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A 2-18GHz RECEIVER FRONT-END FOR ESM APPLICATIONS

A 2-18GHz RECEIVER FRONT-END FOR ESM APPLICATIONS
By: L.M.Devlin*, G.A.Pearson*, A.W.Dearn*, P.D.L.Beasley†, G.D.Morgan†
* Plextek Ltd, London Road, Great Chesterford, Essex, CB10 1NY, UK, lmd@plextek.co.uk
† QinetiQ, St. Andrews Road, Great Malvern, Worcs, WR14 3PS, UK

Abstract
This paper describes the design, fabrication and evaluation of a dual channel 2-18GHz front-end module for Electronic Support Measures (ESM) applications. The module converts signals, received by two external antennas, from anywhere in the 2-18GHz frequency range to an IF suitable for digitisation. Frequency conversion is realised by upconverting to an intermediate frequency at around 22GHz before filtering and then downconverting to the IF. The module contains 12 GaAs MMICs including 5 different designs 4 of which are full custom parts designed by Plextek. The receiver exhibits a gain of 10dB, a noise figure of 7dB and an output referred 1dB compression point of 10dBm.

Introduction
ESM for electronic intelligence (ELINT) applications places stringent fidelity demands on a receiver over a multi-octave bandwidth. Practical solutions must not only meet the electrical specification but also be compact, robust and immune to microphonic effects and thermal fluctuations.
A novel ESM module is described that meets these requirements and provides a dual channel architecture with close amplitude and phase matching. The amplitude and phase match achieved between the channels is the fundamental limitation on the accuracy of the bearing assessment for direction finding techniques and this MMIC based realisation helps ensure close amplitude and phase tracking. The module is essentially a low noise, broadband, dual channel receiver that converts signals anywhere in the 2-18GHz range to an IF suitable for digitisation. The module may not only be used as a generic front end for conventional ESM applications but also for monopulse or interferometry techniques.

Design and Operation
The broadband downconversion is realised by first upconverting, using a swept LO, to around 22GHz and then downconverting the signal to the required IF. The module also includes signal limiting for damage protection, low noise amplification to improve sensitivity and filtering to remove unwanted spurious products. A block diagram of the module is shown in Figure 1.

The module comprises a total of 12 GaAs MMICs (5 different types) together with mixed printed/lumped element filtering, bias conditioning and control circuitry. Four of the MMICs were full custom designs realised as part of this development.
The full custom MMICs are shown in blue outline in Figure 1 and are:
* 0.5-20GHz dual channel limiter
* 2-18GHz dual channel LNA
* 2-18GHz upconverter to 22 23GHz
* 22-23GHz downconverter to 0.1 1GHz
Photographs of these MMICs are shown in Figure 2 and Figure 3. Details of the design and performance of the limiter can be found in [1] and the upconverter in [2]. The SPDT MMICs, shown in green outline in Figure 1, are commercially available parts. A dual channel version of this MMIC has now also been developed and is reported in [3] but is not used in this implementation of the module.

The module architecture is identical for both the bands after the input BPFs, since all of the MMICs cover the entire 2-18GHz range. The first component is the dual channel limiter MMIC that protects the components that follow from damage due to high level input signals. This was fabricated as a single die using the Triquint Texas commercially available Vertical PIN diode (VPIN) process. A distributed topology was adopted with antiparallel pairs of PIN diodes in a shunt configuration. The measured performance shows a small-signal insertion loss of less than 0.8dB over the 0.5-20GHz range with excellent channel matching. The limited output power, defined at 10dB saturation, measures around 15 to 16.5dBm and the CW power handling capability is around 2W.
Following the limiter two dual c

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