US2298774A - Crystal controlled oscillation generator - Google Patents

Description

Oct. 13, 1942. V I p. PARKER 2,298,774

CRYSTAL CONTROLLED OSCILLATION GENERATOR Filed June 28 1940- V l VE TOR Gauge arker mu) ATTORNEY Patented Oct. 13, 1942 CRYSTAL CONTROLLED OSCILLATION GENERATOR George Percy Parker, Chelmsford, England, assignor to Radio Corporation of America, a corporation of Delaware ApplicationJune 28, 1940, Serial'No. 342,930 In Great Britain July 12, 1939 9 Claims.

Thisjinvention relatesto piezo-electric crystal controlled oscillation generators and has for its object to provide improved piezo-electrically controlled generators adapted for satisfactory use on fairly low frequencies, i. e., frequencies of about 500 kc. per. second, or lower.

For crystal oscillators at frequencies lower than about 500 kc. per second, where high stability with'temperature variation is required, the cube type of crystal is superior to the plate type but it suffers, as is well known, from the defects of low activity and. unreliable starting. Hitherto the most reliable form of circuit appears to have been. a two valve relaxation oscillator controlled by the crystal. This circuit is fairly reliable as regards starting, but is apt to give trouble due to parasitic oscillations, particularly ifthe capacity across the crystal is not extremely small. If it be desired selectively to switch any of a number of crystals in circuit in order to change the operating frequency, difficulties are experienced in maintaining the capacities sufficiently low to ensure reliable functioning of the circuit. Even under normal conditions without crystal switching, the circuit is not suitable for keying since the oscillator passes through a range of parasitic frequencies before the crystal finally takes charge and the frequency settles down to the correct value.

If a single valve of high gain, for example, a pentode, is employed in conjunction with a cube type of crystal in the ordinary grid-anode or gridfilament connection, oscillations can be obtained, but starting is not reliable and keying at normal hand speeds practically impossible. Again, the power obtainable for driving a subsequent stage is small, usually making it necessary that such subsequent stage be operated as an ordinary class A amplifier with consequent inefiiciency.

The present invention seeks to avoid the above mentioned defects and to provide a satisfactory crystal oscillator circuit suitable for use at frequencies up to about 500 kc. which is near the maximum frequency for which it is practicable to make the cube type of crystal.

According to this invention a piezo-electrically controlled oscillation generator comprises an electron discharge tube oscillator of the cathode coupled type having its tuned circuit in parallel with a series circuit comprising the crystal and a balancing reactance therefor, the junction of said crystal with said balancing reactance being coupled to the grid of the tube.

The invention is illustrated in Figures 1 and 2 of the accompanying drawing which illustrates two embodiments thereof.

In theembodiment of the invention illustrated in Fig. 1 there is employed a triode'V having its cathode K connected to the mid-point of the inductance L of a parallel tuned circuit, the high tension end of which is capacitycoupled to the anode A of the triode, and the other end of which is connected to earth. Anode potential is applied to the triode through the usual inductance. A piezo-electric crystal P is connected between the grid G of the triode and earth and is in parallel with a grid resistance. The anode is connected to the grid through a balancing condenser C which is in practice adjusted until the feed-back is zero, 1. e., until the voltage fed to the grid through the balancing capacity and the voltage fed thereto through the capacity of the crystal and its holder are'equal and opposite, the said crystal and holder capacity and the said balancing capacity thus forming, as will be seen, a potentiometer across the parallel tuned circuit. With this arrangement self-oscillation at frequencies other than the crystal frequency is prevented by reason of the above described adjustment of the balancing condenser, whereas, at the crystal frequency, the circuit will oscillate strongly since the impedance of the crystal will then be very different from that presented at other frequencies, and the balance of the circuit will accordingly be upset. In the arrangement described the crystal operates in its series resonance mode and presents a comparatively low value of resistance (at resonance) between the grid and the earthed end of the parallel tuned circuit.

In the modification illustrated in Fig. 2 the positions in the circuit of the balancing condenser C and of the crystal P are interchanged. In this case the balancing condenser provides positive feed-back, balancing out negative feed-back effected through the capacity of the crystal and its holder. In this arrangement the crystal operates in its parallel resonance mode giving (at resonance) a high impedance between anode and grid, and under this condition the positive feedback through the balancing condenser causes the circuit to operate.

The main advantages of the invention are (1) very reliable starting characteristics are obtained, (2) the oscillator is capable of being keyed for telegraphy at hand speed keying, (3) the crystal current is extremely low, particularly when it is included between the grid and earth, with consequent lack of heating in the crystal and high frequency stability, (4) a fairly large amount of power can be drawn from the oscillator Without destroying its reliability.

I claim:

1. In combination, a triode discharge tube, a parallel tuned circuit having a high potential end and a grounded end, a series connected piezoelectric crystal and a condenser connected between the anode of said tube and the grounded end of said parallel tuned circuit, said condenser and the crystal capacity constituting alpotentiometer capacity, said triode tube having its anode coupled to the high potential end of said tuned circuit, its grid being connected tothe junction point of the crystal and condensen'and itscathode being connected to an intermediate point on the inductive element of theiparallelttuned.circuit.

2. An oscillation generator comprisingan electron discharge tube having at least a cathode, an

anode and acontrolgrid, agparallel resonant circuit having one end coupled tovsaid anode, the otherend being conductivelyrconnected through a 'resistorlto' said control grid, said cathode being connected solely to anintermediate point on the inductive "element of said resonant circuitand frequency stabilizing means comprising -two-capacitivexelements one of'which is connected between said anode and said control grid and the other: of which is in shunt with said resistor, one of said capacitive elements being constituted as a piezo-electric, device.

3.: *Anzoscillation, generator according to claim 2 wherein the:capacitive'.element whichis constituted asapiezo-electric device is disposed'between the anode: and control grid.

- 4.. Anoscillation, generator according to: claim ,2 wherein .the :.capacitive element which is iconstituted as aipiezo .electric device is disposed in shunt with said-resistor.

5. An oscillation generator comprising'an electronsdischarge tube connected in an oscillatory circuit which. includes inductive and capacitive elements connected in-paralleL- said tube having a cathode connected solely to a mid-tap of said inductive element, an anode coupled to one terminal of said inductive element and a control grid conductively connected through a grid resistor to the other terminal of said inductive element, a capacitive connection between the anode and control grid, a second capacitive connection in shunt with said grid resistor, the two said capacitive connections constituting means for neutralizing the capacitive feed-back energy, and

one of said capacitive connections being constituted solely as a piezo electric device for stabilizing the frequency of the oscillations generated. '6. An oscillation generator according to claim -5=and including a direct current source connected to said anode'through an inductive choke and to agrounded point along the connection between said inductive element and said grid resistor.

'7. An oscillation generator comprising an electron discharge tube having cathode, anode and control electrodes an input circuit comprising a substantially non-inductive grid resistor :interconnectingsaidcontrol grid-and said cathode; an output circuit interconnecting: said cathode and anode, a feed-back neutralizing circuit interelectric device.

8. An oscillation generatOrYaccordingI to :claim '7'wherein said piezo electric deviceistdisposed in said neutralizing circuit.

9. An oscillation generator according toclaim 7 wherein said: piezo electric device isdisposed in said input circuit.

GEORGE :PERCY PARKER.

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