REP005: Dual-band CDMA tuning front-end using inexpensive 85MHz IF SAW devices

Rapid engineering prototypes are actual circuits built and tested by Maxim application engineers in our labs. They are the starting point for new RF engineering designs. They are not intended as assessment testing tools.

Purpose: To verify the performance of a dual-band three-mode CDMA front-end IC using a common digital communication path and an AMPS FM 85MHz IF SAW filter.

This design requires tuning all FM/cellular/PCS mixers to a common 85MHz IF. The reason is to use a cheap IF SAW filter . Since RF image suppression filters and duplexers do not provide sufficient sideband suppression, the _-IF performance at the PCS band will be reduced. However, the high IP2 value of its PCS mixer ensures that the performance of this design is surprisingly good.

The MAX2320 low-noise amplifier (LNA) and mixer is designed specifically for dual-band CDMA mobile phones, but it can also be used in dual-band TDMA, GSM, EDGE, or WCDMA systems. It provides two LNA gain states to meet the required CDMA dynamic range with a certain switching hysteresis margin. The device has three mixers: one for the analog cellular channel and two for digital modulation channels for the cellular band and PCS band. Digital channel mixers have a universal IF output, and they can also provide adequate spurious image rejection by changing the IF once. This performance can replace an IF filter. The MAX2320 features independent buffered VCO inputs and outputs for the cellular and PCS bands, which eliminates the need for additional transmit upconversion VCO buffers.

Schematic of the MAX2320 evaluation kit (PDF, 52kB)
Bill of materials, part 1
Bill of materials, part 2