Microwave Engineering Europe - November 2008 - (Page 19) MMICs 19 longer effective as the required resistance would be too low. A low impedance at the LO port of the mixer would reduce the effective LO voltage swing and degrade both the conversion loss and compression performance of the mixer. In order to address the problem of reducing gate reactance with increasing frequency, an impedance matching network was included at the LO port of the mixer between the LO port balun and the gates. A 200 Ohm terminating resistor across the gate terminals of the mixer was used to provide a resistive load and a passive LC matching network was designed to maximise LO voltage swing across the resistive load (and so across the gates of the mixer). The input capacitance at the gates of the PHEMTs was absorbed in to this matching network, which also serves to provide a reasonable return loss at the LO port. On-chip baluns were required at the RF and LO ports, and the next stage of the design process was to consider the most appropriate balun structure. One well-proven structure, capable of achieving broadband operation with low insertion loss, is the “Marchand Balun” [4]. This was originally a coaxial balun, but printed versions have since been developed. The multiple coupled line version, depicted in Figure 2, has been shown to be suitable for MMIC implementation with octave band operation [5]. Both the RF and LO balun were realised using this type of structure. The approach taken with the balun design was to initially use the multiple coupled line models in a conventional RF simulator to predict performance and balun dimensions. A full EM simulation was then undertaken to improve the accuracy of the simulation and allow the final optimisation of the balun dimensions. EM simulated amplitude imbalance across the 18 to 40 GHz RF band was < ±0.12 dB. The phase imbalance predicted by the EM simulation showed a gradual increases with frequency across the band but was still only 7 degrees away from ideal at 40 GHz. Simulated excess insertion loss was less than 0.85 dB across the 18 to 40 GHz operating band and the input match was better than 14 dB. Measured performance of stand alone balun test structures are not available but a plot showing the EM simulated performance of the RF balun analysed as two port structures, with the outputs differentially combined in an ideal transformer, is shown in Figure 3. The simulated insertion loss is below 0.8 dB across the entire RF band, with a good guard band at the high frequency end. Performance simulations of the entire mixer used EM simulator generated 3-port s-parameter files for the LO and RF baluns. Foundry supplied large signal transistor models were used for mixer transistors and the active bias network. Great care was taken with the layout to preserve symmetry as this would be critical in obtaining good LO rejection. All interconnect tracking, airbridge cross-overs and discontinuities in the LO matching network were EM simulated. The resultant simulated conversion loss versus LO frequency of the whole mixer Microwave Engineering ● November 2008 ● www.mwee.com http://www.feko.info http://www.feko.info http://www.mwee.com
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