2 . 5 . 4 . 1 M ix ers a n d F req u en cy D o u b lers.
The design o f a Microwave FET mixer [14] requires the consideration o f a number o f design requirements. These include isolation between local oscillator (LO), RF port (RF) and IF ports (IF). The isolation o f RF and LO signals is normally achieved by using isolating hybrids. To minimise noise, it is also necessary to prevent the propagation and amplification o f unwanted frequency components through the use o f frequency dependant short circuits, these include the amplification o f IF noise at the input o f the mixer, harmonics o f the local oscillator down converting additional noise. The short circuiting o f unwanted frequencies also can help to reduce the risk o f instability. Filters at the output help to improve LO IF isolation. The use o f various balanced configurations helps to suppress LO breakthrough to the output
There are about three main ways o f using FETs as a mixer. These include using the LO to make the transconductance o f the saturated FET time varying. The bias point and LO power are chosen to maximise the fundamental frequency o f the transconductance. Another approach is to use the FET in the triode region where it behaves as a voltage dependant resistor. This region allows the generation o f a time varying linear resistance, and this configuration tends to have better intermodulation performance than in the saturated region [15]. An extreme strategy is to use the FET in the resistive mode as a switching element, and this approach is associated with the lowest possible noise figure. The resistive mode does not give conversion gain. Variations on the saturated FET mixer include dual gate and distributed designs [16]. W e illustrate the three FET mixing strategies in Fig 2.5.6. The frequency doubler can be thought o f as a special case o f the mixer and has many o f the design difficulties o f a mixer.
IF out RF/LO in—r- IF out LO i n - i - RF in BPF RF BPF S/C BPF S/C RF/LO S/C BIAS TEE BIAS TEE RF/LO MATCH LO BPF & MATCH + --- RF in LO in
y:
5<
IF outFig 2.5.6 FE T M ix er C on fig u ra tio n s, (a) Act ive (Gate) M ix er, (b) Re si s ti ve
(D rain) M ix er, (c) Swi tching M ixer (the th ick lin e FE T s show th e
co m m u tatin g n a tu re o f th e circuit) A lso k n o w n as th e d o u b ly balan ced resistiv e FE T m ix er. (S/C short circu it, B P F h a n d p ass filte r).
2 . 5 . 5 S u m m a rv
In this section w e have reviewed the main RF circuit design techniques used in Microwave circuit design, including narrow and wideband matching and hybrids, linear and nonhnear circuits.
2 . 5 . 6 R e f e r e n c e s
1 C.Bowick, RF Circuit Design, Macmillan Inc, 1st Ed 8th print 1990.
2 T.Edwards, Foundations for Microstrip Circuit Design, Wiley, 2nd Ed 1992, pp. 316-325
4 S j\.M aas, Microwave Mixers, Artech house 2nd Ed 1993, pp.240-244.
5 Fair-Rite Soft Ferrites - Data Book, Distributed by Cirkit Distribution
6 T.Tokumitsu, S Hara, T.Takenaka, M.Aikawa, Divider and Combiner Line-Unified FETs as Basic Circuit Function Modules - Part I, IEEE MTT Trans, Vol. 38, No. 9, Sept 1990, pp. 1210-1217
7 T.Tokumitsu, S.Kara, T.Takenaka, M.Aikawa, Divider and Combiner Line-Unified FETs as Basic Circuit Function Modules - Part II, IEEE MTT Trans, Vol. 38, No. 9, Sept 1990, pp. 1218-1226 8 S.Hara, T.Tokumitsu, M.Aikawa, Novel Unilateral Circuits for MMIC Circulators, IEEE MTT Trans,
Vol. 38, No. 10, Oct 1990, pp. 1399-1406.
9 S.Hara, T.Tokumitsu, Very Small Control Modules with Line Unified FET Configuration for Array Processing, IEEE MTT Trans, Vol. 39, No. 10, Jan 1991, pp.l 17-123.
10 T.Takenaka, H.Ogawa, An Ultra-Wideband MMIC Balanced Frequency Doubler Using Line-Unified HEMTs, MTT Vol. 40, No. 10, October 1992, pp. 1935-1940
11 N.Sclater, Gallium Arsenide IC Technology, Principles and Practice, TAB Professional and Reference Books 1st Ed. 1st Printing 1988, pp. 130-131.
12 R.Lehmann, D.Heston, X-Band Monolithic Series Feedback LNA, IEEE MTT Trans. Vol. 33, NO. 12, Dec 1985, ppl5660-1566
13 J Walker, High-Power GaAs FET Amplifiers, Artech House 1993. 14 S.A.Maas, Microwave Mixers, Artech house 2nd Ed 1993, Chapter 9.
15 S.A.Maas, A GaAs MESFET Mixer with Very Low Intermodulation, IEEE Trans MTT, Vol. 35, 1987 p. 425ff.
16 O.Tang, C.Aitchison, A Very Wideband Microwave Mixer Using the Distributed Mixing Principle, IEEE Trans MTT Vol. 32, 1985, pl470ff.
2 . 6 C o m m o n P ractical L in e a r isa tio n S tra teg ie s
2 . 6 . 1 In tr o d u ctio n
All active devices have significant non-linearity associated with their characteristics. This gives rise to non-linearity when they are used in any circuit. Numerous methods o f circuit linearisation exist, the choice o f a given technique depends largely on the physical environment the system is to be included, together with the type o f signals that are to be handled. For example, in a satellite system or a mobile communications system, volume, weight and power supply requirements are critical. For a terrestrial transmitter, spectral purity may be the most important criterion (e.g. a M obile Radio Base Station).
In section we review several linearisation strategies including: Output Back-Off, Feedforward, Feedback, Cartesian Loop Feedback, Predistortion, Balanced Structures, Filtering, Bias Dependence, addition o f linear components and customised low distortion devices.
2 . 6 . 2 O u tp u t B ack O ff
The simplest technique for reducing distortion is to reduce the input drive level [1]. Or to express it in more practical terms, a more powerful transmitter than that which is needed is purchased and it is operated at a power level significantly lower than that which it is rated
(circa 6-8dB). More powerful amplifiers are generally more expensive, bulkier and
heavier, and uses significantly more DC power (which has to be dissipated) for the same RF output level, making this technique somewhat unattractive.
2 . 6 . 3 F eed F o rw a rd
The technique o f Feed Forward Linearisation was proposed around 1970 by Seidel o f Bell Labs [2,3]. Fig 2.6.1a shows the system architecture for a feed forward amplifier for low distortion [1,4,5,6,7]. The amplifier o f Stage 1 distorts the input signal. An attenuated copy o f the distorted signal is combined out o f phase with original input, through an RF delay t and combiner. This subtraction results in an error signal containing only the
Stage 1
Stage 2
Distorted Signal
V(ol
Error Signal
F ig 2.6.1 (a) F eed F o rw a rd S ystem (b) N eg a tiv e F ee d b a c k S y stem
distortion products. This error signal is amplified to the same level as that in the output o f the main amplifier and fed in anti-phase to the delayed distorted signal through another delay line and combiner. The result is the cancellation o f the distorted products. Since the distortion products are generally quite small (-30dBc), the power dissipation in the error amplifier is small for Class B amplifiers. (For a class A amplifier, the use o f a smaller amplifier is feasible but may cause matching problems).
In practice, there are two main problems, matching the two signal paths in phase and amplitude. A mismatch in phase, gain or attenuation can cause incomplete cancellation o f distortion. In practice the delay lines will be transmission lines, and hence give a Ifequency dependant delay, restricting this technique to relatively narrow band systems. It is also difficult to match the gains o f the amplifiers (a variable attenuator could be used). These problem are accentuated by temperature dependence o f gain and attenuation and phase, especially if the system is non-integrated. In order to reduce the sensitivity to phase and amplitude mismatch, nested Feedforward systems can be used [6]. Feedforward can also be used with other distortion reduction techniques such as predistortion [6]. Kennington [5] has shown that for a given amplifier types, there are optimum coupling factors to maximise the efficiency o f the overall amplifier.
This technique has the disadvantage o f consuming more power, being more complex to build and align and is only suitable for relatively narrow band systems. It is a popular choice for obtaining good linearity in Power Amplifiers.