45 A Reflection-Type Vector Modulator with Balanced Loads Franco Di Paolo, Mauro Ferrari, Franco Giannini, Ernesto Limiti Department of Electronic Engineering, University of Rome Tor Vergata Via del Politecnico 1, 00133 Roma - ITALY Abstract- In this contribution a new vector modulator topology is presented. The new VM allows the reduction of the resulting chip area occupation by using 7 couplers only, as contrasted to traditional reflection-type balanced vector modulators with 9 couplers, and exhibiting comparable performance. A W Band test vehicle adopting such new topology has been designed in monolithic form with 0.13 µm phemt technology, resulting in a die size 2.2 x 1.5 mm 2. The approach to the novel topology and relevant performances of the test vehicle are described in the following. Index Terms- Vector modulator, W band, microwave, MMIC. I. INTRODUCTION AND TRADITIONAL VECTOR MODULATION TOPOLOGIES A Vector Modulator (VM is a subsystem capable of simultaneously changing amplitude and phase of an input RF signal according to a suitable control. Its main applications are in telecommunication systems, where, in direct modulation schemes [1], the use of VM may lead to avoid up-converting chains (mixer, filters, etc... Other VM applications are in FMCW radars used in ACAS (Automotive Collision- Avoidance Systems, in phased-array antennas and in the feed-forward linearization technique, where VMs can be used instead of the attenuator - phase shifter cascade in the nulling loop, thus providing enhanced performances [2]. A large number of VM types have been presented, e.g. VMs simply cascading attenuators and phase shifters [3-4], VMs synthesizing the desired output by means of three [5] or four basic components [6], or the shifted-quadrant VM [7]. Most of these VMs are not suitable at very high frequencies (e.g. W band or their complexity lead to excessive size. For the latter reasons, the Reflection-Type VM has been selected as the VM exhibiting potentially optimum performances in W band. With reference to Fig. 1, a Reflection-Type VM is generally composed by two Biphase VMs (BVM connected via an hybrid junction splitting the input signal into in-phase and quadrature components, controlled according to S I and S Q signals respectively. s of the BVMs are then summed up by a suitable power combiner. Matched termination Fig.1. Biphase VM S I S Q Biphase VM Reflection-type vector modulator Traditional BVM, as reported in ], is based on balancing a simple reflection-type BVM, as schematically depicted in Fig. 2.
46 Biphase VM Γ 90 deg hybrid V G Γ( V G Fig.2. Simple reflection-type BVM Adopting ideal couplers, such connection should exhibit a scattering matrix given by: Fig.3. Simple load realized with a cold-fet and Г(V G @ 80GHz 0 jγ( VG [ S] = jγ( VG 0 (1 where Г(V G is the reflection coefficient of the load, typically realized via two cold-mode FETs controlled by their gate voltage V G. To maintain the biphase transfer function defined by expressions (2, an ideal BVM should fulfill condition (3, i.e. symmetry with respect to its control voltage: S21( VG = k VG π π S21( VG = + sign( VG 2 2 (2 suitable for narrowband applications only; the second one is the balancing technique, actually adopted for broadband applications [10-16]. In the latter methodology, two simple BVMs, controlled by complementary voltages, are combined in a push-pull structure as depicted in Fig. 4: Balanced Biphase VM ( ( S V = S V V (3 21 G 21 G, G FET s parasitics, whose effects are especially evident at high frequencies, do not allow acceptable performance adopting such simple connection, given the lack of symmetry in the device behavior, as evidenced in Fig. 3 where a typical cold-fet reflection coefficient is plotted as a function of the gate control voltage @ 80 GHz. To correct this problem two main strategies are possible: the first one, as reported in [9], is based on parasitics compensation, therefore being Fig.4. Γ( V G Γ ( V G Balanced reflection-type BVM where the total transfer coefficient of the balanced BVM is given by (4: 1 S ( V = S V S V 2 ( ( TOT 21 G 21 G 21 G (4
47 Such connection exhibits two main drawbacks: the first is the need of four couplers and the second is the difficult access to control terminals of the structure. Due to the former, the total number of couplers for the whole VM structure (composed by two BVM and the input coupler becomes nine, while for the latter, access to all control terminals is not possible if the use of underpasses or bridges are avoided, as highly recommended at very high frequencies. II. PROPOSED VM TOPOLOGY The idea underlying the proposed VM is based on the observation that the transmission coefficient S 21 and reflection coefficient Г of the simple BVM are identical with the exception of a 90 deg phase term (the j coefficient in (1. Instead of balancing the simple BVM (as in Fig. 4, the same result may be therefore attained by balancing the loads directly. This principle (Balanced Load is described by expression (5 and is represented by the connection in Fig. 5 (left, where the resulting reflection coefficient is plotted as a function of the gate control voltage (right. TOT 1 Γ ( V = Γ V Γ V 2 ( ( G G G (5 The resulting reflection-type BVM adopting balanced loads is therefore schematically depicted in Fig. 6, while its transfer function is given by (6: TOT 1 TOT S21 ( VG = j Γ ( VG (6 2 Γ TOT Fig.5. ( V G Fig.6. Γ( V G Balanced load and Г TOT (V G representation @ 80GHz Novel Balanced Biphase VM with balanced loads Γ( V G Γ ( V G Γ ( V G Reflection-type biphase vector modulator with balanced loads Expression (6 is fully equivalent to (4 and both verify condition (3: nevertheless, in the Balanced Load proposed approach only three couplers are needed and control signals can be easily accessed on a single side of the structure, thus easing the overall VM routing. III. TEST VEHICLE DESIGN To validate the proposed topology, a test vehicle has been designed in monolithic form making use of a 0.13 µm GaAs phemt technology from
48 OMMIC and selecting, as a load, a 2x15 µm device. The resulting chip, depicted in Fig. 7, occupies 2.2 x 1.5 = 3.3 mm 2 and has been designed to operate @ W Band (75 85 GHz. Fig.8. VM constellation @ 80 GHz Fig.7. Test vehicle VM relative S 21 phase (covering the same range is plotted in Fig. 9 as a function of frequency. The design of the whole VM has been performed making a combined use of both circuit and 2D electromagnetic simulation tools, the latter both to validate passive models (tee junctions, steps, tapers, etc.. and to take into account potential asymmetries and EM interactions arising from coupling between different sections of the circuit. After a 2D EM simulation of the critical parts and passive elements, the final design has been therefore obtained after a subsequent cosimulation of the entire VM. As evidenced in Figure 6, the VM needs two complementary voltages (V G and -V G to control amplitude and phase of each BVM (clearly visible in the upper and lover portions of the circuit in Fig. 7. By varying such controls from -0.55 V to 0.55 V, a possible symbol constellation exhibiting a 360 deg coverage has been obtained and reported in Fig. 8. With the application of a 0.55 V stepped control voltage, 9 states for the VM are scanned and the Fig.9. Relative phase of the VM S 21 by varying control voltages with 0.55 V steps. For the above 9 states of the VM, input and output return loss are presented in Fig. 10 and in
49 Fig.11, where better than 20 db input match and 15 db output match are demonstrated. The insertion loss of the VM, presented in Fig. 12 and in Fig. 13, is always better than 14 db. Fig.10. VM Return loss at port 1 Fig.12. 4-State S 21 magnitude by varying control voltages from -0.55 V to 0.55 V Fig.11. VM Return loss at port 2 Fig.13. 4-Point VM Constellation by varying control voltages from -0.55 V to 0.55 V @ 80 GHz IV. CONCLUSIONS A novel VM topology has been presented, based on the Balanced Loads approach. Such new topology allows the reduction of the overall VM complexity by reducing the number of necessary couplers from nine to seven, as compared to traditional balanced VMs. A W Band test vehicle of the new topology has been designed in MMIC
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