Microwave Engineering Europe - April 2009 - (Page 16)

16 MMICs/RFICs frequencies of 2 GHz, 10 GHz and 18 GHz. Conversion loss increases with RF frequency due to the drain-source capacitance of the PHEMT devices limiting the isolation of the switches. For an LO drive of +7 dBm, conversion loss varies from 7 dB at 2 GHz to 11.5 dB at 18 GHz. The next stage of the design was to consider the most appropriate balun topologies. The RF (21 to 23 GHz) and LO (23 to 41 GHz) baluns were realised as uniplanar Marchand baluns and the IF (2 to 18 GHz) balun as an active long-tail pair differential ampliﬁer. Details of the balun design process are included below. The architecture of the entire upconverter IC is that shown in Figure 25. Marchand baluns are capable of achieving broadband operation with low insertion loss. The implementation ﬁrst described by Nathan Marchand in his 1944 publication [10], was a co-axial structure. Various printed implementations have since been described; the simplest of which is depicted in Figure 26. Increased coupling and therefore increased bandwidth can be obtained from a planar implementation if multiple coupled lines are used, as depicted in Figure 27. This was the balun structure adopted for the RF and LO baluns. The total length of this structure is approximately half a wavelength at the centre frequency. A layout plot of the LO balun is shown in Figure 28. The layout was meandered to reduce the chip size. Electromagnetic simulation of the Marchand baluns was carried out to properly account for the discontinuity and coupling effects of the layout. Figure 29 shows the simulated amplitude and phase difference between the two branches, which is within 0.9 dB and 9° of ideal from 14 to 44 GHz. The excess insertion loss of the balun is less than 0.5 dB. The IF port balun needs to work from 2 to 18 GHz so an integrated Marchand balun was not appropriate, both in terms of the area it would occupy and the difﬁculty of realising tight enough coupling to achieve the required operating bandwidth. The solution adopted for the IF balun was to use a two stage differential ampliﬁer. A simpliﬁed circuit diagram of the ﬁrst stage is shown in Figure 30. The input is a long-tail pair biased with a current source (Q5). One transistor in the differential pair (Q4) has its gate held at ground and the gate of the other transistor (Q3) is the LO input port which is matched to 50 Ω with a broadband lossy input matching network. The drain terminals are biased using active loads (Q1 and Q2) with their gate and source terminals capacitively coupled to hold them at the same RF potential. The output of this ampliﬁer stage is a differential signal, the balance of which is improved by the use of a second differential ampliﬁer stage. The simulated performance of the complete active balun showed a phase difference of 180° ± 2° and an amplitude imbalance of less than 0.2 dB from 1 to 20 GHz. The simulated performance of the complete upconverter, including quad-ring mixer, active IF balun and passive RF and LO baluns is shown in Figure 31. Input return loss and conversion loss is shown for LO drive levels of +8, +10 and +12 dBm. The input return loss is better than 12 dB from 2 to 18 GHz and does not change noticeably with varying LO input power. This is a result of the isolation provided by the active balun. Across 2 to 18 GHz the simulated conversion loss is less than 12 dB for an LO drive of +10 dBm. Fabrication and measured performance (Upconverter) A photograph of the upconverter IC is shown in Figure 32, the die size is 3.04 mm x 3.28 mm. Evaluation of the upconverter was carried out on ICs assembled onto an MIC carrier tile, as shown in Figure 33. This was fabricated from 0.01 inch thick RT/Duroid 5880 with a brass backing to give rigidity. Figure 30: Input stage of IF active balun. Figure 31: Final simulated performance of the upconverter. Figure 32: Photograph of the upconverter IC. Figure 29: Electromagnetic simulation of amplitude and phase difference of the LO balun. Microwave Engineering Europe ● April 2009 ● Figure 33: Photograph of the upconverter MMIC evaluation tile. www.mwee.com http://www.mwee.com

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Microwave Engineering Europe - April 2009 News Contents Comment MMICs/RFICs: MMICs for Broadband Receiver Applications Antennas: Mutual Coupling Reduction in Compact Arrays for Wireless Sensors via a Pre-fractal Defected Ground Structure Repeatable Characterization of Distortion Caused by Nonlinearities in Wideband Communication Systems Products Calendar

Microwave Engineering Europe - April 2009

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