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Active phased-array technology, also known as active electronically scanned array (AESA) technology, has been widely used in commercial, military, and scientific applications.1,2 It supports much faster beam steering than mechanical means, with the capability to prevent interference through the use of strong nulls.3,4

Modern radar systems employ phased arrays that contain thousands of transmit/receive (T/R) modules for electronic beam scanning.2 Figure 1 shows a block diagram of a T/R module. The phase shifter is a critical component, used to shift the phase of incoming RF/microwave signals and to meet certain gain, noise, linearity, and phase-shift specifications.

T/R module system

The phase shifter (Fig. 2) is an important part of the T/R module, with a low-noise active balun (LNAB) comprising the first block of the phase shifter following the input signal port. The balun provides impedance matching (at 50 Ω), low-noise amplification, and single-to-differential conversion to provide a quadrature signal to a quadrature-all-pass-filter (QAF) network at X- and Ku-band frequencies. The LNAB circuit is divided into two circuits. The first stage is an active balun which converts single-ended input signals to quadrature signals. Critical performance parameters include high, flat gain, and low noise figure, with good input-output impedance matching.

Digital phase shifter

A number of structures and approaches have been proposed to achieve these goals for the active balun, including a two-stage cascode configuration with passive inductors.7,8 In one of these designs,7 the LNAB provides S21 (gain) of 10 dB, noise figure of 5.4 dB, and power dissipation of 66 mW at 14 to 15 GHz. In the other,8 the simulated S21 performance is 10.2 dB, the noise figure is 4.6 dB, and the power consumption is 87.5 mW at 13 to 15 GHz. Unfortunately, passive inductors contribute to higher loss and larger circuit sizes, and two-stage cascode active baluns have been proposed with active inductors, although these suffered in linearity.4,6 One design offered simulated input third-order intercept point (IIP3) of -30 dBm at 12 GHz. In another design, differential passive inductors were used in an active balun to decrease size and improve circuit performance.9

In pursuit of improved LNAB performance at X/Ku-band frequencies, a two-stage design was developed where the first stage is a single-stage low-noise amplifier (LNA) with cascode configuration. It employs resistive shunt-shunt feedback and peaking inductor for enhanced bandwidth.10,11 The second stage is an active balun with output emitter-follower to convert single-ended input signals to quadrature output signals and provide wideband output impedance matching. The two-stage LNAB was simulated and optimized using the Spectre RF computer-aided-engineering (CAE) software from Cadence Design Systems based on the use of a 0.18-μm silicon-germanium (SiGe) BiCMOS semiconductor technology.

LNAB circuit elements

In the proposed LNAB design (Fig. 3), wideband input impedance matching was achieved with a shunt-shunt feedback resistor (Rf) in conjunction with a preceding inductive-capacitive (LC) passive network; flat, high-frequency gain was achieved with the aid of the peaking inductor (Ld1 technique. The second stage is a differential cascode pair with differential peaking inductor (Ld2) to provide the single-ended-to-differential signal conversion, additional common-mode rejection, and wideband output-impedance matching. Use of the differential inductors, Ld2 and L3, save chip space and improve linearity.

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