PROCEEDINGS OF THE I.R.E.

Two Simple Bridges for Very-High-Frequency Use *D. D. KINGt, MEMBER, IRE

Summary-Two bridge circuits for impedance measurement atvery-high frequency are described. The first is a hybrid junctiontunable over the 100 to 500-Mc range, and capable of great sensitiv-ity. The second is an untuned Wheatstone type for service over thesame frequency range. Design features and sample performancecharacteristics are given for both units.

I. INTRODUCTION

RIDGE CIRCUITS are basic in most low-fre-B quency measurements. At microwaves, the use of

waveguide bridges is also widespread. However,in the intermediate-frequency range, above 100 andbelow 3,000 Mc, relatively slow progress has been madein applying bridge methods. The advantages of bal-anced or null-type circuits are almost equally great inthis portion of the spectrum, but the required electricaland mechanical design involves problems peculiar tothis frequency band.At frequencies much above 100 Mc it becomes impos-

sible to provide the standard capacitances and resist-ances essential to the conventional impedance bridge.The arrangement of the bridge also presents problemsdue to stray capacitances and inductances. The ulti-mate in conventional circuitry has probably been at-tained by Soderman,l whose bridge is accurate up to afew hundred megacycles. A new null-type admittancecomparator described by Thurston2 promises to covermuch of the vhf spectrum, as does a modified direc-tional-coupler null-indicator credited to Byrne.3 Bothof these ingenious devices furnish direct readings of theunknown impedance in terms of dial settings. Theformer uses a wide-range multiplier, with a coaxial stuband a resistor as standards. The latter relies on broad-band matched terminations.The two devices to be considered here are bridge cir-

cuits of a simpler type. Impedance standards or theirequivalent are not contained in these circuits. Instead,two coaxial terminals are available, to which arbitraryimpedances may be attached. When these two imped-ances are equal, the bridge is balanced. The serviceswhich may be performed by such a device fall in twocategories:

1. Accurately matching one impedance to another,known or unknown.

* Decimal classification: R244.111. Original manuscript receivedby the Institute, May 16, 1949.

t The Johns Hopkins University, Radiation Laboratory, Balti-more, Md.

I Type 1601A VHF Impedance Bridge, General Radio Co., Cam-bridge! Mass.

2 W. R. Thurston, "A device for admittance measurements in the50- to 500-Mc range," presented, 1949 National IRE Convention,March 9,1949, New York, N. Y.

3J. F. Byrne "A null-method for the determination of impedancein the 100-400 Mc range," Proc. N.E.C. (Chicago), vol. 3, pp. 603-614; 1947.

2. Detecting and measuring very small changes in animpedance.

The applications of the two bridges to be describedtherefore do not coincide precisely with those of the im-pedance measuring schemes mentioned above. Thehybrid junction system is considered first.

II. A TUNABLE COAXIAL HYBRID JUNCTIONThe hybrid junction in waveguide form is commonly

used as a "magic tee" bridge. A coaxial version men-tioned by Pound4 served as the starting point for thepresent design. Basic to the hybrid circuit are the seriesand parallel connections made at the same point along aline. In order to obtain external terminals for these con-nections, stub supports are required. The mechanicalstructure devised in the present case makes use of twotunable stubs mounted in concentric fashion. The ar-rangement of the coaxial circuits is shown in Fig. 1.

A-

SECTION AA

SECTION SB

3

,SERIES CONNECTION

-2

CONNECTION

SLIDING SHORTCIRCUITS

Fig. 1-Coaxial hybrid junction.

The main line has terminals 1 and 2 in the figure.Output 4 is in series with this line, and output 3 is inshunt. The outside diameter of the assembly is one inch,hence the electrical length of the actual junction pointis negligible at the highest frequency used. The sliding

4 R. V. Pound, "Microwave Mixers," McGraw-Hill Book Co.,New York, N. Y., p. 268; 1948.

1950 37

PROCEEDINGS OF THE I.R.E.

short circuit in the split inner conductor is part of anadjustable stub in parallel with the series connection 4.The plunger in the main barrel similarly is part of anadjustable stub in parallel with the shunt terminal 3.Terminals 1 and 2 are for the impedances under test;terminals 3 and 4 are for generator and detector. In thepresent case the generator is arbitrarily assigned toterminal 3. The equivalent circuit of the hybrid junctionis shown in Fig. 2. This circuit is obtained by superpos-ing the equivalent circuits for shunt and series tees inlines or guides. The transformer in the representation ofa series tee is then replaced by an equivalent imped-ance Z4. Without generator and detector connections,the bridge acts as a reactive T-network or triple-stubtuner.

Z 4/:, z 4/2

z3

Eg

Fig. 2-Equivalent circuit of the hybrid junction.

Any residual asymmetry at the junction results in anunequal division of Z4. This unbalance is very smallwhen the physical symmetry is good. The shunt imped-ance of the two stubs mentioned above is of courseincluded in Z3 and Z4 of the equivalent circuit. The the-ory and applications of waveguide "magic-tee" bridgesis thoroughly treated in the literature,' and applies inlarge part to the coaxial version just described. A fewpractical aspects are mentioned here.The simplest application of the bridge is in adjusting

one impedance to precisely equal another. Exact sym-metry within the bridge is then unnecessary, if an ad-justable reference impedance is permanently attachedto arm 1. The impedances to be equalized are then at-tached in turn to arm 2. The smallest change in imped-ance which may be detected depends in principle on thenumber of decibels between input power and detectornoise level. The fineness of adjustment possible usuallyis a more practical limitation.

Fig. 3-Circuit for testing bridge sensitivity.

In order to test the sensitivity of the bridge, a beyond-cutoff attenuator may be used in the arrangement shownin Fig. 3. The input loop of the attenuator is a 75-Q re-sistor, the output loop a copper strip. With the attenu-ator set at maximum attenuation, the bridge is balancedby the phase changer and stub in arm 1. The reaction ofthe resonant output circuit in the attenuator on thebridge null is a convenient indication of the sensitivityat balance. As an example of the sensitivity attainable,the following data were taken at 150 megacycles:

AttenuatorInsertion Loss

APR-4 Receiver TuningMeter Reading

(0-200,amp. movement)

56 db

105 /uamp.

>80 db

95 ,uamp.

The maximum reflection coefficient at the input termi-nals of a matched 56-db pad, obtained by changing itsoutput impedance, is of the order of 10-6. The initialbalance of the bridge is the equivalent of a perfect refer-ence match when the pad is set at > 80 db. Consideredas an attenuator between power source and receiver, thebridge at balance presented an attenuation of the orderof 90 db. For such critical adjustment an unmodulatedsignal is required to avoid the frequency modulationpresent in most modulated oscillators.

"Magic tee" bridges are commonly used to measurethe magnitude of the reflection coefficient| r2 directly.Under these so-called "magic" conditions, ZlAZ3=Z4=ZO, (P1-=F2=P3=0) where Zo is the characteristicimpedance of the transmission lines involved. Then theinput to output power ratio is6

P4 r2 12

Ps 4 (1)

Since a shunt stub is an integral part of outputs 3 and4, an external phase changer in these two lines suffices toprovide the required matched conditions. More gen-erally, the values of Z3 and Z4 may be adjusted to equalany desired reference standard ZA. However, the sensi-tivity of the bridge is reduced when Z3 and Z4 differappreciably from the cable impedance used. Efficientpower transfer from source to bridge and from bridge todetector is thereby impaired. To adjust the bridge to the"magic" condition, a reactive load varying at an audiorate may be placed in arm 2. The other arms are thenadjusted to minimize the resultant modulation in theoutput. A motor-driven butterfly coupled to the bridgethrough a pad approximates the desired performancefairly well.Although the hybrid circuit described above is sensi-

tive and flexible in its application, it requires carefultuning at each frequency of operation. This drawbackled to the development of a second bridge of simplerdesign.

' C. G. Montgomery, 'Technique of Microwave Measurements,"McGraw-Hill Book Co., New York, N. Y., Chap. 9; 1947. 6 See p. 530 of footnote reference 5.

38 January

King: Bridges for VHF Use

III. A WHEATSTONE BRIDGE FOR VHF

The balance in a Wheatstone bridge is obtained bysymmetry, and the simple bridge circuit must continueto give a null indication at any frequency at which thesymmetry can be preserved. At microwave frequencies,impedances and terminals may no longer be defined inthe usual way for circuits of practical dimensions. How-ever, at frequencies in the hundreds of megacycles, defi-nite transmission line terminals are available. Accord-ingly, the device shown in Fig. 4 was built to have com-

appears better than the electrical tolerances would indi-cate.The sensitivity attainable is shown by Fig. 5. These

data were obtained by the method shown in Fig. 3. The

45j-

db

4

40 2

00 200 300 400 500

f in mcFig. 5-Attenuator insertion loss for 1-db change in null indicator.

oscillator is, of course, direct-coupled in this case. The

self-contained crystal detector feeds a tuned amplifierand Ballantine volt meter. Although the sensitivity to

impedance change is much lower than that quoted for

the tuned hybrid junction, it exceeds that attainable byslotted lines or directional couplers. The convenientmounting of the instrument is shown in the photograph,Fig. 6.

3

Fig. 4-Wheatstone bridge circuit. Bipass capacitors are 500 ,ufd.

plete physical and electrical symmetry. Any asymmetry

in the structure of the detector crystal is neglected. Theelectrical balance is limited by the 2 per cent toleranceof the matched components.The generator terminal 3 shown in Fig. 4 is designed

to mount directly in a GR type 857A oscillator. Alter-natively, the loop may be mounted in one end of a be-yond-cutoff attenuator. The direct mounting in the os-

cillator provides maximum power input over the entire95 to 515-Mc tuning range of the oscillator. A micro-ammeter at terminal 4 with series multiplier to protect

the crystal serves as a simple null indicator. Greater sen-

sitivity is obtained with a modulated system and tunedamplifier. In either case, the only tuning adjustmentrequired is the oscillator dial.The symmetry of the system is easily checked by bal-

ancing the bridge and then interchanging the loads ofterminals 1 and 2. The symmetry is best at low frequen-cies. It is worst at high frequencies and at very low loadimpedances. Lack of standards prevented a quantitativecheck, but in the range 100 to 500 Mc the symmetry

Fig. 6-Wheatstone bridge mounted on GR 857A oscillator (coverplate removed).

IV. CONCLUSION

Two bridge circuits useful in the 100 to 500 Mc range

have been described. The first, a tuned hybrid junction,is extremely sensitive, and may be adapted to any fre-

quency suitable for coaxial-line stubs. The second bridgeis a modified Wheatstone circuit, operating without tun-

ing adjustment over the entire 100 to 500 Mc frequencyrange. These circuits are applicable wherever sensitiveimpedance comparisons are required in the very-high-frequency spectrum.

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391950

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