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Designing a CMOS synthesizer RFIC

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By Louis Fan Fei, Garmin International
November 16, 2007 -- rfdesignline.com

The three major building blocks in a modern, fully-integrated transceiver are a transmitter (TX), a receiver (RX), and a synthesizer. A synthesizer design is quite different from the TX or the RX. Both a TX and a RX have a higher analog content. The synthesizer design has significantly higher digital content. It challenges the designer's analog and digital skills.

In some instances, what appears to be an analog function is actually implemented with a digital block. For example, a frequency divider and a phase/frequency detector (PFD) are routinely implemented with the digital building blocks. In other instances, what appears to be a digital block needs a carefully analog design, such as a charge pump (CP). In the voltage controlled oscillator (VCO) design, both analog and digital building circuits are needed. The required knowledge of mixed-signal circuits makes a synthesizer a challenging design to work on.

A synthesizer is used to generate a very stable carrier signal for the TX and the RX. It is used to switch a transceiver to a different channel. The important requirements are: integrated in-band phase noise, spur level, channel switching speed, frequency step resolution, local oscillator pulling, power consumption, and low power supply rail.

The two popular synthesizer architectures are an integer-N and a fractional-N phase-locked loop (PLL). If an integer-N PLL can meet the requirement, it should be used because of a relatively simpler design. But in most cases, especially the multi-band, multi-mode transceivers used in cellular applications, a fine frequency resolution and a fast switching time are needed. In those cases, a fraction-N will have to be used. There are many good references on the synthesizer system and the digital control block design. This paper focuses on the detailed analog/RF circuit portion. The CP, PFDs, frequency dividers, VCOs are discussed below. All the pure digital signal processing (DSP), such as delta-sigma (Ä Ó), are left out because it is a topic all by itself.

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