SIEMENS

Grid-Controlled Tubesfor RF-Excited CO, Lasers

At its tubes plant in Berlin (BedinerRöhrenwerk)Siemens has been pro-ducing an extensive range of electrontubes for more than 50 years. Grid-controlled high-p triodes and tetrodesare especially suitable for the RFexcitation of CO2 lasers.

Contents

Techniques Applications andTrends in COrLasers

Stimulated Emission -the Principle of the Laser

The CO, Laser:Applications and Trends

Direct-Current- ExcitedCO, Lasers

RF-Excited CO, Lasers

Stripli neAlVaveguide Lasers 10

Possible lmplementations ofRF Sources for Axial-Flow andTransversal-Flow CQ Lasers

The Tube -the Active Component

The RF Circuitry

The Amplifier as RF Source 15

Amplifiers with Triode Final Stage 15

Amplifiers with Tetrode Final Stage 16

The Self-Excited Generatoras RF Source

Generator with p-1 00Triode 17

Generator with Tetrode

Controlling Powerwith Screen-Grid Voltage 19

Possible lmplementationsof RF Sources forStripline/Waveguide Lasers

RF Sourcesfor StriplineÂffaveguide Lasers

Triode Oscillatorup to B kW and B0 to 100 MHz

Triode Oscillatorup to 35 kW and 100 MHz

Dimensions of High-p Triodesand Tetrodes 23

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19

21

22

22

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Tech n iq ues, APP] icationsand Trends in CO, Lasers

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This information brochure from the

Electron Tubes Division of Siemens

AG is intended to offer you assistance

and ideas when you are selecting cir-

cuitry concePts, tubes and RF cavitY

resonators for exciting CO, lasers.

We offer our customers not onlY the

active comPonent, i. e. the tube, but

also, if they wish, allround consultingon how to design an RF source, right

down to design bluePrints and dem-

onstrations of samPle equiPment'

St¡mulated Emission -the Principle of the Laser

When a Photon imPacts with an

excited atom, the latter will emit a new

ohoton of the same frequency and the

same direction and fall into a lower

excited state.

lf you can invert the population of the

uoper state comPared to the lower

siate, the radiation willbe amplified'

When such an inverted medium is putbetween two mirrors that reflect theemitted light back into the medium,this light will be amplified as long asinversion can be maintained, the twomirrors acting as an optical resonator.lf one of the two mirrors is semi-trans-parent, paft of the radiation can leavethe resonator.

A steady state can be achieved wherethe inversion is just enough for the re-sonator losses to be compensated by

the amplification, Such a process forgenerating and amplifying monochro-matic and directional radiation is called"light amplification by stimulated emis-sion of radiation" or LASER.

ln a CO, laser the gas, a mixture ofHe, N, and COr, has to be ionized.This ionized gas, also called plasma, isproduced by a DC or RF glow dis-charge. So, depending on the type ofdischarge, you distinguish betweenDC or RF plasma - with their physical

differences that mark the advantagesand drawbacks of the CO, laser.

Contours cutting

The COrLaser:Applications and Trends

Since its invention in the mid-60s theCO, laser has caPtured a mass of aP-

plications, especially in the processing

of materials, e.g. cutting, welding,drilling, hardening and suface treat-ment. ln contrast to the mechanicalprocessing of materials, laser lightdoes not wear out, ComPlicatedstructures can be cut and worked pre-

cisely under computer control.

To begin with there were DC-excitedlasers with axial or transversal gas

flow. But today, for light outPut of1.5 kW and more, the emPhasis is on

RF-excited lasers. Typical applicationsare cutting and welding. The reason forthis is the superior pulsing capabilily ofRF-excìted lasers. For some Yearsnow CW power of uP lo 5 kW has

been quite customary.

ln the latest CO, laser developmentseffofts are being made to reduce theefforl involved in the heat exchange ofthe laser gas. There has been successhere in the case of stripline or wave-guide lasers, where excitation frequen-cies of 80 to "100 MHz are called forbecause of the special construction.

Princìple ofDC excìtation

Gas inlet Gas outletHe, N2, CO2

1

llt I

Fig

Laser weldingin the automobileindustry

Ratchet wheel withwelded-on synchro-nous ring

Di rect-Cu rrent-Exc¡tedCO, Lasers

ln DC excitation the laser gas is usual-ly converted into an electrically con-ductive plasma with the aid of an igni-tion. This burning plasma has a fallingcurrent characteristic however, and itsoperating voltage is far below the igni-tion vollage. So, to ensure stableburning, you need a series regulator,which, because of the high DCvoltage, is generally designed with atube (tetrode).

With the bigger lasers a number of dis-charge paths are required, each with atetrode for series control. To makesure that the light power stays con-stant, there is a control to stabilize thedischarge current to the required level.

The screen-grid voltage supply, heatertransformer, monitoring devices andcontrol are on high-voltage potential(Uo approx. 20 k\4. So an isolatingtransformer is necessary for applyingheater and screen-grid power. Opticalfibers are used for control and moni-toring because of the high-voltageinsulation that is required.

Series regulatorwith a tetrode

Regulator tetrode Limiting resistor

Ceramic Ceramicisolator

Laser tube

-20KV groundpotential

Fig.250Hz line lsolating transformer

-------r

RF-Excited CO, Lasers

When the CO, laser gas is exposed toa high-frequency electric field betweenthe poles of a plate capacitor, the RFplasma is created that is required forlasering.

For regulating the mean laser lightpower (you also speak of modulation),there is a very fast and elegant techni-cal solution; you simply switch the RFpower on and off with a variable pulsefrequency and variable duty cycle.

The laser has to be excited with a fre-quency of at least 2 MHz. The reasonfor this is a gas layer, directly on theinside of the wall of the quartz tube,which is inactive in converting energy

Gas inletHe, N2, CO2

I

into laser light. Because of the highpulse frequency that is requiredand the short on time, a minimumoperating frequency of 13.56 MHz hasbecome established.

ln contrast to the cutting of a longstraight seam, when you are cuttingout or welding along complicatedcurves it is necessary to have fastcontrol: as soon as the direction of thewelded seam alters, the light energyhas to be reduced. Otherwise it wilnot stay matched to the lower relativespeed produced by the mass inertia ofthe moving mechanical pads.

Without the loaded Q value of the finalstages being too high, pulse frequen-

cies of approx. 50 kHz with duty cyclesof I0 to 90% and more are possiblewith RF amplifiers and setf-excited RFgenerators at frequencies of 13 and27 MHz. The total power produced bythe RF stage with just one tube canalso be evenly distributed to a numberof laser tubes. There are two conceptsfor this on the market.

ln the first the RF source and the laserare in separate housings, connectedby a flexible or rigid 50-e lead. ln thisway the individual units can be keptrelatively smail and light. There has tobe an appropriate matching circuit toensure that the ignited laser acts like a50-O termination at the end of thecable. This guarantees optimal power

Electrodes

Principle of RF excitation

transfer and the RF cable is notendangered.

ln the second the RF source and thelaser form a compact unit in a singlehousing. The cable and matchingcircuit can be omitted, so weight andvolume are reduced.

Both kinds of RF source can bedesigned with RF amplifiers or self-excited generators. Either high,gaintriodes in a grounded-grid circuit ortetrodes in a grounded-cathode circuitare used in stable ampllfiers withfrequencies of 13.56 and 27.12\r/t\z.

At the moment there is a clear trendtowards self-excited RF generators.

The changes in frequency that occurwith generators can be kept smallenough - by skilful selection of theloaded Q value - to be tolerated bythe overall system, i. e. tube, generatorand laser. Suitable shielding measures,on both the RF generator and laserhousing, make sure that regulationsare maintained that refer to maximumpermissible interference field strength.Generators of this kind are best fittedwith high-gain triodes, but tetrodeshave aiso been experimented with.

StriplineÂrVaveguide Lasers

ln the axial-flow and transversal-flowlasers described up to noq the elec-

Completely Laser gasrellecting out ^

Semi-trans-

mlfror VaCuum-parentmirror

Root'sFig. 4 Heat exchanger blower

Principle of the axjal-flowCO, laser'

trical energy that is applied to the gashas to be expelled again in the form ofheat energy, i. e. in as much as it is notconverted into laser light. This is donewhile the gas passes through the heatexchanger, which, like the gas blower,is a determinant component of a laser.

ln waveguide lasers the light is pro-duced in a 1 to b mm wide gap,limited on both sides by wide, water-cooled plates. But the high energyremaining in the gas is transferred veryfast and efficiently to the immediatelyadjacent, water-cooled plates. ln thisway the amount of gas that is moved- and thus the demands made of theheat exchanger and blower - can be

Fig. 3

10

Comparison of DO-excitedand RF-excited lasers

DCRF

Pulsing Fast power variation through pulse/pause ratio. Pulse repetition frequenciesof up to 100 kHz are achieved.

Comparatively slow power variationbecause of the relatively high energy heldin stray capaciiances, which are afunction of the equipment and cannot beavoided.

Gas impurities There are no impurities because there isno contact between the gas and theelectrodes.

The electrodes are in the laser gas, sothere are gas impurities as a result ofcathode wear.

Ma¡ntenance No machine stoppages because ofelectrode replacement.

ïhe cathode is used after 2000 to4000 h and has lo be replaced.

MTBF A number of excltation paths can be fedfrom one generator (one tube). Theresulting simple, clear circuitry reducesthe risk of failure.

A separate power supply is required foreach excitation path; there can be asmany as eight units per laser, whichincreases the risk of failure.

Accident risk, shielding On the laser tube there is only RF voltageof medium level. The electrode pairs andall parts that conduct RF voliage have tobe shielded.

There can be DC voltage of up to 20 kVon the laser tube, RF shielding is un-necessary.

Power-supply costs

ïable 1

reduced. ln ideal cases, where thepower level is not too high, they couldeven be omitted.

Because of the narrow dischargespace in the form of a gap of1 to 5 mm, the total thickness of theplasma is only slight. So the inactiveplasma barrier layer that is always pre-sent on the outside (quarlz tube wallor water-cooled metal plate) has to bekept as small as possible. This require-ment can be met with a pumpfrequency of approx. 100 MHz.

The power-supply costs are between 2000 and 4000 DM per kW. RF excitation is moreattractively priced than DC excilation for laser light power of more than 1.5 kW

RF supply 50 C) coaxial cable

Principle of an RF-excitedCOrwaveguide laser

Fig. 5

11

Possible lmplementations of RF Sourcesfor Axial-Flow and Transversal-Flow CO, Lasers

The Tube -the Active ComPonent

Siemens offers tetrodes and triodessuitable for equipping amplifiers andgenerators. Our new series of ¡r-1 00triodes for RF laser excitation was spe-cially devised for tough industrial envi-ronments and derives from our tried

and tested range of generator tubes'

All our new ¡r'100 tubes - RS 30'1 1 C'RS 3021 C, RS 3027 C, RS 3041 Cand RS 3061 C - are of fullY concen-tric metal-ceramic design. The tubesRS3021/27/41 C have comPatiblebases. Their water connections areaccordingly also of the same arrange-ment and size. This very much simpli-fies new designs for our customerswhen they move uP to a differentpower category

The RF power range of all modelsextends from aPProx. I to 100 kW

continuous wave. The Pulse Powerthat can be achieved will be one and ahalf to four times higher if the dutyfactor D is 50 to 25%.

ln all models fluctuations of heatervoltage of at least +5%o are permissi-

ble. For the crowbar test a test wire

0.3 mm thick (RS 3061 CJ = O'¿ mm)

is sufficient to protect the tubes whenthere is a flashover. The thickness ofthis test wire is a measure of energy

and defines the shortcircuit currentthat may flow for a certaìn time with-out damaging the tube through flash-

over. The rugged construction of thetubes is a particular advantage whenthe anode power suPPlY has to beconfigured with large capacitancesbecause of long Pulse times.

The anode dissipation in pulsed ope-ratìon (D = 25%) can be increased toas much as four times the permissible

continuous dissiPation for Pulsewidths of max. 0.05 s' lt is similar withthe generously scaled grid dissipation.

A ¡-r of approx. 100 was chosen so

that large grid-cathode spacings can

be used with "thick" electrode wires.

This produces the necessary rugged-ness, a large test-wire diameter, safety

in transport and handling - as called

for when personnel in ìndustry is un-

skilled.

A high p (e. g. of 200) is a hindrance tothe design objectives and makes tubefailures more probable. The advantage

that is frequently spoken of, namely

that a very high p enables You to oPe-

rate an amplifier without a bias power

supply, produces the following draw-backs:

Because of manufacturing tolerancesin the tube characteristics there can

in some cases be an unwanted, idling

anode current (1 A) that cannot be

reduced for lack of a bias Powersupply. This means hìgh dissipationand poor efficiencY, esPeciallY in

pulsed operation with a small pulse

duty factor. The cooling water also

becomes unnecessarilY hot.

A grid-cathode shotlcircuit is notnoticed until high VSWR appears atthe input for RF driving. Trouble-shooting is more difficult because witha lower ¡r a fault of this kind is detect-ed as soon as You switch on, sincethe fixed grid bias voltage breaksdown.

ln tubes with a double p factor forexample (p = 200), the probability ofunwanted self-excitation is twice asgreat. A lower p produces more stableoperating conditions.

ln self-excited operation of a generator

triodes with a lower ¡r Prove them-

Heater, cathode andgrid connector forRS 3021 C, RS 3027 C

und RS 3041 C

12

RS 3021 CJ, RS 3027 CJ,RS 3041 CJ

Amplitude relat¡ons of direct current and harmonics

selves to be especially superior whenit is a matter of high pulse power (high

currents). ¡r-100 tubes do not requiregrid currents that are all that high evenup to the highest anode currents andso they make good use of cathodeemissìon.

There are uniform accessories avail-able for use of the three mid-rangemodels as amplifier tubes in a ground-ed-grid circuit. A tube socket formedof these parts is convenient, fast andsecure for the end-user, electricallyideal and permits an extremely flat de-sign, because no space is required forscrews, tools or moving connections.

The RF Circuitry

Both the tube characteristics calcu-lated from the constant-current dia-gram on the load line and thosepublished in the data book presumethat the AC anode and grid voltagesare purely sinusoidal and, viewedfrom the cathode, are offset from oneanother in phase by 180'.

Certain circuit conditions hqve to bemet to achieve this, The tube (currentsource) not only generates the funda-mental current that is required for theRF power but also currents whose fre-quencies are an exact integral multiple(2,3, 4...,n) of the fundamental. Theseharmonic currents (lnj result from aFourier expansion of the individual RFanode-current pulses. The magnitudeof the current amplitudes is given bythe classes A, B, C and D and thenature of the tube characteristic. Har-monic currents are also produced in

an optimally designed circuit.

The harmonic voltages (U"J generatedby the harmonic currents result fromthe product ln,.Zn. and the type ofcìrcuit desrgn.

Harmonic impedances (Z"J in theequipment create harmonic voltagesthat are superimposed on the

C ClassO=60'

B ClassO=90'

DC anode current

Amplitude of 1st harmonic.

Amplitude of 2nd harmonic

Amplitude of 3rd harmonic

Amplitude of 4th harmonic

Amplitude of 5th harmonic

lA

l1

l2

o.22lA¡ú

0.39 IAM

0.28 IAM

0.14lAM

0.03 IAM

0.03 IAM

0.32 IAM

0.5 IAM

0.21 lA\¡

0

0.04 IAM

0

l3

l4

l5

'Fundamental

Table 2

13

desired sinusoidal voltage of the fun-damental. ln this way efficiency canrapidly drop to as little as 50% andpower gain falls.

The linear curve with a kink is a goodapproximation of the actual tube char-acteristic fT-able 2). Here lo, is thepeak anode current that the tube pro-duces at its upper modulation point.

The table shows these currents forboth C and B class of oPeration. ln

both cases the high value of currentamplitude for the 2nd harmonic is

worth noticing. For class C with@ = 60o the current amplitude of the2nd harmonic referred to the funda-mental is:

tz=tt'o'28 #ñ =o'72't1

For class B with o = 90o on the otherhand it is:

t"^,l" = l. . 0.21 d.5Ç =0.42'1,

It is not until after the 3rd and 4thharmonics that the amplitudes beginto reduce drastically. This means thatespecially low impedances are impor-tant for the current paths of the 2ndand 3rd harmonics.

ln the design of a grounded-grìdcircuit rt is consequently essential toremember that the high cathode cur-rent has to pass through the sensitiveinput circuit. ln a grounded-cathodecircuit on the other hand "only" the

harmonic currents of the smaller grid

current have to be thought of in thesensitive grid circuit.

The required imPedances andfrequency stability can usually beachieved with a suitablY highloaded Q, which also means smallbandwidth however.

A loaded Q of 100 for r¡ = rrl1 Pro-duces a reactance for the funda-mental of:

,, RAx =-"'ì 100

i. e. 1o/o of the load resistance.

The admittance of the circuit for the2nd harmonic is:

Yo=Go +i(2ri,.c-=f-¡.¿ , ,ZA,L,

or

Relative to the fundamental, thereactance for the 2nd harmonic is:

x" =å#=o.0066Rn

or 0.66 % of the load resistance'

The harmonic voltage for the 2ndharmonic resulting from the reactance,referred to the fundamental, for Cclass with o = 60o is:

uz=0.72å# l, = o.oo4e u

i.e. O.4Bo/o of the voltage of the fun-damental.

The same calculation for a loaded Q of10 produces:

Uz = 0.048 U

t. e. 4.8o/o of the voltage of the fun-damental.

You can see that there are soon limitsto reducing the Q any further, becauseof the fast increase in harmonic volt-ages and the subsequent sacrifices in

efficiency.

This applies in particular to self-excitedcircuits, where, in extreme cases, theharmonìc voltage produced on theanode is fed back to the grid with un-wanted feedback factors of 50 to B0%(values found in practice).

With an anode fundamental voltage of12 kV and the suPerimPosed harmon-ic voltage of 576 V (corresponding to4.8% of 12 kV) there is an harmonicvoltage on the grìd of:

0.5 to 0.8 . 576 V = 2B8V to 460 V

These values are certainly of the level

of the AC grid voltages of the funda-mental prqected by a designer andlead to strong distottion.

For this reason the capacitors of thedescribed amplifiers are also mountedin the input circuit so that, viewed fromthe tube, they are in series with an

inductance. At the 2nd harmonic aseries resonance (shortcircuit) is pro-duced and, at a low Q (for the fun-damental), no harmonic voltage canbe created for the 2nd harmonic.

Yz = Go.,å lt,l

Anode circuit withdiscrete components

Anode circuit ofcavity-resonator design

Fig. 6 Fig.7

14

Small Q figures achieved in this wayproduce large bandwidths and thusthe possibility of increasing the pulsefrequency. Fufthermore, in an inputcircuit of low Q there is no need for aretuning device - not even after a tubeis replaced.

The design of the RF anode circuits ispossible with both discrete compo-nents and a cavity-resonator kind ofcombination consisting of tubes andmetal plates. Here at least two parallelmetal plates form the resonant-circuitcapacitance, supported by a straightpiece of tube that also acts as the in-ductance of the resonant circuit. Suit-able discrete components are ceramiccapacitors, vacuum capacitors andbent coils of copper tube,

The Amplifier as RF Source

The major advantages of amplifierscompared to self-excited generatorsare:

- crystal-controlled, load-independentfrequency stability,

- continuous, fast power controlof 0 to 100%,

- simple pulsing on the amplifier inputat the smallest levels with duty factorsof 10 to 90% and up to pulse frequen-cies ofl00 kHz.

Where such advantages of an RFamplifier are not necessary or only inpart, a self-excited RF generator willbe used.

One drawback of amplifiers comparedto generators is the higher costs.These are mainly caused by the extraeffort that goes into matching circuitsin both the input and output circuit.

Amplifiers with Triode Final Stage

ln contrast to the RF final stages ofbroadcast transmltters, which workinto a relatively constant load, namelythe antenna, an RF laser pump generates a glow discharge with a complexload that can fluctuate very consider-ably.

The resulting reactive componentdetunes the output circuit of the amp-lifier, which is basically stable infrequency, so that the tube ofthe finalstage no longer works on a load linebut along an elliptical curue. Thismeans increased anode and grid dis-sipation with uncertainty aboutwhether rated power will be achieved.Power reserves in the tube are thenespecially important.

The detuning described above canalso lead to self-excitation at unwant-

ed frequencies however. To avoid this,the stable grounded-grid circuit ispreferred in triodes. Then self-excita-tion is only possible by way of the verysmall anode-cathode capacitor - quiteunlike the ten to 100 times greateranode-grid capacitor of a commontriode in grounded-cathode circuit.

The advantages of this circuit come atthe cost of a much higher requirementfor driving power however, becausethe AC voltage has to drive not onlythe grid current (grounded cathode)but also the higher cathode current(grounded grid).

To keep the necessary driving poweras low as possible, the power stagesare operated in class B. Then onlyrelatively low AC drive voltages are re-quired and anode efficiency does notdrop too much compared to class C.

Triode in grounded-gridcircuit exempìified byRS 3027 CJ

." iÎ- :+Un

-11100V

-305 V.

+210V .

-95 V

Voltage G : K(referred to cathode)

"i[,-JUpper modulation point

Grid: 305-95 = 210 VAnode: 12000+305-11100 =1205 V

Operating point

flfå", ,rt?Y) reo=0,3AFig. I

15

I-

The most suitable tube for minimumdriving power is one that requires alow AC drive voltage and a low con-trol-grid current. For continuous wavethe limit of the tube is usually set bythe maximum permissible grid and/oranode dissipation.

For RF pulse power with maximumpulse width of 0.05 s, the grid andanode dissipation of our ¡r-'100 triodesmay amount to as much as four timesthe permissible continuous powerduring the pulse. This is assuming thatthe duty factor does not go above25o/o, i. e. averaged by time that thepermissible continuous dissipation is

not exceeded.

Amplifiers with Tetrode FinalStage

lf amplifier final stages are fitted withsuitable tetrodes, they can be oper-ated at frequencies of 27 Mïzin agrounded-cathode circuit and onlyrequire relatively little driving power. Sothe expense of powerful RF driverstages can be saved - although at thecost of more elaborate technology forthe screen-grid power supply and RFscreen-grid bypass capacitor, besidesthe comparatively high price of thetetrode.

Seeing as the driving power of atetrode worked in grounded-cathodecircuit is basically low, it can even beoperated in class C with good efficien-cy - and without the required driverpower becoming too high. ln the caseof triodes operated in grounded-gridcircuit however, much higher drivingpower is called for if you move fromclass B to class C.

For tetrode final stages there is ourproven RS 2012 CJ with CW power of10 kW and pulse power of 30 kW aswell as the RS 2058 CJ with 50 kW ofCW power and 200 kW pulse power.

FrequencyAnode voltageScreen-grid voltageControl-grid voltageCathode currentPeak cathode currentAnode dissipationScreen-grid dissipationControl-grid dissipation

Tube type RS 2012 CJ RS 20s8 CJ

Max¡mum ratings

MHzKV

KV

AAKW

30oÃ1.1

-2506351B20070

30151.5

-35035100901.100150

Operat¡ng data Pulse CW(D = 25o/o)

Pulse(D = 25o/o)

FrequencyOutput powerl)Anode voltageScreen-grid voltageControl-grid voltagePeak RF control-grid voltageAnode currentScreen-grid currentControl-grid currenlAnode input powerDriver powerAnode dissipaiionScreen-grid dissipationEfficiencyAnode load res¡stance

MHzKWKV

AAAKW

KW

%o

2711

6700

-2202702.30. 16

0.2213.8bU2,8130BO

1 300

27 2733 (8.25) 558.2 11

1000 1000-220 -220315 230

5.1 (1.3) 70.5 (0.13) O,20.64 (0.16) 0.0542 (10.5) 77192 15e (2.3) 22500 (125) 20078.5 71

740 910

27210 (52.5)13.31200

-26046022 (5.5)1.7 (0.43)2.1 (0.53)2e3 (73]|

91083 (21)2050(51 0)

345

16

Table 3 'r Circuit losses are not ìncluded

Schemalic of self-excited generator

Fig. I

The Self-Excited Generatoras RF Source

The major advantage of a generatorcompared to an amplifier is the omis-sion of the driving power. lf the rightchoice of tube is made and the cir-cuitry properly scaled, it is possible toachieve RF duty factors of '10 to 90%through grid keying at 13 and 27 lr(Hzand up to pulse frequencies of50 kHz, similar to the case with am-plifiers. At lower pulse frequencies theduty factor ranges from 0 to 100%.

Just like with an amplifier, a self-ex-cited generator permits fast and con-tinuous power control between about20 and 100% at a constant load re-sistance. ln the region of very lowpower the solution with an amplifier is

stillsuperior.

When a self-excited generator worksinto a complex load, the transformedreactive component of the latter de-tunes the frequency-determiningresonant circuit. So the generator tubealways operates into a purely ohmicload resistance. Depending on themagnitude of the transformed loadresistance (R.), the grid or anode dis-sipation increases howevel accordingto whether the anode load resistance(Rf is large or small and as a functionof the frequency response of theselected feedback circuit.

GeneratorwithQ=10

For a given reactive component (X.) ofthe load, the frequency stability of agenerator with a high loaded Q isgreater than in the case of a lower Q.This can be seen from the examples inFig. 10 and '11, where the same loadis supplied with the necessary RFpower once by a generator of low Q(10) and in the other case by one ofhigh Q (100). The transformed reactivecomponent of the load K = 360 Oproduces a frequency reduction of5.7% and 0.6% respectively, referredto the original frequency f,,

Generator with p-100 Triode

ln the operation of a generator the RFoutput power will usually alter bothwìth the voltage fluctuation of the50/60-11z supply network and withchanging load resistance Rt

ln cases where fast control rates arenecessary the relatively slow thyristorcontroller of the high-voltage rectifierwill soon prove to be ineffective. ltscontrol tasks can be handled fasterand less expensively by a controlledgrid resistance. The required settingrange of the grid resistance is ob-tained wilh a controlled series transis-tor through which the DC grid currentflows.

With their relatively low DC grid-volt-age requirement, a prerequisite for the

Generatorwith Q = 100

use of voltage-sensitive and power-sensitive semiconductors, our p-1 00tubes are highly suitable for suchapplications.

lf the grid resistance of an RS 3027 Ctube that is self-excited at Uo = 12 ¡yis increased from 200 to 2000 Q forexample, its output power will dropfrom 30 to 3 kW, and the grid biasvoltage shifts from approx. -300 to-150 V

The grid resistance of apprðx. 200 O,which is especially low under full load(30 kW, is the requisite at the grid endfor fast pulsing. The low biasing re-quirement of our p-100 triodes is alsoa considerable advantage in grid-key-ing circuits fitted with semiconductors.Fig,12 shows the characteristic ofthe tube power, the grid'and anodecurrent and the grid bias as a functionof the grid resistance for a p-20 anda ¡r-100 triode. The four load lines inFig.13 illustrate this kind of powercontrol for a p-100 triode.

Tubes with a substantidly higher ¡r,e. g. 150 to 200, offer no extra advan-tages, because the grid-current re-quirement rises more than proportion-ally, the bias that is needed reducesto an insignificant degree, but thetendency to jump frequency increases.

N =ff=ff; = as o

17

-T

Max¡mum rat¡ngs for CW operationr) RS 3011 C RS 3021 C RS 3027 C RS3041 C RS 3061 CJ

Frequency f 50 150DC anode voltage UA 7,2 5kVDC grid voltage Uc -500 -500DC cathode curent lK 3 3Peak cathode cunent lr. 12 12DC grid current lG 1 0.85No-load DC grid current lc r"", 1.25 1.1

Anode dissipation, RS 30oo CL PA 5 5Anode dissipation, RS 30oo CJ PA 5 5Grid dissipation Pc 350 28OGrid resistance for blocked tube Rc or*¡ 15 15

40 12014 10

-800 -8005525 251.7 1.3

2.'t 1.710 1020 20500 33015 15

40 12014 10

-800 -8006630 302.3 1.72.8 2.215 1525 251000 60012 12

40 'r 1015 10

-800 -80012 1248 483.3 34.2 325 2535 351200 70010 10

3015

-80020804.25.3

5022004

MHz

AAAAKW

KW

ko

Operat¡ng data CW Pulse3) CW Pulseor CW Pulse4r CW Pulseor CW Pulse'r

Frequency f 50 100Output power Pzosz I 13DC anode voltage UA 6.5 6.5DC grid voltage UG -280 -250Peak RF grid voltage Un, 570 660Feedbackfactor K 9-7 11-2DC anode current lA 1.6 2.7DC grid cunent lc 0.78 1.4Grid resistance RG 360 180Anode input power Pen 10.4 18Driver power P1 0.41 0.84Anode dissipation PA 2.'l 4Grid dissipation P6 190 480Oscillator efficiency rìosz 76.5 73Anode load resistance R^ ^o, 2.2 1.3

120 8020 66'10 11

-290 -400500 8755-4 I2,5 I0.9 3.15325 12725 BB

o.42 2.54.4 20160 1300

80 752.1 0.72

120 8032 10410 11.5

-280 -200590 8406-6 8.24.15 12.91.5 4.9185 41

41 .5 1480.83 3.78.7 40400 270(77 70127 05

70 7065 14011 12

-300 -400635 9706.3 8.87.6 15.42.8 6.2107 6484 1841.65 5.517 39790 300(77.5 76o77 0 43

30 30 MHz110 220 kW')12 13.5 kV

-350 -300 v855 1100 V7.8 I o/o

12 22.6 A3.7 7.2 A9542c,144 305 kW2.9 7 kw2r

31 78 kW1600 5000 w76.3 72 " %0.54 0.34 kO

OscillatorAnode loi R¡ osz

Table 4

Fig. 12

RS 3027 C (p=100)K=const.

ìl oszlOOa/o

0,5 1 2kA+ U¡=12kV=const

R¡ = const'

2 5 10 15kç)

Re..-.-*

Control of generatorpower simply by alteringthe grid resistance fordifferent tube u figures

2 4 6 I 10 1214Parameter= l¡- ,^ jUParameter= lo

-Fig. 13

r) Mãimum ratings can be expanded on enqu¡ry for pulsed operat¡on q Ma duty = 50 %a Circuit losses arc not ¡ncluded a) Max duty = 25 7o

24A.

300

200

100

0

-200

-300

1.,:Ue

500

40023002,6

o,2

R6

Load lines whencontroll¡ng generatorpower by altering thegr¡d resistance

I e¡e Inzn

700 0,

lcra lrn RS 3027 C

Pzoszl Ro

1B

r

Generator w¡th Tetrode

ln general self-oscillating generatorscan also be constructed with tetrodes.The main advantage of a tetrode asopposed to a triode is that the powercan be generated with lower drivingvoltages, in other words with higheramplification. This effect is obtained byusing a second grid called a screengrid.

Tetrodes have a particularly importantrole to play in amplification technology,where it is important to minimize thedriving power.

However, in oscillator technology,where high driving power is alreadyavailable, this fact is insignificant;indeed, the sensitive input circuit ofthe tetrodes causes uncontrolledoscillation at undesirable frequencies.

ln tetrodes the electrical field at thecathode produced by the control-gridvoltage is only slightly reduced by theopposing effect of the AC anodevoltage, This is due to the screeningeffect of the second grid.

ln order to compensate for the re-duced influence of the DC anode volt-age on the acceleration of the electricfield, the screen grid is supplied withpositive DC voltage.

Controlllng Powerwith Screen-Grid Voltage

lf one observes the constant currentdiagram of tetrodes at differentscreen-grid voltages, one will noticethat for the same driving voltage thepulse anode current increases as thescreen-grid voltage grows.

Thus, with the help of variations in thescreen-grid voltage, it is possible toinfluence the anode current of thetetrode.

lf the anode load resistance (Ro)

remains constant, as is normally thecase, and one reduces the anodecurrent by decreasing the screen-gridvoltage, then the RF anode voltage,

the feedback peak RF grid voltage atthe control grid and thus the outputpower are reduced.

Since constant DC anode voltage (Uo)

and decreasing peak RF anode volt-age (U",) cause the residual anodevoltage U" to increase, the efficiencyof the oscillators decreases, just like

a triode oscillator in which the gridresistor has been increased:

Uo=Uu'*Un

ln shor1, the fast and continuouspower regulation of the output powerusing the screen grid of a tetrode hasno particular advantage over a grid-resistance-regulated triode.

On the contrary extra costs are in-curred through the necessity of usinga screen blocking capacitor, anadditional screen-grid power supplyequipped with interlock, a morecomplex tube socket and last but notleast, a more expensive tube.

If one wishes to use a tetrode withoutscreen-grid power supply in an oscil-lator circuit, which means the screen-grid voltage is zero or negative, then amore powerful tube must be used inorder to reach the required anodecurrent.

The principle of power regulationcan be demonstrated taking theRS 2058 CJ tetrode as an examplewhere, simply by changing thescreen-grid voltage from 700 V to1 1 00 V the output power can beincreased from 53 kW to 124 kW.

Fig, 14 and 15 show the position ofthe load line in the constant-currentdiagram at screen-grid voltages of700 V and 1 100 V from which theoperation characteristics can bedetermined by graphical integration.

Uez = 700 V ln = Parameter -Ucr =f (Ud lcl =Parameter-.. lcz = ParameterUer

300

200

100

0

-100

-200

-300

Fig. 14

1A

OA

UczUer

Uel

8048A

200

100

0

o12 4 6 8 1012

-)Un

Load line for RS 2058 CJPzosz = 53 kW at:un =10kv Uer=700 vUn =240 V Rer = 2O4 A,Rn =326Q,K =4.1 Vo

= 1''l00 V la = Parameter -

= f (UÂJ let = Parameter -lez = Parameter

604

4 6 8 1012----------'

Un

Load line for RS 2058 CJPzosz= 124 kW at:Un =10kv uoz=1100vUn. = 370 V Rer = 2O4 A,Rn =326Q,K =4j %

-1 00

-200

-300012

Fig.15

1AOA

>ì>-

--.--------__-

19

Operating data for RS 2058 CJ from Fig. 14 and 15

Operating data (Oscillator) RS 2058 CJPzosz/

Pa/

RS 2058 CJ

Pz osz

FrequencyOutput powey')

Anode voltageScreen-grid voltageControl-grid voltageControl-grid voltage (peak)

Anode currentScreen-grid currentControl-gridcurrentControl-grid resistanceAnode input powerDriver powerAnode dissipationControl-grid dissipationScreen-grid dissipationAnode load resistanceEfficiencyFeedback factor

fDr 20szUA

un,UotUn.lA

In,lu.,

Ro,DraAP1

PA

Pet

Pn,RA

î oszK

30TQ

10700

-15024011

1470.74204110164565510332648.54.1

30124101100

-23037016.89831.11204168390441301 080326744.1

MHzKW

KV

AMA

oKW

KW

oo/o

o/o

Pç2 / V'l

lq2 / mA

tA/A

15

600 60

400 40

200

600 1 000400 800 1200 v

Oscillator pow€r P26s2,

operating data asfunction of screen-gr¡dvoltage

120

100

BO

Pez JQQ]{lìosz

%

t;

-)Uce

800

Table 5

20

1) Cìrcuit lossæ are not included Fig. 16

0 200

Possible lmplementations of RF Sourcesfor Stri pl i neA/Vaveg u ide Lasers

Single-ended infeed

Fig.17

Fig. 20

Central ìnfeed,voltage characteristicwith compensatingcoils at ends

RF Sourcesfor Stripline/Waveguide Lasers

As already mentioned, you need fre-quencies of around 100 MHz to excitestripline lasers. The lengths of theselasers are 0.5 to 1 m and the free-space wavelength is 300 cm, so youhave to investigate the voltage dis-tribution along the length of the laser.

This was performed in the laboratoryby simulating a stripline laser from RFaspects and measuring the lengthwisevoltage distribution with RF voltmeters.

Central infeed

100 MHz

Fig.21

Central infeed,voltage characteristicwith opiimally configuredcompensating coils

Looking at the unignited laser: if a60-cm-long "laser" (RF stripline) is fedsingle-ended for example, then, as inthe theory of the unloaded RF line, avoltage maximum forms at the openend at U, and:

U, = Uz cos {soo ffi )

U,=0.31 'U,

So it is better to make the RF infeednot single-ended but central. Then a

Central infeed,voltage characteristicwith ends open-circuit

100 MHz

Fig.19

voltage maximum is created at eachopen end (at U,), but this time:

u, = uz cos (360lquof

U,=0.81 'U,You obtain a more even distributionthan with single-ended infeed.

= 0.81 also applied for ihe volta-

ge distribution on an ignited'laser, thepower at the two ends of the line

Iwould be (¡61) 2 = 1.5 times higher

than at the RF infeed point in themiddle of the laser, leading to a highlyinhomogeneous RF plasma. Problemsof this kind are encountered in plasticwelding aI27 MHz with seam lengthsof more than 2 m, where compensat-ing coils are used to remedy thesituation (Fig. 20). A further improve-ment is produced if appropriatelyscaled inductances are arranged asshown in Fig.21.

ln an ignited laser with its linear ex-pansion, the electric power is not seenby a single resistance at the end of theline but by a large number of resist-ances distributed along the line. Thisis where the graphic illustration fails,because the laser can no longer beregarded as an unloaded, lossfree RFline with a cosinusoidal voltage dis-tribution.

'' Ut" l)2

100 MHz

21

Central infeed,voltage characteristicsfor different R/Zrelationships

The dependence of the RF voltage onlength was investigated and calculatedat 100 MHz between two metal plateswhose spacing a and characteristicimpedance Zwere varied. 16 uniform-ly distributed, low-inductance resistorsof 20 or 80 Çl simulated the "ignited"laser, the result being 1.25 or 5 O.

It is possible to see that, for a certaincombination of characteristic im-pedance of the stripline and resultingtotal resistance, a very constant volt-age can be forced along the laser. TheRF power produced in the volume unitis virtually constant and independentof the location of the volume along theline. lf the characteristic impedancebecomes greater than the optimalcharacteristic impedance Zoot (in theexample 5 or 20 Q), the voltage willdrop towards the end of the line, dueto the strong attenuation along theline. lf the characteristic impedancebecomes smaller LhanZoo, (Q factorincreases), the voltage will increasetowards the end of the line, like in anundamped line.

Triode Oscillatorup to I kW and 80 to 100 MHz

A compact, self-excited generator wasdeveloped for CW power up to B kWin the frequency range 80 to 100 MHz.With only slight modification, both theair-cooled and water-cooled versionsof our ¡r-100 triode RS 3011 C can beoperated in it.

The simplicity of the construction is

worthy of attention, with the possibility

l-lengtn = 60 cm-,1RF central infeed

Fi9.22

of simple change of frequency, of finetuning in operation and of routtng thecooling water - to the anode and backagain - without RF voltages havingany influence on the cooling-waterpipe. The variable feedback, based onthe principle of a TPTG (tuned-platetuned-grid) oscillator circuit, consistsof a workable sheet-metal loop. Thepower of the generator can also bevaried, of course, by altering the gridresistance.

Triode Oscillatorup to 35 kW and 100 MHz

For CW power from 15 to 35 kW andfor a frequency of approx. 100 MHz

RF central infeed

there is another newly developed, self-excited generator available, designedfor incorporation ih 19-inch ¡acks. Be-cause of the base compatibility of ourthree tubes RS 3021/27/41 CJ, allthree of them can be operated in thisgenerator with very little modification.So, if increased power is required,more powerful tubes are needed butonly slight alterations on the RF side.

This concept means that, our custom-ers can progress to a different powerclass quickly, securely and inexpen-sively. A lab model is on hand fordemonstrations.

100

50

n=f$=r.zso

?vvv?>vzvv7r, vvvvvùvvvvvzl,-Lengtn = OO

"r-.1

22

Dimensioned Drawings of High-p Triodes and Tetrodes

RS3011 CL

RS 3027 CJ RS3041 CL

RS 3061 CJ

RS 3041 CJ

23

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Herausgegeben vonSiemens AG, Bereich Passive Bauelemente und RöhrenMarketing KommunikationBalanstraße 73, W-8000 München B0

@ SiemensAG 1992.Alle Rechle vorbehalten.

Published bySiemens AG, Bereich Passive Bauelemente und RöhrenMarketing KommunikationBalanstraße 73, W-8000 München B0

@ Siemens AG'1992.All Rights Reserved.

Ordering No. B6-P500ô-X-X-760OPr¡nted in Germanyws 02923.