US2410389A - Frequency divider - Google Patents

Description

Oct. 29, 1945.

E. NORRMAN FREQUENCY DIVIDER Filed Feb. 26, 1943 INVEN TOR.

ErfyzS/"o frm an, W2# qw Patented Oct. 29, 1946 'FREQUENCY DIVIDER Ernst' Norrman, Delavan, Wis., assignor to George W. Borg Corporation, Chicago, -Ill.,a corporation of Delaware .Application February 26, 1943, Serial No. 477,237

(Cl. 250`3G) l Claims. 1

The present invention relates in 'general to frequency conversion apparatus, and more in particular to so-called frequency dividers which are employed to obtaina desired lowjfrequency output from a. high frequency input. The object of the invention is to produce a newvand improved apparatus of this character.

The invention is useful, for example, in generating standard frequency alternating current for running the constant speed motor Aof `a watch timing device. In such a generator4 a source of constant frequency alternating current is required. A crystal oscillator has a substantially constant frequency and'accordingly is very desirable for this purpose, but the frequency is far too high for. use in running a motor. However, by the usev of a suitable frequency Vdivider the high crystal oscillator frequency (in the ,neighborhood of 90,000 cycles per second) can be reduced to a sub-multiple frequency of 100 cycles per seconder some other desired low frequency, and by suitable amplification ofthe low lfrequency output of the frequency divider the required low frequency current for running the motor may be obtained.

Apparatusof the foregoing character, comprising a crystal oscillator, frequency divider, and amplifier, may be referred to as a constant frequency generator, and has been used extensively in connection with watch timing apparatus. In

this known generator the frequency divider comprises a plurality of multivibrator stages connected in tandem and each operating at a frequency which is a sub-multiple of the frequency at the preceding stage.

VA frequency divider ,using multivibrators operates very satisfactorily. At each stage, however, it requires two tubes or the equivalent, and since the rate of division is rather-low (rarelyexceeding 5 or 6), a considerable number of stages is required. For these reasons the cost of a standard frequency generator using multivibrators is rather high.

A Mspecific object of the present invention, therefore, isa frequency divider using single tube oscillators instead of multivibrators. Another object is a frequency divider having a larger division at each stage than has'been considered practicable in frequency dividers using multivibrators.

' vA well known single tube oscillator is the socalled Hartley oscillator, employing a tuned plate divider, for reasons which-need-not begone into in detail. For one thing, the standard Har'tley oscillator 4is not susceptible to reliable control .at a sub-multiple frequency by ,alternating currents derived from a crystal oscillator. Another difficulty arises from the fact that if it is attempted to control a Hartley oscillator `ata sub-multiple frequency fromla similar oscillator the controlled oscillator tendsl to operate as an amplifier at the control frequency.

.In this situation, I have discovered4 thatwith suitable coupling arrangements which I have also devised a somewhat Vdifferent single tube oscillator can' be made to operate very satisfactorily as a frequency divider. The new oscillator ldiffers from the standard Hartley'oscillator in several respects, as will be explained fully hereinafter in describing its construction and operation. One feature which is important and which may be mentioned at this time is the wave shape of the oscillator grid Voltage. VI have 'discovered that forsatisfactory operation the positive half of the wave must bedecidedly limited inv amplitude, with a characteristic `flat top.

According toanother feature of the invention, two somewhat Adifferent circuit arrangements are employed, the one used in any case ,depending on 4whether the oscillator is to divide by an oddor even number. l I have discovered that in the case of anoscillator which is designed for odd division the4 best results-are obtained when a condenser and leak resistor are used in the grid circuit, whereas inthe case of an oscillator designed for even division these parts are omitted.

,Theinvention and the various features thereof will be fully described hereinafter, reference beingV madek to ,the accompanying'drawing, which shows by means ofthe usual diagrammatic'circuit the apparatus and circuits involved.

Referring to the drawing, the completeequip- 'ment therein shown is a standard frequency lgenerator which may be used, for example, to supply alternating current for running a small motor at constant speed. Theprincipal component parts of the generator are acrystal oscillator, athreestage frequency divider, and a power amplifier. The crystal oscillator and power amplifier arefof known construction. The invention relatesV to the combination of these elements with the frequency divider and more -in particular to the frequency divider per-se, as previously indicated.

The `frequency of the crystal oscillator is rassumed to be 590,000 cycles per second,-as indicated in the drawing. The three yoscillators which comprise the three-stage frequency divider divide, respectively, by 10, 10 and 9, and oscillate at frequencies of 9,000y 900 and 100 cycles per second, respectively. This arrangement, giving an output frequency of 100 cycles per second, is satisfactory for the intended purpose, although it will be understood that the invention is not limited to the employment of any specic frequencies. By selecting a different oscillator frequency, or by adjusting one or more of the oscillators at the frequency divider to divide differently, or both, various other output frequencies may be obtained.

Describing the equipment more in detail, the reference character I indicates a suitable space discharge device, which may be a type 6J7 pentode. This tube functions as a combined oscillator and amplier tube. The control grid and the screen grid are coupled to the crystal, as shown, by means of condensers. The screen grid functions as the plate or anode in this circuit, being supplied with anode potential through the resistor Il, which may have a value of 500 M ohms. The control grid is connected to ground through the resistor I9, which may have a value of one megohm. There is a reactance coil 20, value about 20 milli-henrys, which is connected in series with the crystal I6, and the oscillator is tuned to the exact frequency by means of the condenser I 8. The control and screen grids function also in the amplifier. The suppressor grid is connected to the cathode as shown, and the plate is connected to the plus B lead through the resistor 2l, which may have a value of 500 M ohms.

The first oscillator stage of the frequency divider comprises a space discharge device I I, which may be a type 6F8 triode. Explaining the oscillator circuits briey, the cathode is grounded, and the plate is connected to the plus B lead through the resistor 25, the value of which may be 50 M ohms. There is a tuned circuit comprising the inductance 29 and the condenser 28 which is connected between the plate and ground by means of the condenser 21. This condenser preferably has a value of .001 mf. and is shunted by the 100 M ohm resistor 26. The inductance 29 is one Winding of a transformer the other Winding 30 of which is connected to the control grid of the tube through the l0 M ohm resistor The ratio of primary turns to secondary turns is preferably about 4 to 1, as indicated.

In order to control the oscillator Il from the preceding stage the control grid of tube II is coupled to the plate circuit of tube I0 by means of the resistor 23 and the condenser 22. The resistor 23 preferably has a value of 150 M ohms, while the capacity of the condenser 22 may be 100 mmf. A

The second oscillator stage of the frequency divider comprises the space discharge device I2, which may also be a type 6F8 triode. This oscillator is similar to the first stage oscillator except for diiferent values of some of the circuit elements. Thus the coupling condenser 3l preferably has a value of .01 mf. The ratio of turns in transformer winding 32 to the turns in the secondary winding 33 is 2 to 1. The resistors 34 and 35 preferably have values ofY 50 M ohms and one megohm, respectively.

The third oscillator stage of the frequency divider comprises the space discharge device I3, which may be another type 6F8 triode. The coupling condenser 40 preferably has a value of .1 mf. The turn ratio between the transformer windings 4I and 42 is 2 to 1, and the values of the resistors 44 and 45 are preferably 50 M ohms and 500 M ohms, respectively. There is a condenser 46 (capacity .05 mf.) in the grid circuit, shunted by the leak resistor 41 which preferably has a value of M ohms.

The power amplifier comprises the tubes I4 and I5, which may be of any suitable type, and includes the output transformer 50. The conductors 5I and 52 constitute the output circuit, which may leadrto a small synchronous motor. The amplier is of the push-pull type and the grids of the tubes are controlled from the third oscillator stage of the frequency divider, as indicated in the drawing. In this connection it will be noted that the oscillator transformer has a third winding 43. Windings 42 and 43 together have the same number of turns as winding 4I, whereby equal and opposite voltages to ground are available for application to the grids of tubes I4 and I5. The resistor 55 is the usual grid biasing resistor.

The apparatus having been described, the operation of the complete standard frequency generator will now be explained.

' The operation of the crystal oscillator and amplier stage I0 is well known and need not be described in detail. Suffice it to say that by the operation of the crystal oscillator section of the tube I0 iluctuating currents having a frequency of 90,000 cycles per second are set up in the plate circuit of the amplifier section of the tube, whereby fluctuating voltages of the same frequency are made available at the plate for application over the control circuit to the grid of the tube II of the first oscillator stage of the frequency divider. The crystal oscillator may be tuned to the exact frequency of 90,000 cycles per second by means of the condenser I8.

The first stage oscillator II of the frequency divider has a frequency of 9,000 cycles per second and is driven or controlled at exactly that frequency by voltages transmitted from the plate circuit of tube I0 by way of the condenser 22 and resistor 23 to the grid of tube I I.

The second stage oscillator I2 of the frequency divider oscillates at a frequency of 900 cycles per second. It is controlled at exactly that frequency by voltages developed across the tuned circuit 28-29 of the rst stage oscillator and transmitted to the grid of tube I 2 through the resistor 35.

The third stage oscillator I3 of the frequency divider oscillates at a frequency of I00 cycles per second and is controlled at that frequency by voltages developed across the tuned circuit 36-32 of the second stage oscillator and transmitted by way of resistor 45 and' grid condenser 46 to the grid of tube I3.

The push-pull amplier I 4--I 5 amplies the 100 cycle output of the third stage oscillator, said output comprising equal and opposite voltages to ground which are developed in the winding 4I and windings 42 and 43 of the oscillator transformer. These voltages control the operation of the amplifier in known manner, with the result that alternating current having a frequency of 100 cycles per second is transmitted over the output circuit 5I-52.

The foregoing is a more or less general description of the operation. Attention will now be directed more particularly to the features of the oscillators in the frequency divider hereinbefore mentioned and other features which are desirable or essential to satisfactory performance.

The tuning of the oscillators may first be considered. .Thezoscillator I2, for example,'oscillates undericontrol at a frequencyof 900`cycles per second. It 'may `rbetuned to Vthat frequency, but -is preferably'tunedto asome'what -lower frequency. .The tuning isaccomplished by selecting a. condenser 36-of-the proper capacity ,fand by adding a small trimmer-condenseras indicated, if required. f'I/'he frequency to which the oscillator should be tuned for the best results is determined by ascertainingthe'upper and .lower limits of the natural or uncontrolled frequency which the os-4 vsusceptible to controla-t 900. cycles persecond from the preceding 9,000-cycle oscillator. This is a range of 95 cycles. approximately 32 cycles, which when added to 820 cyclesgives 852 cycles per second as .the proper frequency to which the oscillator should be tuned.

The'reason for tuning to .the lo'w side of the range is two-fold. The operative natural frequency range extends farther belowthe operating frequency of 900 cycles per second than it does above it, and the changes which may be expected I to occur in time, such as changes in condenser capacity or variations in resistance of the resistors, tend to increase the natural frequency of the oscillator.

The condenser 3! is the coupling condenser through which power is delivered from the plate circuit of oscillator I2 to the tuned circuit cornprising condenser 36 and inductance 32. Whereas a large condenser on the order of 2 mf. in capacity is generally used at this point, I -Yhave found it desirable to use a condenser of much lower capacity in order 'to' limit the amo-unt of power transferred. -Otherwise too high a voltage appears across the tuned circuit,.and it is very difiicult to properly proportion the `other circuit elements. YIt will be' notedthat the'condensers such as' 3l have different capacity'values depending on the frequency of the oscillators to which they belong. Condenser 3i has a-capacityof .0l mf., condenser 2'I has a capaci-tyof .G01-mf., while condenser 40 hasa capacity Yof .l mf. These values are notl critical, but it is important to ruse such values that the described limiting effect `is secured.

It has been found that -when 'the condenser-V3I for example, is made of low enoughv` capacity to substantially limit the,power transferred to the associated tunedcircuitit hasta considerablezeffect on the tuning of the tuned circuit, and any change-which might occur inthe capacity of ,the condenser would change. the oscillator frequency appreciably. It is desirable to eliminateas many changeable factors as possible, and `accordingly the condenser 3| is shunted by the high yresistance resistor 3'I. When this shunt is used the capacity of condenser 3| can change by as much as 20% without causing failure of .the oscillator to'divide properly. Asimilar shunt resistance 25 is connected around vthe conden'serlatthe oscillator I I.

One-third of 95. cycles is The tuned circuits at theseveral :oscillators shouldeach have va rather high inductance to capacityv ratio, and for this purpose ironcoresare used in the oscillator transformers.

The fact that the oscillator gridvoltage should have a wave shape characterizedbya low arnplitude flat-topped positive halfl wave has previously beenv mentioned. At the oscillator I2 the required wave shape is secured by means of the high resistance 34 in the grid circuit. When the gridswings positive it starts to draw current, but the resistance 34 is so high relative to the internal resistance of the tube .(cathode to grid) that the major part of the .total voltage in the-grid circuit is developed across vthe resistor'34 andthe .potential of the grid relative to the cathode is limited to a low value. Similar resistors are used in the grid circuits. ofthe .other oscillators in the frequency divider. .This is an important feature, since it is impossible to .properly control .the oscillators at the desiredsub-multiple frequencies unless the resistors ,are presentin the grid circuits.

The ratio of .the control voltage to the oscillator grid voltage is also of paramount importance. Considering the oscillator I2 again, .the grid voltage is thevoltage generated in the secondary winding,33,dueto its coupling lwith the primary winding 32. It is the coupling. between these two windings, one in theplate circuit and the other in thegrid circuit, that makes thetube oscillate. By the term control voltage I mean the voltage that is .transmitted from the preceding stage and applied to the grid of tube I2 .to cause it to .oscillate .at exactly .the -desired frequency. This voltage is transmitted. over a circuit to ground which includes the resistor35, the resistor34 and the winding 33, the latter .having such a 10W resistance that it may be neglected. The gridfof the tube I2 being connected to the junction of resistors 35 .and 34, these resistors operate as a voltagedivider or potentiometer and the ratio of resistance 35 to resistance 313 determines what proportion of the` total voltage available will .be effective at the grid of the tube.

Now. I have found that for the best results the control voltage ,should be approximately 15% of the grid voltage. The desired ratio is secured by a proper proportioning of the factors which affect it, but mainly by selecting a resistor 35 havingfthe `proper resistance value.

In stating that the best results are secured when the control voltage is 15% of the grid voltage, I mean that with this ratio of control voltage to grid Voltage the oscillator may be tuned to any frequency within a maximum frequency range and still be susceptible to control at the desired frequencyof 900 cycles per second, and that if the ratio is changed substantially in either direction the frequency range is,corre spondingly narrowed. This may be illustrated by the results of tests on. the 900-cyc1e.oscillator I2. It was previously mentioned that. this oscillator may have a natural .or ,uncontrolled frequency ranging from 820 cycles per secondto 915 cycles'per second,.but it will be understood. now that. it has this large frequency range only when the control Voltage bears the, optimum relation tothe grid voltage. .I have found that if the control voltageV is raised to 30% of the grid `voltage, the natural frequency range .within which .the oscillator may be tuned becomes approxi- .mately'825 cycles to 880 cycles, while if thecon.-

trol voltage.is. decreased .to v',7.1/% of the grid voltagethe frequency rangebecomes approximately 870 cycles to 920 cycles. kThese values show a considerable narrowing of the range and indicate the importance of maintaining the ratio of control voltage to grid voltage within fairly close limits and as near to the best ratio as is practicable. The same ratio is used at all the oscillators of the frequency divider.

Attention has already been directed to the fact that a, condenser is used in the grid circuit at the oscillator I3, which divides by 9. I have discovered that in the case of an oscillator which divides by an odd number the insertion of a condenser of the proper capacity in the grid circuit approximately doubles the natural frequency range within which the oscillator may be tuned. In the case of an oscillator which divides by an even number, however, the use of a grid condenser is of no avail and actually decreases the range. Accordingly, at any stage where odd division is required the grid condenser is used, b'ut is omitted at other stages.

In the description certain types of tubes have been specied, and actual values have been given for various parts such as condensers and resistors, but it will be understood that this has been done merely by way of example and to assist in the understanding of a concrete embodiment of the invention and the principles which govern its operation. l

The invention having been described, that which is believed to be new and for which the protection of'Letters Patent is desired will be pointed out in the appended claims.

I claim:

1. In a standard frequency generator, a crystal oscillator having a constant high frequency, a frequency divider comprising a plurality of single tube oscillators, each oscillator comprising a space discharge device having inductively coupled plate and grid circuits, means for coupling said oscillators together in tandem relation, said means comprising a conductive connection from the plate of each oscillator except the last to the inductively coupled grid of the next oscillator, tuning means in said single tube oscillators whereby the same are caused to oscillate at progressively lower frequencies which are sub-multiples of the frequency of said crystal oscillator, and means for amplifying the output of the last oscillator.

2. In combination, an oscillator comprising a single space discharge device, a transformer having three windings all connected in series, the number of turns in the first winding being equal to the number of turns in the second and third windings, a tuned circuit including the rst winding coupled to the plate of said device, a grid circuit for said device including the second winding, a push-pull amplifier comprising two space discharge devices having their cathodes connected to the junction of the said rst and second windings, and grid circuits for the space discharge devices of said amplifier, one including the said first winding and the other including the second and third windingsM 3. In a frequency divider, an oscillator comprising a space discharge device having cathode, grid and plate elements, a tuned plate circuit including an inductance and a condenser in parallel therewith connected between said plate and cathode, a grid circuit including a resistor connected directly to said grid and a second in ductance connected to said cathode, said second inductance being inductively related to said i'irst inductance, a control circuit including a second resistor connected directly to said grid and to said Iirst resistor, and means for transmitting control voltages over said control circuit to control said oscillator at a frequency which is a submultiple of the control voltage frequency, the said resistors having values which are so related to each other that the control voltage impressed on said grid over said control circuit is not less than 10% nor more than 30% of the voltage impressed on said grid by said grid circuit due to lts coupling with the tuned plate circuit.

4. In a frequency divider, a first oscillator, a second oscillator, each oscillator comprising a single space discharge device having cathode, grid and plate elements, each oscillator also comprising a direct current plate circuit including a resistor, a tuned circuit which is connected to the plate by means of a condenser, and a grid circuit conductive to direct current, which is inductively coupled to the tuned circuit, the tuned circuit of the second oscillator being so adjusted that the second oscillator is adapted to oscillate at a frequency which is a sub-multiple of the frequency of the irst oscillator, and means for coupling the second oscillator to the first oscillator comprising a resistor connected from the junction of the condenser and tuned circuit at the first oscillator to the grid of the second oscillator.

5. In a frequency divider, an oscillator comprising a space discharge device having cathode, grid and plate elements, a rst plate circuit including a source of current, a second plate circuit excluding said source and including tuning elements and a condenser, a grid circuit conductive to direct current which is inductively coupled to said second plate circuit, a second oscillator comprising a similar space discharge device and having similar plate and grid circuits, except that the second plate circuit is tuned to a diiierent frequency, and means for coupling the second oscillator to the iirst oscillator comprising a connection which is conductive to direct current extending between the grid of the second oscillator and the first plate circuit of the first oscillator.

6. In a. frequency divider, a rst oscillator, a second oscillator, each oscillator comprising a single space discharge device having cathode, grid and plate elements, each said oscillator also comprising a plate circuit including a source of direct current, a tuned plate circuit excluding said current source, and a grid circuit inductively coupled to the tuned plate circuit, a condenser .connecting the tuned plate circuit of each oscilvwith the grid circuit of the second oscillator, and

a resistor connected in shunt of said condenser at the rst oscillator, whereby a positive bias is placed on the grid of the second oscillator.

7. In a frequency divider, an oscillator comprising a space discharge device, a tuned circuit coupled to the plate of said device, a grid circuit for said device coupled to said tuned circuit, whereby current changes in said tuned circuit produce grid voltages in said grid circuit, a high resistance in said grid circuit, a control circuit extending to the grid of said device, a high resistance in said control circuit, said resistances being connected together at said grid and means for transmitting control voltages over said control circuit to control said oscillator at a frequency which is a sub-multiple of the frequency of said control voltages, the values of said resistances being so proportioned that the control voltage impressed on said grid by said control circuit is not more than 30% of the voltage irnpressed on said grid by said grid circuit.

8. In a frequency divider, an oscillatoi` comprising a space discharge device including cathode, grid and plate, a, tuned circuit, a condenser coupling said tuned circuit to the plate of said device, said condenser having a capacity so related to the resonance frequency of said tuned circuit that the power transferred to the tuned circuit is substantially limited, a circuit including said cathode and grid inductively coupled to said tuned circuit, means for transmitting control voltages to the said grid to control said oscillator at a frequency which is determined by said tuned circuit, said frequency being a subqnultiple of the frequency of said control voltages, a control circuit over which said control voltages are transmitted terminating directly at said grid, and a high resistance connected in shunt of said cou* pling condenser to limit the effect which said condenser has on the tuning of the tuned circuit.

9. A frequency divider as claimed in claim 4, wherein the tuned circuit in each oscillator coni prises an inductance and a capacity connected in parallel, and each inductance has an iro-n core to give a relatively high ratio of inductance to capacity.

10. A frequency divider as claimed in claim 4, wherein the condensers which connect the tuned circuits in the rst and second oscillators to the plates of such oscillators have capacities which are inversely proportionate to the frequencies of such oscillators, and the capacity o-f each condenser is 10W enough to substantially limit the power transferred through it to the associated tuned circuit.

ERNST NORRMAN.

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