Microwave Engineering Europe - March 2009 - (Page 21) WIRELESS INFRASTRUCTURE 21 Broadband active filters solve accuracy and matching problems in signal processing applications By James Wong, Product Marketing Manager, Linear Technology Corporation F ew active filters on the market offer bandwidth higher than 2 MHz. Yet today’s new generation broadband communication receivers push several to tens of MHz, requiring very good noise and distortion performance. Additionally, I/Q channel or multichannel receivers have tight matching requirements that are extraordinarily difficult to meet. For the most part system designers must resort to discrete designs which are bulky and consume large amounts of board space. And if a high degree of filter accuracy is required, the design challenge is simply daunting. Linear Technology has introduced a broad selection of wide bandwidth active filters building blocks intended as dropin solutions to ease the design task. These single and dual matched filters provide integrated, highly accurate filtering capability in small footprint. This article examines the different application requirements and design considerations to extract the most performance out of these filters. These filters have tight tolerance on their filter response while providing exceptional dynamic range performance. They are ideal for a wide range of applications such as anti-aliasing filters for high resolution A/D converters, reconstruction filters for D/A converters in wireless communication receivers and transmitters, industrial and medical signal processing of optical and image processing filters, instrumentation and testing, RFID demodulation baseband filters, and all types of filtering in signal processing applications. Figure 1: The LTC6601-1 2nd order differential filter topology. Both bandwidth and gain are pinconfigurable by pin-strapping the appropriate internal resistors and capacitors based on the equations given. Part numbers and related features. A versatile broadband active filter building block The LTC6601-1 contains a low noise, wideband amplifier plus an assortment on-chip resistors and capacitors to form a 2nd order differential input and output lowpass filter. The resistors and capacitors are pinned out to allow the user to mix and match different combinations to form a wide range of lowpass response from 5 MHz to 27 MHz as well as filter gain. These internal resistors and capacitors are laser-trimmed at the factory to a tolerance of ±0.5 percent. Additionally the amplifier’s bandwidth is also laser trimmed. The combination provides a lowpass filter response with unparalleled accuracy that is otherwise difficult to replicate discretely, while ensuring excellent repeatability and saving significant board space. Figure 1 shows the filter topology by pin-strapping different resistors and capacitors to form varying filter responses. Higher order filters with precise response can be easily created by cascading multiple stages of the filter. Even the task of creating matched performance in a multi-channel environment becomes simpler. From this building block come dual matched filters with fixed bandwidth of 7 MHz, 10 MHz and 14 MHz cutoffs, corresponding to the LTC6605-7, -10, and -14, respectively. These dual channel filters are tested and guaranteed for their matched gain and phase performance. They are intended to be drop-in, small form-factor solutions with minimum external components. Broadband pre-ADC driver — filter makes a tough application easy High speed, high resolution ADC inputs are difficult to drive, owing to their inherently wide input bandwidth and their abrupt changes in input impedances resulting from the switching of the input sampling circuit. Particularly troublesome is the instantaneous current ● Microwave Engineering Europe ● March 2009 www.mwee.com http://www.mwee.com
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