Abstract

This Letter presents a novel approach to realising and fabricating a planar balun structure, where conventional bond wires are replaced by conductor bridges supported on polyimide blocks. Three different prototype structures were fabricated on 10 mil thick alumina substrate ( ɛ r = 9.8, tan δ = 0.0001) using thin film fabrication technology. More than double octave bandwidths are reported, from 1.25 to 4.7 GHz with losses lower than 1 dB, an amplitude imbalance (trace separation) below 0.75 dB and phase imbalance within ±5°. Measurements show good agreement with simplified 2D EM simulations. The performance of the balun is assessed for suitability in applications where amplitude and phase balance are of critical importance, such as push–pull or outphasing amplifier topologies.

Highlights

  • Introduction and motivationA balun allows a single-ended, unbalanced ground referenced signals to be transformed into a balanced, differential signal

  • Using the Transmission Line (TL) based balun model shown in Fig.1, which includes effects of the parasitic coupling, the authors derived equations

  • In this figure two sections of the balun are modelled as two sets of coupled TLs with characteristic impedance denoted as Z0 and ZStub and TLs corresponding to parasitic coupling

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Summary

Introduction

A balun (pontprenau of words balanceunbalance) allows a single-ended, unbalanced ground referenced signals to be transformed into a balanced, differential signal. This property makes the balun an indispensable component in many RF and microwave architectures: filters, mixers, antennas and power amplifiers (PA). This work uses the following criteria for determining the bandwidth of the proposed balun; return loss at the un-balanced port < -10dB; losses < 1dB (equivalent to transmission loss > -4dB), along with trace separation less than 0.75dB. The authors in [4] use a back-to-back measurement of their structure to determine insertion loss This method cancels out and hides the unwanted trace separation that is crucial in our application. Work presented in [6] and [7] operate in similar frequencies as the balun proposed in this work, the structures are larger and require greater area due to the different fabrication materials

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