Microwave Engineering Europe - March 2008 - (Page 13)

WIRELESS INFRASTRUCTURE 13 Figure 2: LT5575 I/Q output equivalent circuit. Traditional Spectrum Analyzer 5 seconds IC layout considerations Because of the aforementioned issues, the actual receiver implementation requires optimum placement of the direct conversion demodulator IC and its external components, as well as careful RF layout of the printed circuit board (PCB). Special attention should be given to power supply decoupling, grounding, signal path routing, and high frequency termination of the baseband ports. Linear Technology ’s Demonstration Circuit DC1048A (Figures 3, 4, and 5) illustrates proper implementation of the LT5575 direct conversion I/Q demodulator IC as it might be employed in a high performance receiver application. The evaluation board uses 4 layers comprised of commonly available FR-4 PCB materials with a total thickness of 62 mils. Layers 1 and 2, and layers 3 and 4 are separated by 18 mils of dielectric material. To ensure stability and maintain performance, all Vcc pins must be decoupled from the power supply using bypass capacitors placed as closely as possible to the designated Vcc pins. For the LT5575, Vcc pins 6, 7, and 8 are decoupled using one 1000 pF and one 0.1µF capacitor to ground, while Vcc pin 12 has its own 1000 pF decoupling capacitor. As with all high frequency circuits, proper grounding is essential for optimum performance. Use conservative layout with solid, uninterrupted ground planes on the top and the second PCB layers. Apply plated through-hole vias liberally to connect ground planes on all PCB layers, especially along plane edges. Avoid “ﬂoating” copper ﬁlls as they can act as RF resonators and antennas. The exposed IC back side ground pad is the ground return for the chip and provides heat sinking, and it must be soldered directly to the PCB ground plane for optimum performance. Adding several plated through-hole vias directly under the ground pad will ensure a good electrical ground and promote thermal conduction. As mentioned earlier in this article, RF and LO coupling can lead to “self-mixing”, causing undesirable signals as well as DC offset errors to appear at the baseband outputs, and may degrade IP2 performance. Also, large in-band RF interference signals that leak to the LO port can change the phase and/or pull the LO VCO off frequency, degrading the receiver performance. Moreover, the LO signal, along with its harmonics, leaking into the baseband outputs can degrade the phase balance of the baseband I/Q signals. To avoid such signal coupling, all RF, LO and baseband signal traces should be routed in different layers separated by ground planes, whenever practical, keeping the traces as short as possible. The RF and the LO inputs may reside on the same layer if they are separated by adequate ground plane and ground vias to reduce coupling. However, best performance can Tektronix 5 seconds The two screens represent a traditional swept tuned spectrum analyzer and a Tektronix Real-Time Spectrum Analyzer running DPX™, each detecting an identical signal that changes every 1.28 seconds. Tektronix DPX technology—see what you’ve been missing. Discover what’s real with Tektronix DPX™ technology. Now you can see your time-varying signals live with DPX technology, available on all Real-Time Spectrum Analyzers from Tektronix. DPX displays a live RF view of your signal changing over time. It shows you RF signal characteristics that are practically invisible with prior architectures. See a live RF demo www.tektronix.com/rtsa-dpx © 2008 Tektronix. All rights reserved. Tektronix products are covered by U.S. and foreign patents, issued and pending. TEKTRONIX and the Tektronix logo are registered trademarks of Tektronix, Inc. Microwave Engineering ● March 2008 ● www.mwee.com 011_012_013_014_MWEE.indd 13 20/02/08 12:04:11 http://www.tektronix.com/rtsa-dpx http://www.tektronix.com/rtsa-dpx http://www.mwee.com

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Microwave Engineering Europe - March 2008 News Contents Comment Wireless Infrastructure: A Direct Conversion I/Q Demodulatordrives Favorable Basestation Cost-performance Metrics Wireless Infrastructure: Mobile World Set to Reshape the Internet RF Amplifiers: Latest Advances in RF Amplifiers Include a CMOS PA Operating at 77 GHz and Significant Advances in PAs for WiMAX and Broadband Applications Many Applications Still Require Unique Performance Benefits of BeO ACE Automated Circuit Extraction Returns to Real Design by Exploring Design Alternatives and Changes in Seconds Exceeding the Standard for Wireless Sensor Networks Products Calendar

Microwave Engineering Europe - March 2008

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