US2236532A - Constant frequency generator - Google Patents

Description

April'l, 1941. T. B.

consum' FnnQuENcY Humm-on GIBBS 3 Sheets-Sheet 1 Filed Feb. 4, 1957 aacslvlelstdnn April1, 1941. .13.61555 y 2.236.532

CONSTANT FREQUENCY GENERATOR April 1, 1941.l 1'. B. Glass CONSTANT FREQUENCY GENERATOR Filed Feb. 4, A1937 3 Sheets-Sheet 5 RSM.

Patented Apr. 1, 1941 TES PATENT OFFHCE 2 Claims.

My invention relates in general to constant frequency generators, and the object of the invention, generally stated, is to produce a generator having an output frequency of 60 cycles per second, or other standard or desired frequency, which is exact and not subject to hunting or other variations. The generator may be used to operate an electrical master clock, or in connection with watch timing apparatus, or for other purposes where a constant low frequency is necessary. The invention depends on the known constant frequency characteristic of a crystal, such as a crystal of quartz, when used in an oscillator, as by coupling the crystal `to the grid of a triode having a tuned plate circuit. An oscillator of this type, known as a crystal or piezoelectric oscillator, has a very constant frequency, but the natural frequency is high; and, while it can be regulated within certain limits, the lower limit' in practice is far higher than frequencies on the order of 60 cycles per second.

The problem is presented, therefore, of devising a frequency divider or arrangement, Whereby the output of a crystal oscillator may have its frequency reduced to lthe desired frequency, say 60 cycles per second. According to the invention, this is accomplished by employing a plurality of so-called multi-vibrators in tandem relation and tuned to progressively lower frequencies. If the desired output frequency is 60 cycles per second, the last multi-vibrator is tuned to that frequency, and the crystal oscillator is tuned to a frequency within its natural range which is an exact multiple of 60. The other multi-vibrators are tuned to intermediate frequencies which are also multiples of 60. The multi-vibrators are not constant frequency devices, but the crystal oscillator has a Aconstant frequency and is used to control the first multi-vibrator and keep it in step at a lower frequency. The first multi-vibrator controls the second multi-vibrator to keep it in step at a still lower frequency, and so on, with the result that the constant frequency characteristic of the crystal oscillator is transmitted to the 60-cycle multivibrator. The output from this multi-vibrator, having the desired low frequency and the same constancy as the crystal oscillator, may be amplied to any desired extent.

The foregoing states in brief the broad principle employed in theinvention. There are numerous difficulties in the way of employing the principle in practice, which, however, have been satisfactorily solved. The broad aspects of the invention, as well as the various features involved in the solution of the practical problems, will be dealt with in detail hereinafter, reference being had to the accompanying drawings.

In the drawings, Figs. l, 2 and 3 show the pre-` ferred form of the invention. The 'apparatus is shown in conventional form, and the connections are shown by means of the usual diagrammatic circuit drawings.

Referring to Fig. 1, the reference character IBD indicates a crystal which is suitably mounted in association with the two metal plates lill' and |02. The crystal rests on plate |02, but plate |0| is separated from the crystal a short distance, say a few thousandths of an inch. Associated with the crystal mounting are a heating coil |03 and a thermostat |04, which are provided for the purpose of maintaining the temperature of the crystal-at a point somewhat abcvenormal atmospheric temperatures.

The reference character Il indicates a type 6A6 vacuum tube, which is a double triode, having two plates, two grids, and a common cathode. In discussing this tube, the two halves of the tube will be considered as left and right triodes, for convenience, and the same practice will be followed in connection with other tubes of the same type.

The left triode, tube ll, has its grid |09 directly connected to the upper plate |0| associated with the crystal IBG; and the left triode and the crystal, together with the associated parts and circuit connections constitute a piezo electric oscillator. This oscillator includes a grid leak IDE, a variable condenser ||5 for regulating the shunt capacity across the crystal to adjust its frequency, and means including an inductance Hl and a condenser l I for tuning the plate circuit. The gridleak |06 may have a value of 2 megohms, the condenser may cover the range of '7 to 50 micro-microfarads, the inductance H0 may have a value of l0 millihenrys, and the condenser may have a capacity of '70 micromicrofarads. These values are given by way of example, and for convenience are noted on the drawings. The same practice is followed throughout'the drawings, and representative values are shown wherever such showing will aid in understanding the circuits.

The values of the tuning elements referred to above are of course selected with reference toy the oscillator frequency. In the example shown herein the grid circuit, including the crystal |00, is tuned to a fundamental frequency of y108,000 cycles per secondby proper proportioning of the parts and by means of condenser |05. In an oscillator of this type the oscillator will start and its operation is more stable if the harmonics, especially the second harmonic, make up a large portion of the total output, and accordingly .the plate circuit is not tuned to the fundamental frequency, but is tuned to a frequency about halfway between the fundamental frequency and the frequency of the second harmonic, say about 162,000 cycles per second. The plate circuit of the left triode also` includes a resistor |20, of 60,000 (60M) ohms. A by-pass `condenser is indicated at |2|.

The plate circuit of the left triode, tube |01, is coupled to the grid ||2 of the right triode by means of a resistor ||3 and a condenser ||4. The right triode functions primarily as a filter to eliminate the harmonics, and to this end the circuit of plate I5 includes a closed resonant circuit comprising inductance 6 and variable condenser ||1 and tuned to resonance at a frequency of 108,000 cycles per second. The plate circuit also includes a resistor |l8. A oy-pass condenser is shown at |I9.

The first multi-vibrator MV! utilizes another type 6A6 tube, indicated by reference character |21. The plate circuit of the left triode includes a resistor |3I, and the grid is provided with a grid leak |30. The plate circuit of the right triode includes resistor |33, and the grid has associated with it grid leak |32. The plate |28 of the left triode is coupled to the grid |31 of the right triode by means of a condenser |34, which may have a capacity of 500 rnicro-microfarads. Similarly, the plate |35 of the right triode is coupled to the grid |29 of the left triode by means of a condenser |35, also having a capacity of 500 micro-microfarads. Thus the grid of each triode is inuenced by the plate circuit of the other triode and the device will oscillate in known manner. The frequency is determined by the values of the resistors and condensers and in the case of the first multi-vibrator is as nearly as possible one-sixth the fundamental frequency of the piezo electric oscillator, or 18,000 cycles per second.

The plate circuit of the right triode, tube |01, is coupled to the grid |29 of the left triode, tube |21, or to the first multi-vibrator, by means of a condenser |23 and a resistor |24.

The second multi-vibrator MV2 is similar to the first and includes a type SAG tube |41, but the grid leaks have different Values and the plate grid coupling condensers |48 and |49 are of greater capacity, whereby the frequency is lower. The frequency of the multi-vibrator MV2 may be, for example, one-fifth the frequency of the multi-vibrator MVI, or 3600 cycles per second.

The multi-vibrator MV| is coupled to the multi-vibrator MV2 by a condenser |40, which connects the grid of the right triode, tube |21, with the grid of the left triode, tube |41.

The multi-vibrators MV3 and MV4, including tubes |51 and |61, respectively, are similar to the first two, but are tuned to progressively lower frequencies. The frequency of the multivibrator MV3 may be one-fifth that of the second, or '720 cycles per second, and the frequency of the multi-vibrator MV4 may be one-half that of the third, or 360 cycles per second.

The multi-vibrator MV2 is coupled to the multi-vibrator MV3 by means of a condenser |50. The multi-vibrator MV3 is coupled to the multivibrator MV4 by means of a condenser |60.

Referring now to Fig. 2, the tube 200 is another type SAG double triode, but the plate circuits are independent and the tube functions as two ordinary triodes. Both grids are controlled from tie multi-vibrator MV4 through a condenser |10 and a resistor |1|, Fig. 1. Each plate circuit includes an inductance of fairly substantial value in addition to a resistor.

The tube 2i!! and the tube 220, both type GAG tubes, together with the associated circuit and tuning elements shown, constitute two final stage low frequency multi-Vibrators MV5 and MVG, and are tuned to operate at 12 cycles per second and G0 cycles per second, respectively. These values are, respectively, one-fifth and onesixth of 350, which is the frequency of the multivibrator MV4. The low frequency multivibrators are similar to those shown in Fig. 1, except for the difference in frequency.

The plate circuit of the left triode of tube 200 is coupled to the Z2-cycle multi-vibrator MVS by means of condenser 202, and the plate circuit of the right triode of tube 200 is coupled to the (S0-cycle multi-vibrator MVS by a condenser 204.

The reference character 230 indicates a triode which may be a type 76 tube. The grid of this tube is controlled from the multi-vibrator MVS by means of a voltage divider or potentiometer comprising the resistor 2|2 and the resistor 23|, the latter resistor functioning also as a grid leak and being shunted by a condenser 232. The plate circuit of tube 230 includes a closed resonant circuit comprising inductance 233 and condenser 234 having such values that the circuit is resonant at a frequency of '72 cycles per second. The cathode 23E is not connected direct to ground, but the ground connection includes a resistance 231 shunted by a by-pass condenser 23 The latter may be of the electrolytic type,

l having a capacity of 10 microfarads or more.

' vshown is a type 635 amplifying tube.

The arrangement, including resistor 231, is a customary arrangement for making the cathode positive with respect to ground when the tube passes current, which has the effect of making the grid 235 negative with respect to the cathode 230. In other words, this arrangement provides negative bias for the grid.

The reference character 240 indicates another triode which is similar to triode 230. 'I'he arrangement is the same as that just described, except that the closed resonant circuit in the late circuit is tuned to cycles per second instead of '72 cycles per second. The grid, of course, is controlled by the 60-cycle multivibrator.

From the plate circuits of tubes 230 and 240 extend the 72-cyc1e and (l0-cycle output leads 230 and 240. A tap is taken oi the (iO-cycle output lead 240, and is' connected by the means shown to the grid 25| of tube 250, which may be a type 75 triode. The plate circuit of tube 250 is coupled by means of a transformer 253 to the grid 28| on the input side of tube 260, which as This tube has two grids and two plates, and also has two cathodes which are connected by a 10,000 ohm resistance inside the tube. This is a somewhat special type of tube, but its construction is known and is suiiciently indicated in the drawings.

The output leads 239 and 249 extend by way of resistors 210 and 21| and condensers 212 and 212 to the switch S, Fig. 3. In its upper closed position switch S grounds output lead 239 and connects output lead 249 to ground through the voltage control rheostat R. In the lower closed position of switch S these connections are reversed.

The reference character 304 indicates a potentiometer, which is bridged across the output leads 239 and 249 in series with the resistors 302 and 303. The movable contact of the potentiometer is connected tothe grid of the tube 300, which may be a type 59 pentode operating as a triode. This tube has the same arrangement for obtaining a negative grid bias as was described in connection with tubes 230 and 240, Fig. 2.

The plate circuit of tube 300 is coupled by means of a transformer 30| to the push-pull power amplifier comprising tubes 3|0, 3H, 3|2 and 3|3. These may be type 2A3 tubes. As clearly shown, tubes 3|0 and 3|2 are connected in parallel, and likewise tubes 3| and 3|3. Grid bias is supplied by means which will be pointed out.

The plate circuits are coupled by means of transformer 320 to the A. C'. power output leads 330 and 33|. Across these leads may be connected any device that requires a constant frequency power supply. The drawings indicate a motor M. The apparatus is all operated from an A. C. commercial power line. To this end a plug 350,

Fig. 3, may be provided, which can be plugged' into any convenient receptacle. A switch SI serves to extend the connection to the power bus bars |98 and |99.

In Fig. 1, the crystal temperature control arrangement, including heater coil |03, is bridged across the bus bars |93 and |99. Also bridged across these bus bars is the primary winding of the transformer |80. This transformer and the rectier tube |84 constitute a known type of fullwave rectier for furnishing plate current to the oscillator and multi-vibrator tubes in Fig. 1, also those tubes to the left of the dotted line in Fig. 2. Winding |8| is the rectifier winding and its mid point, which is the negative pole of the rectifier, is connected to ground. The cathode of the tube |84, which is the positive pole, is connected to the plate supply lead |85, also labeled in the drawings. A suitable lter is inserted as shown. Winding |82 supplies current for heating the cathode of tube |84; and winding |83 supplies current for the heaters in various oscillator and multi-vibrator tubes in Figs. 1 and 2. The heater connections have been omitted to avoid complicating the drawings, but it will be understood that all heaters labeled X-X are" connected to winding |83.

In Fig. 2 is shown another full-wave rectifier i comprising transformer 280 and rectifier tube 284. This rectifier supplies plate current to the tubes 250 and 260. Transformer 280 also has a winding labeled Y-Y for supplying heater current to these tubes as indicated.

In Fig. 3 is shown a transformer 380, the primary winding of which may be connected across the bus bars |98 and I 99 by means of a switch S2. The transformer has a winding 38| which, together with rectifier tube 384, constitutes a fullwave rectifier for supplying plate current to tubes 300 and 3|0 to 3|3, inclusive. A winding 383, whose terminals are labeled Z-Z, furnishes current for the heaters of these tubes as indicated. Transformer 380 has another winding 381 which, together with rectifier tube 386, constitutes a fullwave rectifier for supplying a negative grid bias to the tubes 3|0-3|3 of the power amplifier. In this connection it will be noted that the cathode or positive rectier terminal is grounded, while the mid point of Winding 361, which is the negative terminal, leads by way of a suitable filter and over .the conductor 339 to the grid circuits of the power amplier tubes 3MB-3|3,

It will be understood that, while certain specic types of tubes have been mentioned, this is merely by way of information to aid in practicing the invention and is not intended as a limitation or to mean that other types of tubes cannot be used. For instance, wherever a double triode type 6A6 tube is used, two ordinary triodes may obviously be substituted. The same is true as regards the other items of apparatus, including the resistors and condensers whose values have been specifically stated, with the object, however, of merely giving one set of values that has been found to be satisfactory.

The operation of the constant frequency generator shown in Figs. 1, 2 and 3 will now be described. For this purpose it will be assumed that plug 330 is inserted 'in a commercial A. C. power outlet and that switches Si and S2 are closed. It will be assumed also that switch S is closed to its upper position. Switches Sl and S2 render active the rectiers land the temperature control arrangement for the crystal, while switch S associates the Gil-cycle output lead 249 with the power amplifier. The rectiers being in operation, the tubes are supplied with heater and plate current, and the crystal oscillator and the multi-vibrators begin to function. These devices are unstable and begin oscillating spontaneously due to any slight mechanical jar or to `a slight fluctuation in the plate supply voltage such as is constantly occurring.

In Fig. l, the oscillator comprising the crystal i90 and the left triode of tube 07 begins toV operate and will settle down to a xed constant frequency of 108.000 cycles per :second as soon as the crystal is raised to the predetermined temperature by the heater |33.'y It may be assumed that, the frequency has already been adjusted at this temperature by means of condenser |05.

As a result of the operation of the oscillator,

, fluctuating currents flow in the plate circuit of the left triode, and the positive potential impressed on the resistor |13 and on con-denser ||4 from the plate circuit rises and falls. Accordingly, the condenser Ht charges and discharges in series with the resistor E22, and alternating voltages are impressed on the grid ||2 of the right triode. These alternating voltages have a fundamental frequency' of 108,000 cycles per second, but as stated before, the harmonics, and especially the second harmonic, are pronounced, and voltages of all these frequencies are therefore impressed on the grid H2. Now the plate circuit of the right triode includes a closed resonating circuit which, as previously mentioned, is tuned to resonate 4at a frequency of 108,000 cycles per second. This rescnating circuit offers very little impedance toy plate currents produced by the grid voltages of harmonic frequencies, and consequently such plato currents find a low resistance path to ground through the resonant circuit and condenser lit and impress very smal?. volt.. es the condenser 23. The resonant circuitdoes, however, offer a very high impedance to plate currents of the fundamental frequency, and consequently the drop across this impedance, fiuctuating with the fluctuations of plate current produced by voltages of the fundamental frequencyV at the grid |52, applied to the condenser .i 23. The eective voltage changes at the condenser 23 therefore are made upof components of very low amplitude due to the harmonics and a component of high amplitude due to the fundamental frequency. The harmonies are thus effectively eliminated.

The operation of a multi-vibrator such as is shown herein is in general well understood and consequently it will be unnecessary to go into this in detail. The rst multi-vibrator MVI oscillates at a frequency of 18,000 cycles per second; that is, it is so tuned that its natural frequency is 18,000 cycles per second, or as near as may be to that value. If oscillating independently, the frequency would tend to vary somewhat due to temperature, supply voltage changes, etc. When connected as shown in Fig. 1, however, multivibrator MV l receives alternating voltages of a frequency of 108,000 cycles per second due to the charging and discharging of condenser |23 and pulls into step and operates at a frequency of exactly 18,000 cycles per second.

An explanation of just why the multi-vibrator does this is somewhat complicated, and a detailed discussion is unnecessary to an understanding of the invention. It may be stated, however, that in the operation of the multi-vibrator the grid |29 goes alternately positive and negative (grid |31, of course, does the same, the voltage changes being displaced 180 in phase from those of grid |29). The alternating voltages due to the charging .and discharging of condenser |23 are impressed on grid |29. These impressed voltages alternately aid and oppose the voltages at grid |29 which are produced by the multi-vibrator operation, as they have six times the frequency of 4the latter; but as the impressed high frequency voltages are of relatively low amplitude they cannot reverse the multi-vibrator grid voltages ex cept when the latter are changing near the zero line. At these points the impressed high frequency voltages apply a corrective influence. If the multi-vibrator or local grid voltage tends to change too early, due to the multi-vibrator running too fast, the change is opposed each time by the high frequency voltage impressed at that instant and the shift across the Zero line is delayed. On the other hand, if the local grid voltage tends to change too late, due to the multivibrator running too slow, the high frequency voltage has the effect of speeding up the change each time. The control thus exercised is suiicient to keep the multi-vibrator in step, notwithstanding fairly substantial variations in the frequency at which the multi-vibrator tends to operate.

The second multi-vibrator MV2 is tuned to operate at a frequency of 3,600 cycles per second, which is one-fifth the frequency of the rst multi-vibrator. The changing potentials at the grid |31 of the first multi-vibrator cause the condenser |40 to charge and discharge and apply alternating voltages of a frequency of 18,000 cycles per second to the grid |42 of the second multi-vibrator. These control voltages serve to keep the second multi-vibrator in step at exactly 3,600 cycles per second, in as just above described.

In a similar manner the third multi-vibrator MV3 is controlled by the second and is kept in step at a frequency of '720 cycles per second, While the fourth multi-vibrator MV4 is controlled by the third and is kept in step at a frequency of 360 cycles per second.

The operation of the fourth multi-Vibrator MV4 causes the condenser |10 to charge and discharge, producing alternating voltages having the same way a frequencyv of 360 cycles persecond which lare impressed on the grids of the double triode 200. Fig. 2. The plate circuit of the left triode of tube 200 is coupled to the multi-vibrator MVS by means of condenser 202, and consequently the multi-vibrator MVB receives S60-cycle control voltages which keep it in step at the sub-harmonic frequency of '72 cycles per second. The right triode of tube 200 has its plate circuit coupled to the 60-cycle multi-vibrator MVG by means of condenser 204, and thus the (S0-cycle multivibrator is kept in step also.

The tube 20) may be referred to as a buffer tube. Although it has an amplifying function, its main purpose is to couple the fourth multivibrator to both the 'l2-cycle and the (iO-cycle multi-vibrators in such a manner that neither one of the latter multi-vibrators can react on the other and interfere with the control exercised by the fourth multi-vibrator.

We may consider now the function oi the tubes 230 and E458, one of which is associated with the 'l2-cycle multi-vibrator MVS and the other with the (iO-cycle multi-vibrator MVG. As the grid 2|| of rnulti-vibrator tube 2|0 goes alternately positive and negative, alternating currents are produced in the circuit which includes resistors 2|? and 23|, and alternating voltages are applied to the grid 235 of tube 230. The alternatng voltages at grid 2li are characterized by sharp peaks and steep wave fronts, which are undesirable for amplication purposes, and accordingly the resistor 23| is shunted by a condenser 232. This condenser charges in series with resistor 2|2 when the impressed voltage 1s rising and discharges in series with resistor 23| when the impressed voltage falls, and has the veffect of flattening out the peaks and making the voltage changes less abrupt.

The tube 230 has a negative grid bias when passing current, as already explained, and acts as an amplifier. The grid circuit just described improves the wave form of the input voltage, but it does not eliminate the harmonics, which are therefore amplied along with the fundamental 72-cycle frequency and appear in the plate circuit. In order to eliminate these harmonies, there is a closed resonant circuit comprising inductance 233 and condenser 234 inserted in the plate circuit. This resonant circuit is tuned to 72 cycles per second and offers low impedance to the plate currents of harmonic frequencies and high impedance to plate currents of the l2-cycle or fundamental frequency. Only the latter currents, therefore, result in enough drop to impress voltages of substantial amplitude on the output conductor 239. The closed resonant circuit has another effect which is of importance at this stage, which is to store energy on rising voltages and return it on falling voltages, being effective therefore to further improve the output to such an extent that the wave form of the voltage on conductor 239 beyond the condenser 2H is approximately a sine curve.

The circuit arrangement at tube 240 is the same as that at tube 230, and the operation is the same as that just described. except of course that the output at conduct-or 268 has a frequency of 60 cycles per second.

A portion of the Gil-cycle output may be tapped off through a condenser 254 and a resistor 255 and suitably amplied to provide power of consta-nt E50-cycle frequency for operating a clock, as, for example, a master clock. As shown in the drawing, the resistor 255 is connected to grid 25! of the amplifying tube 250. The output of this tube is transmitted by means of the transformer 253 to the input side of the power amplifying tube 260, and the output of the latter is coupled to the output leads 26'! and 268 by means of the transformer 266. The tubes 250 and 252 function in known manner, and their operation need not be explained in detail. rl`he clock, or other device requiring constant frequency (iO-cycle power, is connected across output leads 26'! and 2558.

Attention may now be directed to the power amplier shown in Fig. 3. The 72- and 60-cycle output. leads 239 and 2li9 extend through resistances 3&3 and 352, respectively, to the opposite sides of a potentiometer 3M, and they also extend to the movable blades of the switch S. Since the switch is assumed to be in its upper closed position, the 'l2-cycle output lead 239 will be grounded at the switch. The resistor 210 and condenser 272 prevent overloading the tube 230 under these conditions The output lead 249 extends to ground over two parallel paths. 'One path extends through resistor 302, potentiometer 304, and resistor 353 to ground on lead 239, and current iiow over this path is effective to impress alternating voltages of E50-cycle frequency on the grid of tube y380. The amplitude of these voltages can be adjusted by means of the potentiometer 304. The second path extends through the switch S to ground at the rheostat R, and is a shunt of the first path which is an additional means provided for regulating the input voltage at tube 300. The rheostat R obviously provides for producing any Voltage adjustment from zero to maximum. In practice the potentiometer is used to balance the 72- and 60-cycle outputs and after being once adjusted need not be disturbed, the voltage adjustments being made at rheostat R.

The 60-cycle voltages thus impressed' on the grid of tube 200 are amplied by tube 300 in well known manner. The output of tube 300 is coupled to the power amplifier comprising the four tubes 3MB-3&3 by means of the transformer I30|. The power amplifier under discussion is a typical push-pull class B amplifier and operates in known manner. It will be sufficient to state, therefore, that the plate circuits are coupled by means of transformer 320 tothe power output leads 330 and 33d, which may be extended to any desired point and afford a source of constant frequency fifi-cycle alternating current. This power supply may be used for any purpose as required. The drawings indicate by way of example a motor M, which may be a synchronous motor used to drive watch timing apparatus.

It will be understood that 'l2-cycle alternating current. may be obtained if desired, instead of 60-cycle current, merely by reversing the switch S. This operation grounds the (S0-cycle lead 249, and renders the 'l2-cycle lead 239 effective to apply alternating voltage of 'l2-cycle frequency to the grid of tube 300.

The reason for providing the switch S2 associated with the rectifier in Fig. 3 will now be apparent also. The power output at leads 330 and 33|, Fig. 3, will usually be of intermittent application, whereas the output at leads 261 and 268, being used to drive a clock, must be on continuously. The switch S2 enables the apparatus in Fig. 3 to be shut down when not needed, without disturbing the operation of the apparatus in Figs. l and 2.

The multi-vibrators described may be referred to as of the direct coupled, grid drive, single grid control type. The equipmentl operates satisfactorily, but is somewhat diiiicult to adjust at the start. The diiiiculties are due at least in part to the fact that' a low frequency multi-vibrator being controlled or governed by one of high frequency tends to react on -the latter and interfere with its operation, which makes it necessary to use rather small condensers for coupling purposes between the successive stages. These small condensers offer greater opposition to the low frequency reactions than they do to the high frequency control, and consequently have the de.. sired selective effect, but they also decidedly limit the control power that can be transmitted through them.

The applicant has found that the undesirable reactions from a low frequency multi-vibrator to the high frequency multi-vibrator by which it is being controlled can be entirely eliminated by the insertion of a buffer tube. An example of this is the tube 200, Fig. 2, which is inserted between the multi-vibrator MV4, Fig. 1, and the two low frequency multi-vibrators, Fig. 2. The tube 200 is primarily provided to enable the output of the fourth multi-vibrator to be split, as already describe-d, but it has the added function of preventing any feedback or reaction from the low frequency multi-vibrators and enables appreciably more power to be used for controlling them. The condensers 202 and 204 therefore may have a capacity of 250 micro-microfarads, whereas the condenser |60, through which the fourth multivibrator is controlled, has a capacity of only micro-microfarads. For these reasons it may be stated that coupling the successive stages by means of buffer tubes is somewhat more satisfactory in operation than direct coupling, although it is also more expensive.

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 frequency divider, a multi-vibrator, two other multi-vibrators operating at different frequencies each of which is a sub-multiple of the frequency of said first multi-vibrator, and space discharge means for causing said first multi-vibrator to control each of said other multi-vibrators independent of the other, said means including grid circuits coupled to said first multivibrator an-d plate circuits coupled to said other multi-vibrators, respectively.

2. In combination, a crystal oscillator of constant high frequency, frequency dividing means comprising a plurality of multi-vibrators in tandem controlled by said oscillator, a plurality of other multi-vibrators having different frequencies, means including a space discharge device whereby the low frequency tandem connected multi-vibrator controls each of said other multivibrators independently to keep them in step with said oscillator and at the same time prevent interaction between them, a power amplier, and

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