US2429656A - Balanced transmission line oscillator - Google Patents

Description

OIIIL 284, 1947. E, o, w|| 0UG|-|BY y 2,429,656

BALANCED. TRANSMISSION LINE OSCILLATOR Filed Feb. 27, 1942 ,5; @ww @M from/Ey y elements.

Patented Oct. 28, 1947 BALANCED .TRANSMlSS/,IQN i oscmLA'ron- I Eric- Osbome Willoughby, v Londonf. Englandif. as.. `signolgoto, International Standard .Elec/triatlon peraiicnewcllt N Yr;

y aclantis. (cresci-sc),

1 The present inventionfrelatesmte short- :ware @adentrashrt-Waveescllaters andrartcularly unbalancedbscillators, i. efioscillator usfizvigfa single Valve lor-two or more valves. inparalljjel-,fand according. thereto, a trimming Acondenser tied; to

the Plaie :lineisarrfvided i0 :balance fthe endend plate transmission-linesi0.-@Meede- 'The detention will l iba Ebene-ri understood :from thefffollowingdescriptions-staken orliunctien WithV the awmneeyine drawiriesf.;Which.;i1lus trate. ultraeshcrt .WaY-eSQiIlatQrS: It: Willibe undersiflod; howeverithat. the inventif). is equally :applicable toshQrt-iwavetoscillators :with larger valves,

In the drawings,

Figs. 1a, 1b and 1c illustratefknewn,:types of ultra-short wave oscillatorsutilising lengths? of transmissiondine together- .with the interelec .ode capacity of a thermionicgyalyeffas theI tank ci.:` it The, earth f is. nermallyj thctscreening case of the oscillator. Y

Figure 2 :shows ,schematicallya tank` cir-cuit for an. ultrashort WaVeoscillator-embodying the inventiorriwfandl Figure 3 rshowsa .sligl-itly- ,modied arrange: ment.`

In, Fig. 1a, the :oscillator Valve ,..ontubet Twine4 cludes aI cathode.-.C; grid-. Gn and man, anode eA.: One-quarter. Hvrfaifelen'gthI transmission line iconsductors` GL and are `connected..arespectiytelyr Withthe. grid. and anode.l of.yalved'iatK theright hand end of line AL,GL,= thelefthandiendloffithe line being 4effectively short circuitedithrough the condenser plates E. The lineAL; GL incombmaf,

- tion With the. inherent capacity -.Cyc;.betweentlie.

grid G and cathode.C'randrxwithvthe inherente capacity I Cpc. .bet'\ife'en` the .anode .A rand.. cathode; C, servesv as a tank circuit.. A positive hignitena sioni/voltage` source connected-through the radio frequency chokecoil BFC with the left end of line AL for supplying anode current to the valve reconnection :iseprevded firent@ grclmd through grid leak resistance Myvithhelefk endof line GL for the grid G. A line. CI-,LO adjustable length, connects from ground with the cathode C. The screeningcanaor shield Kaprow vides an earth, or groundconnectionV torathe' circuits; ofthe oscillator.

In Fig. la, valve, T is connected at the-right hand end of line.AL. GL, ythe lett. end of the line being open, and the connections for coil RFC and leak M being made at thecenterof the liner..

In Fig.` lcT the.valveffwis;.connected-atllthe center of line AL, GL, the connections for coil RFC and leak M being made near the right end of the line.

Referringfto 1a,- 1b 'and flc, in-'suclfrv cire` cuits .as illustrated-'the grid cathodecapacity Cgcis .inherentlylarger than the plate cathode capacity C'pc,. and there is an unbalanced current to earth-through-the cathode return. lead :CL which isccmmonly tuned to the oscillationy by varyingitheiengthfof transmission line attached te the filament cathode terminals of the valveand returned to the screening can or shield K-o'f the' oscillator.

ItI now appearsrthat lsuch-tuning of .thecathode' line can have no appreciable eiiect. on balancing the oscillator tank circuiti-and in carrying out this.v invention a `low .impedance is provided-in the cathode to earth' pathlisuch for= instance-.as may often be obtained by directly short circuiting the cathode to oscillator. screening canK): .and

the circuit is tuned by means of a small trimming condenserfbeiween platel lead and earth or screen lcamthereby; providingaJ parallel reactance between plate andmscrevenr can or earth which Vre-V -actance can begmade to balance. the plate and grid `transrnissionv lines of the voscillatortank circuit about earth or the screening can.

' Referring to yFigure 2,; the; tank circuit may be' modiiied to any of the transmission line arrange-Y4 ments known to the art and typicd by Figures 1a., 1b and 1c.

The anode of thevalve.4 is indicated at A, contml.. aridi:v at.:.G,aa1id the. cathode (indirectly heatcdfoverffthe@circuit frQm; the 10Wv voltage; source LT to ground) at C. Anode andlgridlcads; are shown respectively at ALvand GLaslengths of concentric transmission line and thecathode lead as CL, itsy inducta'nce being represented by L. Outputfrom-.the valve. is obtained by means of a coaxialzline LO. The small trimming condenser canfis indicatedat ,Cba This condenser may be.

connected between. lthe pla-te lead ALI-and'. cathode directly iii-:itis practically more convenient.- The outer'-t-:ondu'ctorsy of the. plate and grid transmis--` sion.- =linesy @are earthed for vhigh frequency'. cur-- rentsthrcughfzpl-'ates EV` forming vcondensers-.with the x'screen-ingA caln-Kf, a; sheet. .of .mica N` being inserted desired ,between E' andi-Kt.

Thisrbalancingg.adjustment of condenserv `Cb gives' "strongestfoscillationand maximum output and in practice .thebalancing poi-rit.is-:shownv most markedly y.bythegrid.l current. which reaches a maximum value corresponding.. tok maximum plate and..gridf.vol'tage swings..the grid vleak being increased,iiinecessary, to :maintain .the grid dish ssination.- at. a reasonable xvalue...

It is well to note that the essential adjustment is an increase of plate-cathode capacity by an amount corresponding to Cgc-Cpc, the adjustment being affected by the inductance of the leads to the electrodes inside the Valve, andin practice the adjustment amounts to a balance f plate-to-cathode and grid-to-cathode reactances of the valve and tank circuit in the oscillator.

Since the grid line GL is already damped by the grid leak M and since the trimming condenser Cb adjusts reactance of the plate line AL for an unbalanced output, it is advisable to tap the output lead LO directly on the plate line, for'if the transmission line is correctly matched at the load end, due to the high ratio of volt-amperes to watts in the short wave circuits, the reactance adjust-l `ment of the trimming condenser Cb will necessarily be small. It should be noted that although push-pull arrangements of valves are convenient for oscillators, they result in relatively high capacity across the transmission lines and limit the frequency that can be obtained, while use of inverted valves, that is, of the grounded grid type, in pushpull although reducing this capacity effect inherently reduces the ratio of alternating anode voltage to Adirect current anode voltage and their eiective eiiiciency, hence the above balancing circuit for single valve oscillation often is the most satisfactory solution of the problem of obtaining a desired output with a given value at a relatively high frequency. v

Theory y Referring to Fig. 2, the unbalance current through AL to earth is: mgl/ l(Xcpc--X-cgc) (Kw-Xen) (XL-Kapu) (XL-Xeye) Since Cgc is inherently greater than Cpe from the construction of the valve, the balancing method must be resorted to, to balance the transmission line tank circuits to earth. It isv important to notice, however, that since L the inductance of the cathode lead carries the unbalance current, so long as its reactance is reasonably small'such as often can be obtained merely by shorting the cathode to the screening case of the oscillator, it will have little effect on the amplitude of the grid voltage and the only tuning necessary becomes the balancing adjustment provided the transmission lines are correct to length, otherwise a small trimming condenser will be required across the lines for tuning the valve.

The balancing and trimming condensers may be combined as two series condensers in an arrangement as indicated in Fig. 3 in which C1 and Cz are two condensers in series across the plate 1 Cz namely,

is the trimming or tuning adjustment.

Magnification factor (n) of valves The type of oscillator shown in Figs. 1a, 1b

Y. and 1c is eminently suitable for low a tubes, and

the grid voltage swing obtained by the arrangement according to this invention is higher than is desirable for a good output with high values of ,u., the resistance of the grid leak having to be increased to an extent that makes the operating angle of the valve very small.

In this case a capacitative reactance placed in the cathode and filament leads (i. e. in series with the inductance L) is necessary to reduce the drive to the grids to a satisfactory value, the unbalance current to ground due to Cg being larger than Cpe will then result in the grid-cathode voltage swing being reduced and the drive brought to a satisfactory value without too high a value of grid leak, resulting in too small an operating angle. A length of transmission line or a small condenser in the cathode lead will then be satisfactory. This, however, does not do away with the need of balancing and the balancing condenser Cb is still necessary.

What is claimed is:

1. An oscillator for short and ultra-short waves including a valve having grid and plate electrodes and a cathode, a grounding impedance for saidA cathode, saidimpedance having someinductive reactance inherent therein at the frequencies involved, the grid-cathode internal capacity of the valve being greater than the plate-cathode internal capacity, transmission line circuits coupled to said electrodes, and a trimming condenser, having a capacity corresponding to the diierence between said capacities, connected externally of the valve between the plate line and ground, arranged to balance said transmission lines with respect to the cathode, and to tune the out-of-balance current to ground through said inherent cathodegrounding impedance.

2. The oscillator of claim 1 wherein the transmission line circuits coupled to said electrodes comprise anode and grid leads terminating in grounded condensers and wherein the output of said Oscillator is taken at a point on the anode lead in advance of said grounded condensers.

ERIC OSBORNE WILLOUGHBY.

REFERENCES CITED The vfollowing references are of record in the ille ofthis patent:

UNITED STATES PATENTS Number Name Date 1,995,175 Gill Mar. 19, 1935 2,068,990 Kolster Jan. 26, 1907 2,243,574 Nickle May 27, 1941 FOREIGN PATENTS Number Country Date 659.531 Germany May 5, 1938

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