Mixers

1. D. Robertson'

Singlechip transmitters and receivers are the most desirable MMIC products, preferably packaged with only four pins (input-output-ground-DC) and costing less than €l! The major applications are mobile communications, wireless LANs, satellite TVROs, and wireless local loop outdoor units. Frequency translation is the fundamental function of these MMIC transmitters and receivers: i.e. the function of converting baseband signals to and from the microwave carrier. Other functions, such as amplification and filtering have to be introduced to mask the mixer's shortcomings.

As an example, the DBS receiver chip is a very basic downconverter: the LNA is usually off-chip using discrete HEMTs and the oscillator circuit is partly off-chip. The mixer is the central part of the chip's functionality and any reduction in its layout area is of great commercial benefit. However, traditional microwave mixer technology is unfortunately not well suited to MMICs. The ubiquitous double-balanced diode ring mixer is illustrated in Fig. 1. The baluns are typically realised using wire-wound transformers or using suspended-stripline baluns 111. These conventional baluns can be difficult to implement monolithically, and so a wide range of new techniques has been developed for the MMIC mixer. The transmission-line circuits, couplers, and fdters used in conventional microstrip mixers are at best wasteful of space and therefore expensive, and at worst they are non-planar and hence not realisable. Only at frequencies above about 10 GHz can normal microstrip couplers be used readily, and even at these frequencies they are a major obstacle to keeping integration levels high. One approach is to use folded and spiral couplers instead, but ultimately stray coupling limits how compact the couplers can be made. Another approach is to replace them with active circuits; FET and HEMT mixers have been extensively investigated, and HBT mixers are starting to receive more attention.

2. Active Mixers

Diodes, MESFETs, HEMTs, BJTs or HBTs can all be used to realise a MMIC mixer. The choice of device is largely determined by the requirements of other circuit functions such as amplifiers. Diodes on MMICs are most often created using some derivative of the transistor structure: For example, a Schottky diode can be created from a FET by connecting the source and drain fingers together. Transistor-based mixers have the advantage of compatibility with amplifiers, oscillators, etc., and may offer higher performance than these suboptimal diodes. Dual-gate FETs provide an attractive solution for MMIC mixers because the RF and LO can be input to separate gates, and so the number of passive components can be reduced 12-51. This approach is commonly used for DBS receivers and excellent results have been obtained. In the low microwave frequency range active mixer designs based on the Gilbert multiplier have been implemented monolithically by several companies, and operation up to over 4 GHz has been reported 16-91. Fig. 2 shows an actively-matched double-balanced FEiT mixer circuit.

* Department of Electronic & Electrical Engineering King's College, Strand, London WC2R 2LS 0 I. D. Robertson

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3. Lumr>ed-Element DesigaS

In the 2 to 18 GHz frequency range, the all-transistor approach runs out of steam but couplers are still too large. Here, lumped-element splitters and combiners can be used [lo- 131. Fig. 3 shows the circuit diagram of a lumped-element 90 degree phase splitter, for example.This could replace the Lange or branch-line coupler used in image-rejection mixers. Thomson [I4] have reported a 6.5 to 9 CHz monolithic balanced mixer using a 180 degree hybrid made up of lumped elements. TRW [15] have designed a mple- balanced monolithic X-band mixer using this approach, and have fitted it into 2.5x2.7 mm. Lumped spiral transformers, consisting of inter-wound spiral inductors, can be used in balanced diode mixers in place of conventional bi-filar wound transformers. They can perform bias injection, matching, DC blocking and phase-shifting all at once. This technique can result in very high packing density, with even the space in the centre being used by a transistor 1161. The frequencies that this technique can be used at are limited by the size and DC resistance at low frequency, and the inter-spiral capacitances at higher frequencies.

4. w i v e @litters -Combiners

Instead of using lumped element balms, the phase inversion of the FET can be used for balancing, and Fig. 4 shows an active balun design which uses a pair of FETs, one in common-source and one in common-gate configuration [*‘I. In-phase splitters and combiners are rather straight€orwarcf and use pairs of transistors with common inputs or outputs, respectively. Active splitters and combiners have useful advantages such as reverse isolation, gain, and high isolation between inputs or outputs.

5. Millimetre-wave Mixers

At frequencies above around 20 GHz, the same balanced mixer designs can be used as for hybrid mixers, because the couplers become relatively small. Thus mixers can be realised easily at frequencies like 30 GHz [1q, or even 94 GHz 191, using diodes and couplers in conventional circuits. Fig. 5 shows a typical mm-wave balanced MMIC mixer layout. The advantages of using MMICs are potential low cost and easy manufacture, and also the device parasitics are reduced and accusately predictable, The disadvantage is that state-of-the-art noise figures are not usually achievable.

Hybrid HEMT mixers have already been reported at frequencies as high as 94 GHz [20]. For mm-wave ICs, coplanar waveguide has many advantages over microstrip. Monolithic CPW mixers have already been reported [21]. In this latter paper, the HEMT structure is actually used as a diode, which keeps the mixer simple whilst enabling it to be integrated with a HEMT LNA. Another way of employing MESFETs and HEMTs is as switches [Z 231.

6. Distributed Mixerg

The distributed MESFET mixer was first demonstrated by Tang and Aitchison [*41 in hybrid form, using single-gate FETs with a passive coupler to combine the RF and LO signals. The principle is similar to the distributed amplifier with the gate and drain capacitances incorporated into artificial transmission-lines, so that multi-octave bandwidth is achieved. Dual-gate FETs can also be used so that the coupler is not needed w-1: The mixer then has a gate-line for the RF, a gate-line for the LO and a drain-line for

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the IF, as shown in Fig. 6. Because this has some in-built L O W isolation, it is very suitable for MMIC'implementation 1261. Alternatively, the three-terminals of a normal FET can be used for each signal separately, and techniques such as the distributed source mixer can be realised as MMICs.

By having a distributed drain-line, very high IFS are in principle possible, although a way of obtaining LO and RF to IF isolation is needed. A 1 to 18 GHz active balun has been demonstrated in MMIC form [271. This uses the common-source/common- gate technique in distributed form, and excellent amplitude and phase balance is achieved. Double-balanced MMIC distributed mixers are possible using this type of circuit.

Peferences

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S. Maas, 'Microwave Mixers', Artech House, 1986

S. C. Cripps, 0. Nielsen, D. Parker and J. A. Turner, 'An experimental evaluation of X-band mixers using dual-gate GaAs MESFETs', The 7th European Microwave Conference Proceedings, 1977, p.101

C. Tsironis, R. Stahlmann and R. Meierer, 'Dual-gate MESFET Mixers', IEEE Trans. M'IT-32, no.3, March 1984, p.248.

C. Kermarrec. et al.. The First GaAs Fullv Intemated Microwave Receiver for DBS applications at 12GHz', hoc. 14th EGopean"Microwave Conference, Liege, 1984.

T. Sugiura, K. Honjo, and T. Tsuji, '12 GHz band GaAs dual-gate MESFET monolithic mixers', IEEE Trans. MlT-33, no.2, February 1985, pp.105-110.

M. Shigaki, et al., 'Onechip GaAs monolithic frequency converter operable to 4 GHz', IEEE Trans. M1T-36, no.4, April 1988, pp.653-658.

R. Van Tuyl, 'A Monolithic GaAs IC for Heterodyne Generation of RF Signals', IEEE Trans., ED-28 February 1981, pp.166-170.

M. West, et al., 'Monolithic GaAs blanking upconverter', 19th European Microwave Conference Proceedings, September 1989, pp.725-730.

K. Kanazawa, et al., 'A GaAs Double-Balanced Dual-Gate FET Mixer IC for UHF Receiver Front-end Applications', IEEE MlT International Microwave Symposium Digest, 1985, pp.60-62.

GUPTA, R. K., and GETSINGER, W. J.: 'Quasi-lumped-element 3- and 4-port networks for MIC and MMIC applications,' IEEE Int. Microwave Symp. Dig., 1984, pp. 409-41 1.

VOGEL, R. W.: 'Analysis and design of lumped- and lumped-distributed-element directional couplers for MIC and MMIC applications,' IEEE Trans. Microwave Theory Tech., MlT-40, pp. 253-262, Feb. 1992.

HIROTA, T., MINAKAWA, A., and MURAGUCHI, M.: 'Reduced-size branch-line and rat-race hybrids for uniplanar MMICs,' IEEE Trans. Microwave Theory Tech., M'IT-38, pp. 270-275, Mar. 1990.

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13. G. B. Beech, C. W. Suckling, and J. R. Suffolk, 'An S-band Image Rejection Receiver Front-end Incorporating GaAs MMICs', hoc. European Microwave Conference, 1985, pp.1019-1024.

14. I. Telliez, P. Chaumas, C. Rumelhard, and G. Pataut, 'GaAs Monolithic balanced mixer for C-band direct demodulation receiver', 19th European Microwave Conference Proceedings, September 1989, pp.731-736.

15. T. N. Ton, et al., 'An X-band monolithic double double-balanced mixer for high dynamic receiver application', IEEE M"T-Symposium, May 1990, pp. 197-200.

16. D. Ferguson, et al., Transformer Coupled High-Density Circuit Technique for MMICs', BEE Microwave and Millimetre-wave Monolithic Circuits Symposium Digest, 1984, pp.34-36.

17. L. C. Liu, et al., 'A 30GHz Monolithic Receiver', IEEE Trans., MTT-34, December 1986, pp.1548-1552.

18. P. Bauhahn, et ai., 'A 94GHz Planar GaAs Monolithic Balanced Mixer', E E E Microwave and Millheter-wave Monolithic Circuits Symp. Digest, 1984, pp.70 - 73,

19. B. Adelseck, et al., 'A monolithic 94 GHz balanced mixer', IEEE MTT- Symposium, May 1990, pp.193-1%.

20. P. D. Chow, et al., 'Design and Performance of a 94GHz HEMT Mixer', IEEE M l T International Symposium, June 1989.

21. T. H. Chen, et al., 'A Q-band monolithic balanced diode mixer using AlGaAs/GaAs HEMT and CPW hybrid, IEEE l"T-Symposium, May 1990, pp.895-898.

22. S. Weiner, D. Ned, and S. Spohrer, 2 to 8 GHz double balanced MESFET mixer with +30 dBm input third order intercept', IEEE M1T-Symposium, 1988, pp.1097- 1100.

23. S . J. Nightingale, ''Mixers", Chapter 6 of MMIG Design, ed. I. D. Robertson, pub, IEE, 1995.

24. 0. S. A. Tang and C. S. Aitchison, 'A Practical Microwave Travelling Wave MESFET Gate Mixer', IEEE M?T Int. Microwave Symp. Dig., 1985 , pp. 605-608.

25. T. S. Howard and A. M. Pavio, 'A Distributed 1-12GHz Dual-Gate FET Mixer', IEEE MTT International Microwave Symposium Digest, 1986, pp.329-332.

26. T. S. Howard and A. M. Pavio, "A Dud-gate 2-18 GHz Monolithic FET distributed mixer', IEEE 1987 Microwave and Millimeter-wave Monolithic Circuits Symposium Digest, pp.27-30.

27. PAVIO, A. M., et al.: 'Double balanced mixers using active and passive techniques', IEEE Trans. Microwave Theory Tech., vol. MTT-36, Dec. 1988, pp. 1948-1957

Fv. 1. Double-bolnnced diode mixer

Fig. 2. Double-balanced dual-gate FET mixer

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Fig. 3. Lumped element 90 degmtplitlu

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Fii 4. Active balm

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Fig. 5. Typical mm-wave diode mixer

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Fig. 6. Dual-@& FET distributed d X W

i Fig. 6. Dual-@& FET distributed d X W