US2289399A - Crystal oscillator system - Google Patents

Description

y 14, 9 c. w. WINTER, JR., ETAL 2,289,395

CRYSTAL OSCILLATOR SYSTEM Filed Aug. 18, 1939 l'l'l'l' HIGH VOLT/76E SUPPLY INVENTORS mar/ls W Mam/2 B'v/[ wlll 1/ Aqzo awzvs Patented July 14, 1942 CRYSTAL OSCILLATOR SYSTEM Charles W. Winter, J r., and Edwin M. Hinsdale, J r., Miami, Fla.

Application August 18, 1939, Serial N0. 290,724

3 Claims.

This invention relates to oscillation generators and has particular reference to oscillator circuits of the piezo-electric crystal controlled type suitable for use in radio transmitters and. receivers and frequency measuring devices.

The crystal oscillators now commonly used in radio transmitters and receivers are generally difficult to adjust and stabilize at one desired frequency, and more especially so when the desired frequency is a harmonic of the fundamental crystal frequency, for the reason that they have a tendency to oscillate on frequencies other than the desired frequency, such as, for example, frequencies not in harmonic relation to the crystal frequency, but frequencies governed by the circuit constants of the oscillator circuit. Some of these will oscillate even with the crystal disconnected from the circuit, such as may happen when faulty contacts are had in the crystal circuit. Such prior oscillation generators have other disadvantages, such as being too critical in adjustment and too slow in starting to permit operation at high keying speeds when used in transmitters.

An object of the present invention is to provide crystal oscillators of very simple construction and having relatively few parts, which are stable, easy to adjust and suitable for telegraph keying at speeds as high as fifty to one hundred words per minute when used in transmitters.

Another object of the invention is to provide crystal oscillator circuits which may readily utilize crystals having average characteristics and which will operate at the fundamental frequency of the crystal or harmonic frequencies thereof without any tendency towards instability.

Another object of the invention is to provide a crystal oscillator circuit which has a high power output with a minimum plate power consumption by the oscillator vacuum tube and without subjecting the crystal to high current flow.

The objects of the invention have been attained by the provision of a simple oscillator circuit which may suitably include a crystal in conjunction with a beam tetrode type of tube and which may be adjusted to operate at fundamental frequency and. frequencies as high as the sixth harmonic or higher, by tuning the usual plate tank circuit and by varying the capacity of only one condenser in the circuit.

Conventional types of crystals such as X and AT out crystals may be used in this circuit without special selection as to starting speed so as to permit fast keying and very stable operation at the desired frequency.

For a better understanding of the present invention, reference may be had to the accompanying drawing, in which the single figure discloses a typical oscillator circuit embodying the invention. In the circuit illustrated, a beam tetrode tube H1 is used as an oscillator, although it will be understood that other types of tubes may be used. In this circuit, the key II is located in the plus high voltage lead and is connected in series with plate or anode I2 of the tube [0 through a radio frequency choke 26 and the plate tank circuit which consists of a variable condenser l3 and an inductance coil l4, connected in parallel. It will be understood that keying may be accomplished in other parts of the circuit, as for example the screen grid or control grid or cathode.

The crystal I5 is excited by means of regeneration voltage. This voltage is obtained by the radio frequency voltage drop across condenser 16 which is connected between the low potential end of the tank coil 14 and ground. The crystal I5 is connected to the grid l8 of the tube In. The screen grid I9 is connected in series with a resistance 20 to the plus high voltage lead and is maintained at radio frequency ground potential by electrical connection to ground and the oathode 2l, through a by-pass condenser 22 of relatively high capacity. The condenser 23 is connected between grid I8 and cathode 2|; its value is selected for best stability of the oscillator; a value of 50 micromicrofarads has been found satisfactory for the type 807 vacuum tubes. Resistor 24 is connected between grid is and cath- Inasmuch as the cathode 2| is grounded, a filament tube of the type described above may be used.

The plate tank coil [4 may be inductively or capacitively coupled to a load of any desired type, as for example an amplifier or an antenna. It has been found that an oscillator circuit of the type described above will drive two type 807 tubes (class C amplifiers) to full output with an input power to the oscillator plate and screen of two watts.

Satisfactory keying with this circuit may be obtained with condenser l6 having values between and 500 micromicrofarads. The circuit may be caused to oscillate at the fundamental frequency of the crystal or harmonics thereof, by varying the value of the condenser l6 and by adjusting the plate tank condenser 13 and inductance I4. Three interchangeable or interchangeably connected fixed capacity condensers l6 are sufficient for fundamental and harmonic operation up through the sixth harmonic.

The condenser I'l, connected from grid to plate, is used to prevent spurious and transient oscillations when the plate tank circuit is tuned to the third or higher harmonic of the crystal frequency. Its approximate value is micromicrofarads but the exact value will of course depend upon tube type, crystal holder capacity and circuit layout. The oscillator functions normally also on the fundamental and second harmonic of the crystal frequency with this condenser in the circuit.

The circuit described above is simple and requires but few interchangeable parts in order to permit wide variation in its operating frequencies. The oscillator is particularly suitable for use in airplanes because it may be readily tuned or adjusted to many frequencies Without requiring careful adjustment or the interchange of a great number of parts. Likewise, its power output is such that it is useful for communication over long distances and when used as a telegraph transmitter permits such communications to be carried on at high speed.

Parallel plate feed operation may be used if desired, in which case the top end of radio frequency choke 26 is connected directly to the plate l2 of vacuum tube l0, instead of to the bottom end of inductance M as shown in the accompanying drawing. In addition a direct ourrent-isolating condenser would be connected between the plate l2 of vacuum tube l3 and the top end of inductance I4, instead of the solid Wire lead shown. The only advantage of this arrangement is that direct current voltage would be removed from condenser 13 and inductance M.

It will be understood that this circuit may be varied considerably, without departing from the invention, and the circuit disclosed herein, therefore, should be considered as illustrative only and not as limiting the scope of the following claims.

We claim:

1. An oscillation generator comprising a; vacuum tube having a cathode, an anode, a control element and a screen element; a tank circuit connected in series with and between the anode and a radio frequency impedance, a piezo electric crystal connected between the control element and said tank circuit and connected through a capacitance to ground for controlling energization of said crystal, a capacitance and a resistance connected in parallel between said cathode and said control element; a capacitance connected between the control element and thd anode; a resistance in series with said screen element connected to the side of said impedance remote from said anode, and a capacitance connected between said screen element and said cathode, said cathode being connected to ground.

2. In an oscillation generator and amplifier for radio signalling, the combination of a tube having a grounded cathode, an anode and a control element; a first oscillatory circuit comprising a crystal having one side thereof electrically connected directly to said control element and connected through a capacitance to said cathode and the other side of said crystal connected through a capacitance to ground, a second oscillatory circuit comprising a tunable tank circuit having one end thereof connected to said anode and the other end connected directly to said other side of said crystal, and a capacitance connected between said anode and said control element for causing said crystal to operate as a parallel resonant circuit and at its resonant frequency, only, and for maintaining the output frequency of said generator and oscillator at the fundamental frequency of said crystal or a multiple thereof and means for maintaining the tube at operating potentials.

3. In an oscillation generator and amplifier for radio signalling, the combination of a tube having a grounded cathode, an anode, a screen element and a control element; a first oscillatory circuit comprising a crystal having one side thereof electrically connected directly to said control element and connected through a capacitance to said cathode and the other side of said crystal connected through a capacitance to ground, a second oscillatory circuit comprising a tunable tank circuit having one end thereof connected to said anode and the other end connected directly to said other side of said crystal, and a capacitance connected between said anode and said control element for causing said crystal to operate as a parallel resonant circuit and at its resonant frequency, only, and for maintaining the output frequency of said generator and oscillator at the fundamental frequency of said crystal or a multiple thereof, means for maintaining said screen element at radio frequency ground potential and means for maintaining the tube at operating potentials.

CHARLES W. WINTER, JR. EDWIN M. HINSDALE, JR.

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