THE DESIGN OF SIGNAL GENERATORS FOR CENTIMETRE WAVELENGTHS*

By D. C. ROGERS, Graduate.!(The paper was first received 5th February, and in revised form 4th April, 1946.)

(1) INTRODUCTION. AND SUMMARYThe function of a signal generator is to provide a test signal

of known frequency and amplitude. In the design of suchinstruments for centimetre wavelengths, problems chiefly asso-ciated with the determination of the signal amplitude havenecessitated some departure from conventional practice at lowerfrequencies. The main difficulties encountered have been in thedesign of a suitable output attenuator, together with a monitorby which the output may be maintained constant at somereference level, and the provision of adequate screening for thegenerator. This paper seeks to acquaint the reader with someof the methods that have been evolved to meet these problems,and includes a brief description of a typical signal generatoroperating in the 6-cm wavelength region.

The paper is not intended to be a complete survey of thework which has been carried out on this subject during the war,but it is hoped that it will be of value in indicating some of thelines along which development has taken place.

(2) OSCILLATOR, MODULATOR, AND POWER SUPPLIESA description of the many types of r.f. oscillator available is

obviously beyond the scope of this paper; there are, however,a few general points which may be mentioned.

To facilitate examination of the receiver under test, it isusually. desirable that the output of the signal generator bemodulated at some low frequency. At centimetre wavelengthsit is desirable not to employ the normal form of sine-wavemodulation, since most oscillators are, at present, of the velocity-modulated type, and oscillate at a frequency which is dependenton the voltages applied to their electrodes. This makes itdifficult to obtain amplitude modulation without some attendantfrequency modulation. It is, therefore, customary to use 100%square wave, or "chopped c.w." modulation, in which theoscillator operates at constant amplitude during one half of themodulation cycle, and is completely cut off throughout theremaining half. Any frequency change occurring will then takeplace during the "changeover" period, and its effects can bemade insignificant by using a steep-fronted waveform. Square-wave modulation also avoids the possibility of distortion of ther.f. envelope due to non-linearity.

In view of the dependence of frequency on electrode potentials,the r.f. oscillator is almost invariably operated from a stabilizedpower supply unit.

(3) THE OUTPUT ATTENUATORThe most reliable attenuators at centimetre wavelengths are

of the so-called piston type, utilizing a wave guide operating ata wavelength longer than the critical value. Under these con-ditions the electromagnetic field decays exponentially withdistance along the guide, and by using a movable electrode asa pick-up device, a linear db scale is obtained.

The attenuation at a free-space wavelength Ao is given by

a = 2TTV[U/AC)2 - (1/Ao)2] nepers/cm . . (1)

* Radio Section paper. t Standard Telephones and Cables, Ltd.

where Xc is the cut-off wavelength for the particular wave modeinvolved.

It is usual to employ one of the two lowest orders of mode,viz. Hj or Eo, in a circular guide. These have a lower rate ofattenuation than the higher orders, and if, therefore, any highermode is present as an impurity, it is more rapidly attenuatedthan the desired wave, and its contribution to the output becomesnegligible at short distances down the guide.

For the Hj and Eo modes, eqn. (1) becomes1 6 / / , H'65r2\ . . .

Hi wave: = —• /( 1 — -—r-_—I db/cm . . (2)rM\ A2, /

Eowave: =20-9

TTJ— ) db/cm (3)

where r is the radius of the guide (cm).Even when using these wave types, considerable care is

necessary in the design of the launching and pick-up electrodesto exclude the excitation of unwanted modes, if the attenuatoris to obey the theoretical law at low values of attenuation.

The Eo wave, on account of its circular symmetry, is ofteneasier to obtain in pure form than the Hj mode and is preferableto the latter, in spite of its somewhat higher rate of attenuation.Experience has shown that one of the best methods of obtaininga pure wave is to excite the attenuator directly from the field ina resonant cavity. Typical methods of doing this are shownin Figs. 1 and 2 for the Eo and Hl waves respectively.

—

7&Flat disc-

Piston/

^Resonator

Fig. 1.—Resonant cavity for exciting EQ wave in attenuator.Lines of magnetic force. • • Lines of electric force.

In the case of the H{ wave, E waves can be excluded by theuse of an electrostatic screen consisting of a series of fine parallelwires orientated parallel to the magnetic field, and in contactwith the tube wall at their extremities.

The pick-up electrode is mounted on a movable piston, andconsists of a loop in the plane of the electric vector (Ht wave)or an axial wire or disc (Eo wave). The piston should makegood electrical contact with the wall of the tube, thus eliminatingerratic output changes due to varying reflection from the pistonface. A suitable mechanical arrangement, such as a screwthread and rotating head of the micrometer type, must be pro-vided for moving the piston. If its bore is sufficiently small,the attenuator can be calibrated directly in decibels, even ingenerators tuning over a wide frequency-band, since the secondterm under the radical sign of eqns. (2) and (3) is then negligible,and the attenuation is independent of frequency.

[ H57 ]

1458 ROGERS: THE DESIGN OF SIGNAL GENERATORS FOR CENTIMETRE WAVELENGTHS

Resonator Loop istoti

Tungsten wirefilament

0"7cm long0-001 cm dia.

0-036cindia.

Roccsscontuirvmj filament

Fig. 3B.—Improved type of bolometer lamp.

L;imp

Fig. 2. Resonant cavity for excitingwave in attenuator.

Lines of electric force.... — — Lines of magnetic force.

Fig. 3A.—Lamp suitable fornoise measurement bythermal method.

The impedance presented by the pick-up electrodes is highlyreactive, and unsuitable for direct connection to the receiver.It is usual, therefore, to fit a buffering length of attenuatingcable permanently to the output of the generator. The impe-dance seen by the apparatus under test at the end of this cablewill then be approximately equal to the characteristic impedanceof the cable.

(4) THE OUTPUT MONITORIt is necessary to maintain the exciting field at the mouth of

the attenuator at some constant or fiducial level. An elementarymethod by which this can be done in the case of the HOi wavewas proposed by C. N. Smyth early in 1940. A high-frequencycurrent from the oscillator was passed through the filament ofa small lamp, the filament, which consisted of a straight wire,being placed diametrically across the mouth of the attenuator.The current was increased, by adjusting the input power to theoscillator, until the filament glowed just visibly red. Thecurrent value at which this occurred was very sharply defined,and tests carried out using d.c. to heat the filament showed thatdifferent observers could set the current to an accuracy ofabout j 2%.

The filament may conveniently be viewed through a tubesoldered into the wall of the generator screening case.

Fig. 3A is a sketch of a suitable lamp, having a tungstenfilament 0-7 cm long, and 0001 cm in diameter; the bulb isevacuated. The filament becomes visibly red at a current ofabout 35 mA.

Satisfactory results were obtained with this lamp, but it wasnot practicable for large-scale use as it had to be made by hand.The design of Fig. 3B was therefore introduced, in which asimilar filament was used, but in a construction suitable formanufacture by conventional valve production methods. Thefilament was located in a recess in the moulded end of thetubular bulb, and fed from a parallel-wire transmission lineformed by the two support rods. The moulded recess enabledaccurate location of the filament within the bulb during manu-facture and also of the bulb itself at the mouth of the attenuator.In this way large variation in the output power on changing themonitoring lamp was avoided, although, of course, the accuracywas not sufficiently great to make re-calibration unnecessary.

At the shortest wavelengths, this lamp was enclosed in a metaltube to prevent radiation loss from the parallel-wire line.

The lamp of Fig. 3A can also be used with an Eo attenuatorusing the circuit of Fig. 3c in which the r.f. voltage developedacross the filament is used as the reference standard.

H.F. input line

Fig. 3C.—Circuit arrangement for launching EQ waves.

Although it is technically satisfactory to set the filamenttemperature by eye, the method lacks the convenience of opera-tion offered by a meter, and it is sometimes desirable, particu-larly in instruments intended for field use, to include the filamentin a pre-set Wheatstone bridge circuit and set its temperatureby the change in electrical resistance.

In such an arrangement, a thermistor forms a convenientalternative to the lamp as a temperature-sensitive resistance,since it can have a resistance that will terminate the r.f. input-line correctly, and thus give a system which is satisfactory overa wide frequency-band. Some form of compensation forambient temperature change is necessary when using thethermistor, owing to its large negative temperature-coefficient.

The disadvantage of the methods so far described is that thecoupling efficiency between the monitor and output attenuatoris low. This low efficiency, together with the minimum attenua-tion necessary to bring the attenuator on to the linear part ofthe scale, and the attenuation of the buffer cable, result in themaximum output being only a few microwatts, even when somehundreds of milliwatts are available in the oscillator. Theoutput may be raised to a level more convenient for calibration,if the field at the mouth of the attenuator is increased by reso-nance, using, for example, the circuits of Figs. 1 and 2.

The thermojunction of Fig. 4, which was designed specificallyfor building into the walls of coaxial lines and wave guides,forms a suitable monitor for circuits of this type.

As will be seen from the figure, the bulb of the thermojunetionis of the disc-seal type; beneath the glass dome on the upperside of the disc is a small loop of wire, approximately rectangularin shape, one side of which is formed by the couple itself. Ther.f. field induces a voltage in this loop, which is incomplete ford.c , but is completed at high frequency by a small by-passcondenser built into the disc; the r.f. current is thus confinedto this side of the disc. D.C. connection to the couple elementsis made via the disc and the connector cap on the lower halfof the disc. Owing to the small size of the loop, the thermo-couple is substantially aperiodic at frequencies up to 6 000 Mc/s,beyond which resonance may occur.

The couple itself has a resistance of about 20 ohms, and givesan e.m.f. of about 12 mV for a current of 10 mA. The disc isnotched in its periphery to provide location of the plane of theloop with respect to the circuit.

It is possible in some cases to use the resonator of the valveitself to excite the attenuator. This gives a considerablesimplification in both layout and operation, since there is thenonly one frequency-discriminating element in the system.

ROGERS: THE DESIGN OF SIGNAL GENERATORS FOR CENTIMETRE WAVELENGTHS 1459

Bypasscondenser

Thermocoupleelements

Disc seal

Getter

Connectorcap

Fig. 4.—-Thermojunction for u.h.f. monitoring (Type T2H/15JA).

Crystal rectifiers have been used successfully in laboratoryinstruments, and have the merit of a high sensitivity; they arenot recommended for field use, however, as their rectificationefficiency is affected by temperature variation.

(5) SCREENING AND FILTERINGWhere the generator is intended for use at very low output

powers—e.g. in the region of noise level, the stray radiationfield must be about 140 db below the power level obtaining inthe r.f. oscillator. The screening problem is more acute than atlow frequencies, since radiation will take place from any holeor crack larger in any dimension than a small fraction of awavelength.

Screening cases have usually been fabricated out of coppersheet, with all seams carefully soldered, or alternatively havebeen cast in an aluminium alloy. The lid must be held in placeby a large number of closely-spaced bolts, preferably not morethan i\ apart, in a wide flange, or some equivalent clampingarrangement. The mating faces are machined to ensure a leak-proof joint.

Leakage arising where control spindles enter the case can beprevented by making the spindles of insulating material, andpassing them through metal tubes soldered to the wall of thecase; these tubes should have an internal diameter not greaterthan about {X in the dielectric and a length equal to four orlive times the diameter. If the requisite mechanical strengthcan only be obtained by using a metal spindle, a "packing"gland, stuffed with steel wool, may be used.

Radiation may also take place via the power-supply leads tothe oscillator, and some form of filter must therefore be fitted.Filters may be divided broadly into two classes:

(a) Non-dissipative wave filters.(b) Filters which rely on dissipative loss for their attenuation.A method by which non-dissipative filters may be constructed

at these wavelengths was suggested by R.R.D.E., and by theG.E.C. Ltd., and consists of sections of coaxial transmissionline of alternately high and low characteristic impedance con-

nected in tandem. The filter thus formed has multiple rejectionbands, the first occurring when the length of the sectionsapproaches £A. The attenuation and the width of the rejectionband both increase as the ratio of the two characteristic impe-dances is made greater. This ratio is usually limited by practicalconsiderations to about 10 : 1 , when the maximum attenuationis 20 db per section. An alternative form for the line elementsis found in the flat-strip line; the centre conductor can thanconsist of copper strip cut as shown in Fig. 5A, and can beclamped to the wall of the screening case between two sheetsof dielectric, as in Fig. 5B.

Fig. 5A.—Shape of conductor in u.h.f. strip-type filter.

Generator

rrn rm

V~I lamping

Filter strip plate DielectricFig. 5B.—Clamping arrangement for filter strip.

Filters using resistive loss are usually in the form of coaxiallines having a very high-loss material as a dielectric. Onematerial that has been found satisfactory is a jelly of coppersulphate solution first introduced by the Clarendon Laboratory,Oxford. Another, first employed in America, is the iron-dustmaterial used in magnetic cores at radio frequencies. Both needto be enclosed in a tube of insulating material to prevent break-down when the supply potentials are applied. The iron-dustmaterial is usually preferable to copper sulphate, as it is moreable to withstand the temperature variations likely to be en-countered overseas; it is readily obtained in a suitable form —namely, in small cylinders about $in diameter with an axialhole to admit the centre conductor of the line. An attenuationof 20 db/cm at 5 000 Mc/s has been obtained using an iron-dustfilter with the dimensions given in Fig. 5c.

^ Paxolin tulu*insidi' brasstube

Fig. 5C.Dust-iron cylinders

Iron-dust filter construction.

As an alternative to making a large screening case to containall the working parts of the generator, it is sometimes possibleto enclose the oscillator valve itself in a tight-fitting metal case,the internal structure of which may form part of the r.f. circuitassembly. The leak-proof joints are then all made on relativelysmall machined parts, and a sound joint is consequently mucheasier to obtain. Since the outer box is then no longer requiredto be leak-proof, its cost and weight are considerably reduced.

(6) CALIBRATIONIt is customary to calibrate the generator output in terms of

power rather than voltage, since the measurement of voltage atcentimetre wavelengths presents some difficulty. Even if thiswere not the case, a power calibration has a more fundamentalsignificance than one of voltage, since the signal/noise perfor-mance of a receiver can be expressed explicitly in terms of itsband-width, and the available power in the source necessary to

1460 ROGERS: THE DESIGN OF SIGNAL GENERATORS FOR CENTIMETRE WAVELENGTHS

give unity signal/noise ratio at the receiver output. The avail-able power of the source (here, the signal generator), is defined asthe power which the generator will deliver into a matched load.

The output power is usually measured with the aid of abolometer, in which the r.f. power is absorbed by the filamentof a small lamp, thereby raising the temperature of the filament,and increasing its d.c. resistance; the r.f. power is equal to thed.c. power which causes the same change in resistance.

It is convenient to use the circuit of Fig. 6A. The lamp

Piston

Controls

Mica condenser

Fig. 6A.—Circuit of bolometer bridge.

forms part of a Wheatstone bridge network and is mounted ina resonator that serves to match it to the output impedance ofthe generator. The direct current flowing to the bridge isadjustable, and is measured with the meter I.

The bridge is first balanced at a low value of direct currentwith the r.f. power applied and with the resonator adjusted formaximum transfer of energy into the lamp filament. The r.f.power is then removed and the bridge re-balanced by increasingthe bridge current. The increase in d.c. power dissipated in thelamp is equal to the r.f. power removed and can be deducedfrom the known values of the bridge current and of the resis-tances i?j, i?2 and R3.

An interesting variation of this circuit by W. A. Montgomeryas shown in Fig. 6B which was developed to enable calibrationto be carried out by less skilled operators. It will be seen thatthe two arms Rj, R4 of the bridge are simul-taneously adjusted by means of a gangedcontrol. If these arms are made to bear afixed ratio k, it can be shown that at balancethe ratio of the power in the arm R2 to thatin the bolometer R3 is also k. The arm R2can therefore include a meter calibrateddirectly in units of power, and no calcula-tions are necessary.

With the most sensitive lamps that can beproduced at present, the smallest powermeasurable using standard 2i-in panel-mounting meters is about 200 /xW. Usinga mirror galvanometer as a balance indicatorenables measurements of powers in the 1 /xWregion to be made. It is evident, therefore,that in generators for use outside the labora-tory the attainment of adequate output poweris a matter of considerable importance.

Calibration must be carried out at variousfrequencies throughout the operating range,as the efficiency of the monitor and theattenuator-exciting electrode is rarely in-dependent of frequency.

The question of frequency calibration willnot be discussed here, as the problem offrequency measurement has received ade-quate attention elsewhere. It may be men-tioned, however, that many generators havebeen fitted with a built-in wavemeter, since,at these frequencies, the oscillator valve will

Fig. 6B.—Direct-reading bolometer bridge circuit.

contain at least a part of the r.f. circuit, and any calibration ofthe oscillator controls will become invalid when the valve ischanged.

(7) TYPICAL GENERATORSThe author has been associated with the design of some of

the centimetre-wavelength signal generators manufactured duringthe war for use in the Government Research Establishments andin the Services. The one shown in Fig. 7 was designed in 1941,and was intended primarily for laboratory use. It operated inthe range 8-8 to 11-8 cm, and used a current indicator lamp asthe output monitor, exciting an Hrwave attenuator. To ensurecomplete freedom from leakage, two screening boxes, made ofaluminium, were used.

A more recent signal generator, incorporating many of thefeatures described in this paper, was designed early in 1944 forthe field testing of a communication receiver operating in the6-cm wavelength region. This generator operates at a fixedwavelength. The high-frequency unit, which consists of theoscillator valve and its associated resonant circuit, the outputattenuator, and the supply lead filters, are shown in Fig. 8.The oscillator is a CV228 type magnetically-focused coaxial-linevelocity-modulated valve, mounted in a cylindrical resonantcavity having a diameter of 2 | in and a length of 1 in. As thegenerator operates at a fixed frequency, no tuning is provided;it is found that the wavelength does not deviate from the nominalwavelength by more than 1 mm on changing valves.

Fig. 7.—View of 8 • 8-11 • 8 era generator.

ROGERS: THE DESIGN OF SIGNAL GENERATORS FOR CENTIMETRE WAVELENGTHS 1461

Fig. 8.—High-frequency unit of signal generator for 6-cm wavelengths.

The attenuator is of the Eo type, and is excited directly fromthe resonant cavity; its mouth lies in the axis of the cavity,opposite to the CV228 antenna. As already mentioned, thismethod of excitation gives an extremely pure Eo wave, and itis possible, using a 10-db length of buffering cable, to obtainan output of 300/^W without observable deviation from thetheoretical law.

The attenuator piston is driven by a wedge-shaped stainless-steel plate actuated by a rack and pinion. The wedge carriesthe attenuator scale, which is viewed through a window in thefront panel of the generator. The scale is adjustable relativeto the cam, and by the process of calibration is set so that acertain reading on the scale corresponds to an output power of300 /u,W, as indicated by the calibrator unit. This arrangementenables the scale to be calibrated so that it reads the receivernoise factor directly, if the attenuator is adjusted to give unitysignal/noise ratio at the receiver output, as seen on the oscillo-graph screen. To make this calibration valid, the receiveroutput noise must be measured in a known band-width. Thisis achieved by limiting the band-width of the vertical deflectionamplifier of the oscillograph to lOOkc/s. It is known thatmeasurements made after the detector of the receiver are subjectto an error dependent on the law of the second detector, but,since the generator is intended for testing one specific type ofreceiver, a suitable allowance can be made the in scale cali-bration.

After calibration, the constancy of output is observed bymeans of a thermojunction monitor of the type described inSection 4, mounted to project through a hole in the peripheryof the resonator.

The body and base of the oscillator valve are enclosed in ascreen integral with the resonator. All joints in the h.f. unitare leakproof, thereby eliminating the need for an outer screeningcase. The supply leads to the valve, and the output lead of thethermojunction, pass through filters of the dust-iron-core type.

Square-wave modulation is provided at a repetition frequencyof 1 000 pulses/sec. The modulating voltage is obtained fromtwo pentodes operating in a multivibrator circuit, and is appliedto the grid of the h.f. oscillator. The use of a large value ofgrid leak and coupling condenser ensures that the grid voltageduring the modulation ON period is determined by the pointat which the grid commences to pass current, and so is inde-pendent of variations in the modulation voltage.

Safety devices are fitted to prevent damage to the oscillatorby overheating in the event of any failure of the modulator.

The calibrator is in the form of a separate unit, mountedadjacent to the main generator chassis. Its purpose is to enablethe output power of the generator to be set to 300 /uAV. Itcomprises a platinum-filament bolometer lamp in a wave-guidematching circuit, and a bridge of the special type described inSection 6. The resistance of the bolometer is about 25 ohms.

The equipment operates from a 230-V, 50c/s mains supply,and has a total power consumption of about 80 W.

(8) ACKNOWLEDGMENTSThe design work on which this paper is based, and the develop-

ment of the valves used, were carried out in the laboratories ofStandard Telephones and Cables, Ltd., except where otherwiseindicated in the text. The generators were designed by theauthor under the direction of C. N . Smyth.