Lumped Components for Microwave Frequencies

Philips tech. Rev. 32, 305-314, 1971, No. 9/10/11/12 305

Lumped components for microwave frequencies

C. S. Aitchison

Introduetion Microwave circuit functions have been performed in Lumped microwave circuit elements are a possible the past by combinations of individual components answer to this situation, but not necessarily the only built from transmission lines such as hollow waveguide one. Another possibilityis the use of microstrip technol- or coaxialline, and carefully manufactured with preci- ogy, which is dealt with extensively in other articles in sion tolerances. Such components are large compared this issue [11 [21. Both techniques use a substrate on to with thewavelength and are described as "distributed". which the conductors are evaporated; however it is They contrast with the alternative lumped components, important to differentiate between the two to avoid used at much lower frequencies where the component confusion. Microstrip is a distributed transmission-line dimensions are very small compared with the wave- medium in which a conductor is spaced from a ground length. plane by a materialof low microwave loss and high Traditionally the transition from the use of lumped dielectric constant so that most of the microwave to distributed components has taken place in the region energy is confined to the dielectric substrate. Thus the of 500 MHz to 1 GHz; but there is no reason why substrate has to have acceptable electrical and mechan- components for microwave frequencies, i.e. above ical properties. A typical microstrip substrate will be 1 GHz, should not be constructed in lumped form - some tens of square centimetres in area. other than the possible physical inconvenience of so In the lumped case the main function of the substrate doing. Recent developments in photo-etching and is to support the evaporated layers. The microwave vacuum-deposition techniques have made it possible energy within the substrate is small; thus the impor- to deposit lumped-circuit elements on a suitable back- tance ofthe substrate electrical and mechanical proper- ing surface (substrate) by evaporation, Both high- ties is considerably reduced. Typical substrate areas conductivity metals such as gold and copper and micro- are one square centimetre or less. wave dielectric materials such as silica can be de- Viewed in the light of these facts lumped microwave posited. circuits bear promise of an even smaller size and The advantages of fabricating microwave circuits in greater economy of production than is offered by lumped form are the extremely small dimensions of the microstrip circuits. This article describes a first survey circuits thus produced and the rather inexpensive of the field, consisting of the experimental assessment technology - even more so because microwave semi- of the properties of individuallumped components - conductor devices are mounted into the lumped circuits the five basic circuit elements are the inductor, the as unencapsulated chips. It is interesting to consider capacitor, the resistor, the transformer and the gyrator the cost of the existing sequence of making a semi- - and of simple circuits built up from these compo- conductor chip and combining it with traditional distri- nents with the addition of semiconductor devices. buted components. Normally the chip, costing a few The ultimate object of our work is to demonstrate the tenths of a pound sterling, is encapsulated in a special feasibility of manufacturing on one side of a glass (or microwave encapsulation which usually costs more than similarly cheap material) substrate a complete micro- the chip, and it is the encapsulated chip that forms the wave circuit with one or more microwave inputs, a output from a semiconductor factory and is purchased number of d.c. connections and an output at a con- by the customer. He constructs around it a three- venient intermediate frequency. Such a circuit should dimensional distributed circuit which normally costs be completely contained on not more than one square many times more than the encapsulated chip. Thus the centimetre of glass and typical examples might contain: typical microwave circuit is expensive - so expensive a mixer with one or two Schottky-diode chips, a tunable that its use can only be considered in professional ap- local oscillator with a Gunn device and a Schottky plications where the performance is more important diode, a preamplifier (parametric amplifier, tunnel than the price. diode or transistor amplifier), a number of d.c. con-

[1] J. H. C. van Heuven and A. G. van Nie, Microwave integrated circuits; this issue, page 292. e. s. Aitchison, B.Sc., M./IISt.P., A.R.e.S., is with Mullard [2] M. Lemke and W. Schilz, Microwave integrated circuits on Research Laboratories, Redhill, Surrey, England. a ferrite substrate; this issue, page 315. 306 C. S. AITCHISON Philips tech. Rev. 32, No. 9/10/1 i/12 nections isolated through low-pass filters, and an en- convenient for use with existing unencapsulated micro- capsulation for the complete circuit, together with one wave semiconductor chips since they combine with the or more appropriate microwave connectors and a num- intrinsic and stray capacitances and inductances of ber of d.c. connectors (this encapsulation is not required these chips to give resonances in the range of 4 to to have microwave properties). The successful con- 12 GHz, at least. struction of a circuit at the research laboratory should Lumped resistors are readily formed using evapor- enable the cost of quantity production to be estimated. ated nickel chromium, which is, in any case, used as a Preliminary factory estimates of a simple 9 GHz Dopp- seed layer for the evaporation of gold and copper ler radar including a circulator and Schottky detector conductors. suggest a factory selling price of about one fiftieth of Work on the lumped transformer is still in the the price with conventional components. experimental stage and will therefore not be described In the following sections the results obtained with here. Initial results are not discouraging. individual passive lumped elements, measured at fre- The characteristics of these lumped components are quencies up to 12 GHz will first be described. Secondly, most conveniently measured by combining them in some simple filter circuits and a few active circuits will either a series or parallel circuit resonant between 4 and be described: the mixer, tunable oscillator and pre- 12 GHz. Fig. 3a shows a series resonant circuit and fig. 3b amplifier mentioned above. Finally an account will be shows a parallel resonant circuit. The parallel circuit given of the Doppler radar. is particularly convenient for measurement purposes because it presents a high impedance at resonance and Passive lumped elements thus is less dependent on contact losses which may

Substrate and measurement technique The components made for the measurements are mounted on a quartz substrate, although the ultimate aim is to use a less expensive material. Quartz was used at the beginning of the investigations to avoid the complications that would arise with a less well known microwave material. In fig. I the arrangement is shown which has been adopted for examining the microwave characteristics of most of the passive lumped elements. On a quartz disc 9 mm in diameter and 0.5 mm thick, inner and outer gold connections are deposited so that the disc can be placed across the end of a coaxial line. This enables conventional microwave measurements to be made using standard coaxial measuring equipment. The element to be examined is evaporated within the area AA'B'B. Fig. I. A quartz disc, 9 mm in diameter and 0.5 mm thick, on Inductor, capacitor and resistor which the experimental lumped passive elements are evaporated (inside the rectangle AA' B' B) for measurement purposes. The A typical inductor is shown in fig. 2a. It consists of disc is provided with evaporated contact areas which enable it to be placed directly across a coaxial 50 (2 line for connection with one turn of evaporated metal. Both the diameter and the measuring equipment. track-width can be varied to give a range of inductance values from less than 1 to more than 3.5 nH. The lumped capacitor is formed by the fringing field between an interdigital gap as shown in fig. 2b. The capacitance values which are obtainable range from 0.01 pF to 1pP. Larger capacitance values can be obtained by means of a metaljsilicon-dioxidejmetal sand- wich. These are avoided where possible since they involve more processing stages than the simple interdigital capacitor. g_ The inductance and capacitance of elements pro- Fig.2. a) Single-turn lumped inductor. b) Lumped interdigital duced in this way have values that are particularly capacitor. The gap is 20 (J.111 wide. Philips tech. Rev. 32, No. 9/10/11(12 LUMPED MICROWAVE CIRCUITS 307 occur because the sample is mounted in the coaxial measuring jig. The parameters of these resonant circuits are deter- mined by measuring the reflections they cause when

Fig. 3. a) Series-resonant circuit. b) Parallel-resonant circuit. Fig. 4. Admittance plot of a lumped parallel-resonant circuit on a Smith chart. The Smith chart is essentially a polar plot of the reflection coefficient of the circuit, which is what is usually meas- connected to a transmission line of 50 Q characteristic ured; however, it is calibrated in such a way that the real and impedance. A microwave generator whose frequency is imaginary parts of the admittance can be read off for any of the frequencies of measurement. The admittance is normalized to swept from 4 to .12GHz feeds a signal into the line. The the characteristic admittance of the transmission line on to which reflection coefficient of the circuit under test is plotted the circuit has been connected, in this case 0.02 S [4]. On the horizontal line the imaginary part of the admittance is zero; above on the well known Smith chart [3J. Fig. 4 gives an the horizontal line it is positive, below the line it is negative. At example of a Smith-chart plot for a parallel resonant about 6.7 G Hz the measured curve intersects the horizontal line at a minimum admittance value, indicating that this is the fre- circuit having a Q (quality factor) of 80 and resonating quency of the parallel resonance. At this frequency the admit- at 6.7 GHz. The resonant frequency or frequencies can tance is about 0.0 I x 0.02 S; from this value and the capacitance and inductance, the Q (quality factor) can be calculated to be be directly read from the plot as they are the frequen- 80. Between 10 and II G Hz there is a series resonance with an cies at which the curve intersects the horizontal axis; admittance of more than 20 x 0.02 S. fig. 4 shows, besides the parallel resonance at 6.7 GHz, a spurious series resonance at about 10.5 G Hz. Addi- tionally, the inductance L or the capacitance C and the Q can be deduced from the plot. Q depends on the losses of the circuit; these losses have been found to be almost exclusively due to the inductor.

For a parallel-resonant circuit the admittance representation is the more convenient one; in this case the admittance is y = C + j(wC- l/wL) = C + jB. In this expression C is a parallel conductance which expresses the losses of the circuit. It can easily be shown that (dB/dw)wo = 2C; the value of dB/dw at the angular resonant frequency Wo can be found graphically from the Smith-chart plot and gives us the value of e. Since the resonant frequency is known L can now be calculated from wo2 = 1/Le. At resonance Y(wo) = C, whose value can also be directly read from the plot. Q is then found from Q = (C/L)tC. The losses of the circuit may be expressed by a series resistance R for the inductor. R is not equal to I/C but can be easily calculated from L, C and Q. In fig. 5 an example is given of a Smith-chart admittance plot of a parallel-resonant circuit in series with a lumped resistor designed to have a value of 50 Q. Fig. 5. Smith-chart admittance plot of a parallel-resonant circuit [3] P. H. Smith, Transmission line calculator, Electronics 12, with a lumped series resistor with a design value of 50 0; the Jan. 1939, 29-31; An improved transmission line calculator, admittance is again normalized to 0.02 S. The resonant frequency Electronics 17, Jan. 1944, 130 el seq. is 6.9 GHz; at high andlow frequencies the curve approaches the [4] In this article and elsewhere the siemens (S) has been adopted centre of the diagram, which means that the admittance ap- for mho or Q-1 in accordance with international recornmen- proaches a pure conductance of 0.02 S, the design value for the dations. ~ resistor. 308 C. S. AITCHISON Philips tech. Rev. 32, No. 9/10/11/12

To assess the losses associated with the lumped in- can be seen that the resistance achieved is approxi- ductors 20 parallel-resonant circuits were measured. mately twice as large as theory predicts; it is likely that The capacitance was nominally equal to 0.35 pF in this difference arises because the calculation is based each case and the track-width and outside diameter of on the normal bulk values of resistivity for the con- the single-turn inductors were varied. Assuming the ductor metals without taking into account the particu- losses in the resonant circuit to be exclusively due to lar structure of the evaporated and plated metal. 8Q'.------~ 8Q'r------~ • 0.6 mm 7 o 0.8 7 Il 1.2 6 6 R o o f : 1.2mml I • I Il • I 3 I 3 I I I 2 Il 0 • I • • Il I Il M Il o I Il I Il 0 I 1 Il o o /. '"" o~----~----~--~4 6 8 1IJGHz °0~--+1--~2~~3~~4nH Q -'a 12 -L 4n~'r_------_, 0'6pFi.------.

0.5 3 o 0.4 o c o 0 o Il go • • 0 Il Il • • • •o t 0.3 •• 0.2 1 0.1

qo~~~~*I~~~~I' ~~--~6~--~8----~WGHz 2 5 100 2 5 1000pm g_ -w -fa Fig. 6. Measurements on 20 lumped resonant circuits. The inductors of the circuits are divided into three classes with outside diameters of 0.6 mm (dots), 0.8 mm (circles) and 1.2 mm (tri- angles). a) Series resistance R of the inductor for circuits of various resonant frequencies fo. b) Series resistance R for inductors ofvarious inductances L. Computed curves are included for comparison. R turns out on average to have twice the computed value; the large experimental values may be due to the fact that the conductors are evaporated and plated. c) The inductance L for different track-widths w. d) The measured capacitance C for circuits of different resonant frequency fo. All the capacitors were nominally identical and the design value was 0.35 pF. A large part ofthe scatter in the measured values is accounted for by the measurement error which is estimated at ± 25 %, so that the reproducibility of the capacitors appears to be fairly good, the inductor, the inductor resistance could be deduced The results demonstrate that lumped-element paral- from the Q-factor of the circuits. Results are presented lel-resonant circuits with single-turn inductors can be in figs. 6a and b; in fig. 6a the series resistances that fabricated for operation from 5 to 10 GHz with resist- were measured at different resonant frequencies are ance values of about one ohm or less, corresponding plotted; three classes of inductors with outside diam- to Q values of between 10 and 90. To understand the eters of 0.6, 0.8 and 1.2 mm were involved. In fig. 6b significanee of the resistance values measured for the the resistance is plotted as a function of the inductance inductors it should be kept in mind that for applications and theoretical curves are included for comparison. It where a low noise figure is required the loss in the in- Philips tech. Rev. 32, No. 9/10/11/12 LUMPED MICROWAVE CIRCUITS 309 ductor should be small compared with the loss in the Filters semiconductor. This condition is normally met with When lumped-circuit elements and semiconductor lumped components. The sarne applies for conditions chips are combined to form an active circuit it is nec- in which power handling is of importance. essary to supply and remove d.c. or low-frequency a.c. The measurements on the twenty resonant circuits energy without disturbing the microwave energy. This provide us with more data. In fig. 6c the measured de- pendence of the inductance on the inductor track- width w is shown; from fig. 6d an impression can be gained of the reproducibility of the interdigital capacitors. The scatter of the measured capacitance values is not much larger than the scatter accounted for by the limited measuring accuracy. In conclusion, capacitors, inductors and resistors have been made which exhibit lumped-element characteristics up to 12 GHz (this is not the limit ofthe technique). The loss associated with the lumped reactances has been adequately small and is compatible with their use with semiconductor chips.

Fig.7. Lumped circulator consisting of a symmetrical pattern of Gyrator conductors on ferrite. Outside the circle the conductors are 50 r2 microstrip lines; inside the circle the ground plane has been re- The gyrator, which is the fifth basic circuit element, moved. Bonded wires bridge the gaps for d.c. interconnection. appears in practical circuit form as the circulator and the isolator [21. At microwave frequencies junctions of ------1dB transmission lines, including ferrite material, form circulators and these are distributed structures. At frequencies below 3 G Hz lumped circulators have been made by using a block of ferrite and winding a coupled inductances on this with a geometrical spacing of 120°. At microwave frequencies this design of lumped circulator is not attractive; a corresponding structure with planar conductors has been designed and is shown in fig. 7. It consists of a symmetrical pattern of intersect- 4 6 8GHz ing conductors on ferrite; the gaps at the intersections -f

are bridged by bonded wires to remove the d.c. isola- Fig.8. Pen recordings of insertion loss a1~2 and isolation a1->3 of tion. This pattern terminates three 50 Q microstrip lines the symmetrical lumped circulator at frequenciesfbetween 4 and 8 G Hz. The optimum performance is a1->2 = 1.2 dB, a1->3 = 18 and is contained within a circle of 0.8 m111diameter. dB; it is available over the band 5 to 7 GHz by variation of the The performance of this circuit is shown infig. 8 as a applied magnetic field. function of frequency showing that an isolation, a1--->3, of 18 dB is obtained with an insertion loss, al --->2, of less than 1.2 dB. The performance is available over the band 5 to 7 GHz by variation of the applied magnetic field. A simpler unsymrnetrical pattern is shown in fig. 9 where it can be seen that two conductors pass from one port to the other two ports, giving a single intersection. The performance is similar to that obtained with the symmetrical structure. The details of the operation of these circulator con- figurations are being studied. Later models of the symmetrical and unsymmetrical circulators have been made without the d.c. isolation so that there was direct Fig.9. Unsymrnetrical lumped circulator, also for a frequency contact at the junctions. No significant change III band centred on 6 GHz. It shows an insertion loss < 1 dB, an microwave behaviour was observed in either case. isolation> 20 dB and a bandwidth of about 2 %. 310 C. S. AITCHISON Philips tech. Rev.. 32, No. 9/10/11/12 is conveniently done by means of simple low-pass filters OdBr---"Iiiië':::::::::::------, with lumped inductors and capacitors for the reactive components. A simple three-element filter, as indicated in the inset oîfig, la, has been constructed with a series inductance of 1.9 nH and a shunt capacitor of 1.0 pF. The circuit 10 consists oftwo single-turn inductors and an interdigital capacitor. With a 50 Q termination this should give a a 3 dB cut-off frequency of 5.5 GHz and 20 dB isolation 1.9nH 1.9nH at 10.5 GHz. Fig. 10 shows reasonable agreement be- ~ tween the computed and measured values. P 20 F The circuit diagram of a simple band-stop filter is r1.O o 0 shown in the inset of fig. 11; it consists of a cascaded combination of a series-resonant circuit in shunt and a shunt-resonant circuit in series. The 2 Q series resistances of the inductors result from the loss in these elements. The insertion-loss performance is shown in fig. 11 together with the computed value with the -f Fig. 10. Insertion loss a plotted against frequency f for the system terminated in 50 Q. An adequately high value lumped low-pass filter shown in the inset. The dashed curve gives of insertion loss (30 dB) is obtained at 9 GHz. the computed frequency response. OdB~=_~__~__------,-_ -

/ / / 10 \ / J \ / a \ / 1.6nH 2.0 \ / \ / ;c?T \ I O.2pF 20 \ I \ I \ I \ I \ I 0 0 \ 30 , I \ I

2 4 6 8 10 12GHz f

Fig. lt. Insertion loss of the lumped 9 GHz band-stop filter shown in the inset. The dashed curve gives the computed frequency response.

Active circuits In fig. 12 a circuit diagram is shown of a tunnel- diode amplifier connected to a microwave system by Tunnel-diode amplifier means of a circulator. There are three basic circuit The first active circuit which was constructed with blocks that make up the tunnel-diode amplifier. Block 1 lumped elements and a semiconductor chip was a is a transforming network which transforms the stand- 4 GHz tunnel-diode amplifier. This circuit was chosen ard impedance of the transmission line (usually 50 Q) because of its simplicity. to the value required to give the desired gain. Block 2 Philips tech. Rev. 32, No. 9/10/11/12 LUMPED MICROWAVE CIRCUITS 311

1------1 I I ~~~~~_rl I I I I I I I I I

I I I I I : L _j L __ J L _j 1 2 3 Fig.12. Circuit diagram of lumped tunnel-diode amplifier connected to a circulator. I transforming network matching the amplifier to the line impedance. 2 tunnel diode with resonating inductor. 3 stabilizing network. is the tunnel-diode chip resonated by an inductor, and block 3 is a stabilizing network which removes the negative conductance of the tunnel diode at all frequencies except in the band over which gain is required. The stabilizing network consists of a resistor of appropriate value in series with a parallel-resonant circuit Fig. 13. Lumped tunnel-diode amplifier on standard 9 mm disc. which resonates at the operating freq uency, th us remov- I transforming network. 2 area for mounting tunnel diode and bonding wire serving as an inductor. 3 stabilizing network with ing the loss from the tunnel-diode circuit at this fre- nickel-chromium resistor. quency (the Smith-chart plot of fig. 5 applies to a stabilizing network of this type). Fig. 13 is a drawing of the amplifier in the coaxial configuration; a photograph is shown infig. 14. For measurements of the gain-frequency response the amplifier was connected to a broad-band circulator. Fig. 15 is a pen recording showing the gain variation over the band 2.6 to 4.2 G Hz with 3, 9 and 12 dB calibration lines. These are recorded on the chart by intro- ducing a known fixed attenuation. It can be seen that the response is double-humped; this effect is produced by the impedance variation ofthe wide-band circulator. The peak gain is approximately 12 dB and 9 dB of gain is obtained over almost all of the band from 2.8 to 4.1 GHz. The measured noise figure is 6.2 dB. Fig. 14. Lumped tunnel-diode amplifier. In this example the transforming network has only one capacitor. It is concluded that the technique of lumped elements and unencapsulated chips is suitable for the construction of practical microwave tunnel-diode amplifiers.

Varactor-tuned Gunn oscillator G The frequency of a Gunu-effect oscillator r5] can be varied by connecting a varactor (a voltage-dependent r capacitor) in series with the Gunn device and varying the capacitance of the varactor by means of the bias voltage applied to it. This provides a simple circuit for a tunable oscillator; an equivalent-circuit diagram incorporating the strays is shown in fig. 16. Calculation enables the parameters of the varactor to be specified 25 -f [5] G. A. Acket, R. Tijburg and P. J. de Waard, The Gunn effect; this issue, page 370. Fig. IS. Pen recording showing the gain G of the tunnel-diode J. Magarshack, Gunu-effect oscillators and amplifiers; this amplifier as a function of frequency f The dashed curves are issue, page 397. calibration curves recorded for the measuring arrangement. 312 c. S. AITCHISON Philips tech. Rev. 32, No. 9/10/11/12

2 21mm

-G 6mm

Fig.16. Equivalent-circuit diagram ofvaractor-tuned Gunn oscillator, incorporating stray components. J varactor. 2 Gunn device. -G is the negative conductance of the Gunn device.

for this application. ln practice, for frequencies about Fig.17. Varactor-tuned Gunn oscillator with lumped low-pass filters F for connection to d.c. bias. V varactor. C overlay capaci- 9 G Hz, existing varactors such as the Mullard CXY JO tor providing d.c. isolation. G Gunn device mounted on heat- and other varactors with a similar high figure of merit sink tab. are suitable provided they can be mounted in chip form. Means must be provided for supplying a d.c. bias to both the Gunn device and the varactor chip. The lumped-element low-pass filters previously described are inserted in the bias leads; they enable us to realize the whole tunable oscillator circuit within very small dimensions. Fig. i7 shows the actual component dis- position; the Gunn chip G is mounted on a heat-sink tab and a series overlay capacitor C is provided for d.c. bias isolation. For the purpose of measurements this circuit was connected to a microstrip circulator used as a microwave isolator by loading one of the arms with 50 Q. A photograph of the arrangement is shown in fig. 18. This showsclearly the two low-pass filters as well as the- Gunn and varactor chips. The circuit arrangement here is slightly different from fig. 17 so that the overlay capacitor can be omitted. The tuning range obtained Fig. 18. Varactor-tuned Gunn oscillator for frequencies of about with this arrangement is 1.1 G Hz together with a maxi- 9 GHz. A lumped oscillator circuit and a microstrip circulator mum power output of 20 mW. Fig. 19 shows the varia- are integrated on the same substrate. The overlay capacitor shown in fig. 17 can be omitted here. tion of frequency and power with varactor bias. The conclusion is that the lumped-element technique yields an extremely compact Gunn-oscillator circuit of satisfactory performance. 11GHz 30mW

f P Degenerate parametrie amplifier -f ~ Another simple circuit that has been fabricated in i 10 20 i lumped form is a degenerate parametrie amplifier. This I consists simply of a series-tuned circuit formed from a varactor and a resonating inductance. Fig. 20 shows the J i-- p- 9 -. 1o experi mental arrangement which was used: a varactor C and a three-turn lumped inductance L mouoted on the standard 9 mm diameter disc. The degenerate parametrie amplifier requires a o o source of "pump" power at a frequency which is twice +5 o -5 -10 -15 -20 -25 -30V the signal-circuit resonant frequency. The pump volt- -Vvor age periodically changes the capacitance of the varactor Fig. 19. The variation of frequency f and output power P of the by which meaos amplification is obtained [6] [7]. The varactor-tuned Gunn oscillator with varactor bias Vvar. Philips tech. Rev. 32, No. 9/10/11/12 LUMPED MICROWAVE CIRCUITS 313

ofthe transmitter signal in a mixer circuit. If the objects move either towards or from the aerial the frequency of the reflected wave is shifted towards higher or lower values respectively. This is known as the Doppler effect. In this case a difference frequency proportional to the velocity of the moving object in the direction of the aerial appears in the mixer circuit. Doppler radar is thus very suitable for the detection of moving objects and for the measurement of their speed; a common application is in the measurement of the speed of mo- tor vehicles [8J. Our Doppler-radar circuit consists of three parts (fig. 21): a Gunn oscillator G, a mixer stage with a

Fig.20. Experimental conAguration for lumped-element degenerate parametrie amplifier constructed for standard 9 mm test mount. C varactor. L three-turn coil. circuit is said to be degenerate because it is a special and simplified case of the general parametrie amplifier which includes an "idling" circuit tuned to the difference frequency of pump and signal source. In the degenerate case this difference frequency coincides with the signal frequency and a separate idling circuit is not required. The gain of the lumped parametrie amplifier was lower than planned; the cause of this was found to lie with the type of varactor used. Nevertheless, the lumped-element technique appeared to be a satisfactory way of making a parametrie amplifier.

Our parametrie amplifier was equipped with a Schottky-barrier varactor. After it was mounted in the circuit this varactor turned out to have only a moderate figure of merit (3 to lOG Hz, which is low compared with 40 GHz for the CXY 10 mentioned above). Fig.21. Lay-out of Doppler-radar circuit. G Gunn device. C circulator. D Schottky-barrier diode of mixer stage. The right-hand As a result of this the gain was lower than planned; 8.0 dB of gain port of the circulator is connected to the aerial. was obtained at 3.1 GHz with a bandwidth of 210 MHz. The measured amplifier noise température [7] was approximately 200 K. A conclusion resulting from this experiment is that the figure of merit obtainable from Schottky barriers is low and it would therefore be sensible to use a mesa diode which is bonded to the circuit and etched to the required value on the circuit.

Doppler radar A Doppler-radar circuit operating in the band 8 to 11 GHz was the first attempt to make a complete sub- system. It was chosen because of its simplicity and also because of its market potential. A Doppler radar transmits a beam of continuous- wave energy. Energy reflected by objects in the path of the beam is received by the aerial and mixed with part

[6] B. Bollée and G. de Vries, Experiments in the field of parametric arnplification, Philips tech. Rev. 21, 47-51,1959/60. [7] C. S. Aitchison, Low noise parametrie amplifiers, Philips tech. Rev. 28, 204-210, 1967. [S] K. L. Fuller and A. J. Lambell, Traffic-flow analysis by radar, Philips tech. Rev. 31, 17-22, 1970. Fig. 22. Doppler-radar circuit. 314 LUMPED MICROWAVE CIRCUITS Philips tech. Rev. 32, No. 9/10/11/12

Schottky-barrier diode D, and a circulator C. The first been optimized for maximum sensitivity of the mixer two are constructed from lumped elements; the third which receives about 100 IJ.W of local oscillator power; was made in microstrip, even though lumped-element the transmitted power is typically 10mW at 10 GHz. circulators were available. The decision to proceed in The system is sufficiently sensitive to detect the return this way resulted from the wide bandwidth available signal from a walking man at 30 metres. A photograph from microstrip circulators and the advantage in the of the Doppler-radar circuit is shown infig. 22. initial research work of using such a wide-band cir- The satisfactory performance of this complete sub- culator. system confirms what was concluded from the previous The Gunn oscillator at this frequency band has al- examples. It not only turns out to be feasible to make ready been described. This is incorporated directly lumped capacitors, inductors and resistors for fre- though the varactor tuning facility has been removed quencies up to 12 GHz, but the technique of com- and the d.c. filter is placed on a different port of the bining these lumped circuit elements with semi- circulator. conductor chips also appears to be a viable method of The Schottky-barrier mixer circuit was specially de- making active microwave circuits. signed for this application and is matched to the circulator by a reactive network consisting of a capacitor and an inductor which is connected in series with the Summary. In electrical circuits the use of lumped components, small compared with the wavelength, has traditionally been con- Schottky diode. In fig. 21 the bonding wire connecting fined to frequencies below 1 GHz. Nowadays planar techniques the Schottky diode D to the circuit serves as an inductor. are available that enable lumped components to be made for frequencies up to at least 12 GHz. These components, and the Inductor and diode form a shunt branch; following circuits built from them, are considerably smaller and cheaper this shunt branch is a low-pass filter consisting of a than the microwave components in current use, which are built from hollow waveguide or coaxial line, and smaller even than capacitor-inductor T-network. circuits made in microstrip. Active circuits can be made by A small amount of the output power from the Gunn mounting microwave semiconductor diodes in the lumped circuits as unencapsulated chips. Individual components and both device is required as a local oscillator signal for the passive and active circuits have been fabricated and give satis- mixer; this is supplied through the circulator in the factory performance; among the examples described are low- pass and band-stop filters, a tunnel-diode amplifier, a tunable isolation direction. The attenuation thus obtained has Gunn oscillator and a Doppler-radar circuit.