hybrid circuits for microwave radio links

Electrocomponent Science and Technology1977, Vol. 4, pp. 79-83

(C)Gordon and Breach Science Publishers Ltd., 1977Printed in Great Britain

DEVELOPMENT AND PRODUCTION OF HYBRIDCIRCUITS FOR MICROWAVE RADIO LINKS

PER CHR. MALMIN

NERA AS, Division Bergen, PO Box 53, N-5032 MINDE, Norway

(Received May 30, 1977)

During the last few years, the use of hybrid integrated circuits in microwave radio links has increased significantly.This paper reports on a range of microwave hybrid circuits now in production at our plant. These include transistorhigh power amplifiers, low noise mixers and various other circuits. The hybrid technology has proven to be costcompetitive with the more conventional waveguide and coaxial technologies, and gives quite a few other advantagesas well. Among these, the standardization of packages and modules is discussed in some detail. The trend is now touse the hybrid technology in VHF circuits as well. This opens up the possibility of integrating related functions inone package. The production cost in this case must be related to printed circuit boards, and it remains to be seen ifthere is anything to gain in this respect. However the reduction of the size of modules and the time for circuitalignment will certainly give an improvement in the overall system cost. In order to reduce the cost of microwavehybrid circuits, thick film technology is currently being investigated for use up to 15 GHz. There is probably nospecific upper frequency for thick film circuits, but the possibility of using conventional thick film technologydepends on the actual circuits under consideration. Preliminary results of this work are reported, together with acost comparison between thick film and thin film microwave hybrid circuits.

1. INTRODUCTION

This report will discuss the use of hybrid circuits inmicrowave radio links. It is mainly concerned withmicrowave hybrid circuits, using microstrip lines, andat first a survey of the principles of these circuits isgiven. Several examples of hybrid circuits currentlyin production at our company are shown. The use ofVHF hybrid circuits, and the possibilities of integratingvarious SHF and VHF circuits are discussed. We willalso report on investigations on thick film technologyas a potentially cheaper way of making SHF hybridcircuits.

semiconductor, packaged or in chip or beam leadform, must be matched to the 50 ohms characteristicimpedance of the microstrip line. This is done byusing distributed circuit elements, i.e. sections ofdifferent transmission lines.

2. MICROWAVE HYBRID CIRCUITS

At microwave frequencies the wavelength is of thesame order of magnitude as the circuit elements.As a consequence, one cannot simply connect diodesand transistors by conductors of arbitrary shape, theyhave to be interconnected through transmission linesof a specific characteristic impedance. Microstrip linesare the more often used transmission lines in hybridcircuits. When designing circuits with microstrip lines,the linewidth, or characteristic impedance, must bekept within close tolerances. The impedance of any

79

FIGURE High power amplifier. Output power 5W,Bandwidth 10%. Centre frequency 2 GHz.

80 PER CHR. MALMIN

w k/v

0 2 4 6 8 10 12 14FREQUENCY (GHz)

FIGURE 2 Noise figure of hybrid circuit mixers.

FIF 70 MHz, NFIF 1.5 dB

16

Use of thin film hybrid technology has manyadvantages when designing microwave circuits.For example:

close dimensional controlease of component attachment, use of chips avoidsproblems due to package parasiticsstandardization of module size independentof frequencyhigh reliability and reasonable costsize and weight reduction, important inmobile systems.

However, it must be kept in mind that not all circuitfunctions are suited for hybrid circuit technology.The most typical example is perhaps filters withhigh Q-value.

FIGURE 4 Low noise mixer with adjustable attenuatorfor LO power adjustment. Frequency range: RF & LO" 7.4to 8.4 GHz IF" 70 MHz. NF 6 dB

FIGURE 3 Low noise mixer.NF 6 dB (incl. 1.5 dB IF noise). Frequency range RF &LO: 3.4 to 4.2 GHz IF" 70 MHz

FIGURE 5 Integrated mixer and IF preamplifier, completewith adjustable attenuator and detector for LO power level.Frequency range: 4.4 to 5.0 GHz

HYBRID CIRCUITS FOR MICROWAVE LINKS 81

FIGURE 6 Two versions of a 2-PSK modulator. On theleft hand side is the waveguide version, with a 4-portcirculator. On the right hand side is the microstrip version,with a separate coaxial isolator.

112 14FREQUENCY (GHz)

FIGURE 7 Attenuation in Microstrip lines with 50 ohmscharacteristic impedance. Different technologies.

The use of a computer in the circuit design maybe extremely helpful. However, a serious limitation isthe lack of available data for the semiconductorsinvolved. High power devices are usually poorlycharacterized. This is at least partly due to the factthat impedance measurements on these devices maybe more or less meaningless if performed at a powerlevel and in a circuit very different from the actualapplication. Therefore, a somewhat pragmaticapproach has to be taken in the design phase.One calculates an initial circuit layout usingsimplified models for the semiconductors involved.This circuit is then measured, and if necessary,changed by removing or adding metallization.These changes are often done by applying smallamounts of silver conductive coating. This usuallyhas the desired effect, because at microwavefrequencies, every line or part of the conductor area,may be considered a circuit element.

Finally, the modifications are included in the goldmetallization pattern by making a new mask.

3. EXAMPLES OF MICROWAVEHYBRID CIRCUITS

For our radio link transmitters we have developedhigh power, broadband transistor amplifiers, as shownin Figure 1.

Hybrid circuit technology has proved essentialfor the development of these amplifiers. Thin filmgold, on alumina substrates, provides both over-the-edge metallization for ground plane connections, and

a favourable length to width ratio for low impedancelines. We are now producing a range of amplifierswith more than 5 W output power over a bandwidthof 10% at 2 GHz. The power transistors are notmounted on the substrates, but directly to thebottom of the box, in order to obtain proper heatsinking. It has been necessary to tune these amplifiersindividually. This is done by locally applying a silverconductive coating while inspecting the frequencyresponse on an oscilloscope. The tuned amplifier isnext given a stabilization bake before final testingis made.

For radio link receivers, mixers with low noisefigure must be used. We have developed a range ofbalanced microstrip mixers with noise figures asshown in Figure 2. Each circuit covers at least oneradio frequency band. This broadband operation isobtained in spite of the fact that a 180 "rat-race"coupler is used to separate the LO and RF signals.A typical mixer circuit, operating over the

frequency range from 3.4 to 4.2 GHz is shown inFigure 3. The shape of this box was determined bythe requirement that this mixer should replace anolder waveguide mixer in an existing system. On theother hand, this made it possible to produce hybridcircuit low noise mixers, covering all the radio linkbands from 3.4 GHz and upwards, and all fitting thissame box. Type HP 5082-2768 Beam-Lead Schottkydiodes are used for all these circuits. In addition tothe mixer, there is a detector circuit on the samesubstrate. This circuit is used to monitor the LOpower level.

It is often convenient to have a variableattenuator for the LO power within the same box.

82 PER CHR. MALMIN

38

0 37

FIGURE 8amplifiers.

THIN FILMTHICI<FIL

1950 2000 2050 2100 2150FREQUENCY (MHz)

Frequency response of microstrip power

THICK

12 13

THIN FILM

14 15FREQUENCY (GHz)

FIGURE 9 Mixer noise figure. The same circuit in thinand thick film technology.

This attenuator is used to adjust the LO drive levelto obtain minimum noise figure. With this attenuator,the complete circuit may look as shown in Figure 4,which is a mixer covering the 7.4 to 8.4 GHz frequencyrange. At microwave frequencies one cannot usepotentiometers, so the attenuator element is a ferritedisc which can be moved up and down above themicrostrip line. The closer the ferrite is to the line,the greater the attenuation. This part of the circuit isproduced on a PTFE based substrate with trade nameEpsilam 10, which is cheaper than alumina and iseasily machined with ordinary workshop equipment.

Figure 4 shows the general design of the boxes.We are using a U-shaped profile with in. internalwidth. This profile may be cut to any desired lengthto accommodate 1,2 or 3 substrates. The box iscompleted by soldering two end wails to the profile,using a high temperature solder. The connectors maybe located according to the application. Thus a highdegree of standardization of the production processis obtained, while flexibility in circuit layout ismaintained.

4. INTEGRATION OF SHF AND VHFHYBRID CIRCUITS

We have started to work with hybrid circuit tech-nology in a special way, namely with circuits formicrowave frequencies. We are now moving down-wards in frequency, and hope to see advantages fromintegrating, for example a thin film mixer and a thickfilm IF-preamplifier in the same box. An example ofthis is shown in Figure 5. The requirements on thepreamplifier are first of all a low noise figure of lessthan 1.5 dB, a fiat frequency response up to around

100 MHz, a gain of approximately 25 dB and outputVSWR of less than 1.1. This goal has been achievedby using a 2 stage transistor amplifier with currentfeedback. Encapsulated transistors are used in orderto simplify packaging and testing problems. Tran-sistors, and chip capacitors and inductors, are solderedinto the circuit.

This unit has several advantages. It requires aminimum of alignment, confines the lowest signallevels within one box, has a broadband frequencyresponse, both SHF and VHF, and saves a lot ofspace in the equipment. One does not here see anycost saving in the production of the thick filmamplifier compared to the former printed circuitboard version. However, the reduction of system set-up time is very important.

5. POSSIBILITIES FOR COST REDUCTION

A few possibilities for cost reduction with microwavehybrid circuits will be discussed.

One example of what has been obtained with thinfilm circuits, is a two-phase shift modulator for a13 GHz digital radio link, Figure 6. The originalwaveguide version, a reflection type circuit, had touse an invar cavity to get sufficient temperaturestability. The new microstrip circuit is of the"Switched-line" type on alumina substrate.In production the microstrip circuit is approximately25% cheaper than the waveguide version. This isprimarily due to lower material costs. This costadvantage is obtained even if the hybrid circuitversion of the modulator has four chip diodes whilethe waveguide version has only one diode.

Another possibility is using thick film technologyfor microwave hybrid circuits. Here two problems

HYBRID CIRCUITS FOR MICROWAVE LINKS 83

must be taken into consideration. The technicalquestion if the line definition and other propertiesof thick films are good enough for microwaveapplications, and then the economical question ifany cost reduction is really obtained.

Figure 7 shows loss per unit length of 50 ohmmicrostrip lines made with different technologies.These data are calculated from measurements ofQ-values of ring resonators. The difference betweenthin film and thick film gold conductors is about20%. The significance of this difference will dependon the application. It is also seen that the material"Epsilam 10" has higher losses than both thinand thick films.

Figure 8 shows the frequency response of thinfilm and thick film versions of the 2 GHz poweramplifiers. While the thin film units are well abovethe specification limits, the thick film units give alow power output at the higher frequencies.

Figure 9 shows the noise figure of receiver mixersin the frequency range 13 to 15 GHz. The noise figureis approximately the same for both the thin and thickfilm circuits, except at the higher frequencies. This hasbeen shown to be due to mismatch, rather thanohmic losses in the thick film.

These examples indicate that it is not possible tostate in general terms for instance a maximumfrequency limit for using thick film technology.

Returning to the economical aspects of thisproblem, the result is totally dependent on theproduction volume. For a specific case, namely thecost of establishing a conductor pattern on the

in. x 2 in. ceramic substrates for the 2 GHz

amplifiers previously mentioned, the comparisoncomes out as follows, for a production volume of afew hundred substrates:

Using thin films, this cost is approximately $25per substrate, of which the metallized substrate itselfamounts to $18.NERA AS has still no printing facilities for thick

film circuits, but has them made at the CentralInstitute for Industrial Research in Oslo. The totalcost for a printed substrate is approximately $11,thus a cost reduction of more than 50% is obtainedcompared to thin films.

However, in order to maintain this cost reductionthrough the rest of the production process, one mustnot run into thick film related problems later onwith bonding, encapsulation or degraded electricalperformance. This is why the microwave propertiesof various thick film pastes are now under investiga-tion.

6. CONCLUSION

This report has tried to show that the use of hybridcircuit technology can be very advantageous inmicrowave radio links, even if the volue of productionis quite small. There are at least two reasons for thisconclusion. Firstly, regarding production costs, onedoes not compete with printed circuit boards, butwith relatively complex waveguide or coaxial circuits.Secondly, the development or prototype lab needed,will have a considerable capacity, which provedsufficient for our production volume.

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