected respectively to the blocking condenser II and t0 the piezo-electric crystal I2. The other terminals of the two last-mentioned elements are connected together and to the ground 8.

With the switch 21 connected to the capacitor II, the oscillator operates as a Colpitts oscillator and the output frequency may be continuously varied by adjustment of the inductance I4 and the tuning capacitor I6. With the switch 21 connected to the crystal I2, the crystal oscillates at its natural frequency and the inductance I4 and the tuning capacitor I6 may be adjusted to select this frequency or any reasonable harmonic of it, the crystal controlled frequency thus selected being fed to the desired load through the coupling capacitor 23.

In Figure 3 is shown a simplified form of the invention utilizing the triode type of electric discharge tube. The tube 21 including the emitting cathode 28 with its associated heater 29, has its control grid 30 connected to the junction of the grid resistor I0 and the blocking capacitor II` while the anode 3| is connected to one terminal of the tapped inductance I4. The circuit elements not associated with the space charge grid 5 of Figure 1 are retained in this form of the invention and are connected in substantially the same manner.

With the switch I3 in the closed position the circuit of Figure 3 performs as a Colpitts oscillator whose operating frequency is determined by the resonance frequency of inductance I4 with the capacitors I5, I 6, I8 and I9.

With switch I3 in the open position, the negative resistance appearing in the input circuit by virtue of the capacitive cathode load and the capacitance 9 causes the piezo-electric crystal I2 to oscillate at its natural frequency, producing in the anode circuit of tube 21 alternating current components of the crystal frequency and the harmonics thereof. By properly tuning the parallel resonant circuit the desired harmonic may now be fed into the output load.

In practice, it is obvious that the capacitor 9 may be dispensed with when employing tubes inherently having sufficient grid-cathode capacity for the satisfactory operation of the circuit.

For the sake of simplicity, I have omitted the source of energy for the heaters from the drawing, but it is understood that this may be sup# plied according to the type of electric discharge device employed.

It will be obvious that many changes and modifications may be made in the invention without departing from the spirit thereof as expressed in the foregoing description and in the appended claims.

What I claim ls:

1. In a high frequency oscillator system, the combination of an electric discharge device having a cathode, a control grid and an anode, a parallel resonant circuit comprising inductance and capacity, one terminal of said circuit being connected to said anode, a capacitance voltage divider connected in parallel with said circuit, a connection between said cathode and an intermediate point on said voltage divider, an electromechanical resonator connected between said control grid and the other terminal of said circuit, and means for disabling said electro-mechanical resonator and maintaining said control grid substantially at the alternating current potential of said other terminal of said circuit.

2. In a high frequency oscillator system, the combination of an electric discharge device having a cathode, a control grid and an anode, a parallel resonant circuit comprising inductance and capacity, one terminal of said circuit being connected to said anode, a capacitance voltage divider connected in parallel with said circuit, a connection between said cathode and an intermediate point on said voltage divider, a capacitor effectively connected between said cathode and said control grid, an electro-mechanical resonator connected between said control grid and the other terminal of said circuit, and means for disabling said electro-mechanical resonator and maintaining sai-d control grid substantially at the alternating current potential of said other terminal of said circuit.

3. In a high frequency oscillator system, the combination of an electric discharge device having a cathode, a control grid and an anode, a parallel resonant circuit comprising inductance and capacity, one terminal of said circuit being connected to said anode, a capacitance voltage divider connected in parallel with said circuit, a connection between said cathode and an intermediate point on said voltage divider, an electromechanical resonator, and a switch having a .plurality of operating positions, one operating position of said switch connecting said electro-mechanical resonator between said control grid and the other terminal of said circuit, and another operating position of said switch connecting cir- 4. Ina high frequency oscillator system, the

combination of an electric discharge device having a cathode. a control grid and an anode, a parallel resonant circuit comprising inductance and capacity, one terminal of said circuit being connected to said anode, a capacitance voltage divider connected in parallel with said parallel resonant circuit, a connection between said cathode and an intermedi-ate point on said voltage divider, a piezo-electric resonator connected between said control grid and the other terminal of said circuit, and means for disabling said piezo-electric resonator and maintaining said control grid substantially at the alternating current potential of said other terminal of said circuit.

5. In a high frequency oscillator circuit, the combination of an electric discharge device having a cathode, a control grid an-d an anode, an inductance, one terminal of said inductance being connected to said anode, a capacitance voltage divider connected in parallel with said inductance, a connection between said cathode and an intermediate point on said voltage divider, an electro-mechanical resonator connected between said control grid and the other terminal of said inductance, and means for disabling said electromechanical resonator and maintaining said control grid substantially at the alternating current potential of said other terminal of said inductance.

6. In a high frequency oscillator system, the.

combination of an electric discharge -device having a cathode, -a control grid, a space charge grid and an anode, a parallel resonant circuit comprising inductance and capacity, one terminal of said circuit being connected to said anode, a capacitance voltage divider connected in parallel with said circuit, a connection between said cathode and an intermediate point on said voltage divider, means for maintaining said cathode and said space charge grid at substantially the same alternating current potential, an electromechanical resonator connected between said control grid and the other terminal of said circuit, and means for disabling said electro-mechanical resonator and maintaining said control grid substantially at the alternating current potential of said other terminal of said circuit.

7. In a high frequency oscillator system, the combination of an electric discharge device having a cathode, a control grid, a space charge grid and an anode, a parallel resonant circuit of inductance and cap-a-city, one terminal of said circuit being conn-ected to said anode, a capacitance voltage divider connected in parallel with said circuit, a connection between said cathode and an intermediate point on said voltage divider, means for maintaining said cathode and said space charge grid at substantially the same valternating current potential, means for maintaining said space charge grid at a positive potential with respect to said cathode, an electro-mechanical resonator connected between said control grid and the other terminal of said circuit, and means for disabling said electro-mechanical resonator and maintaining said control grid substantially at the alternating current potential of said other terminal of said circuit.

8. In a high frequency oscillator system, the combination of an electric discharge device having a cathode, a control grid, a space charge grid and an anode, a power source, a parallel resonant circuit connected between the anode terminal of said power source and said anode, a capacitance voltage divider effectively connected in parallel with said circuit, a connection between said cathode and an intermediate point on said voltage divider, a capacitor effectively connected between said cathode and said control grid, means for maintaining said cathoder and said space charge grid at substantially identical alternating current potentials, means for maintaining said space charge grid at a positive potential with respect to said cathode, means having high impedance to oscillatory currents for completing a direct current circuit from said cathode to the cathode terminal of said power source, a piezo-electric resonator connected between said control grid `and said cathode terminal of said power source, and means for disabling said piezo-electric resonator and maintaining said control -grid substantially at the alternating current potential of said cathode terminal of said source.

` ROBERT B. EDWARDS.

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