Abstract: This reference design (RD) is for a dual-band, triple-mode CDMA front-end IC tuned for use with IF SAW filters. The RD modifies the FM/cellular/PCS mixers to a common 85MHz IF. The design uses a low-noise amplifier (LNA) with mixer, the MAX2320, that is also useful for TDMA, GSM, and EDGE applications. Schematics and bill of materials are shown.
Rapid Engineering Prototypes are real circuits that Maxim application engineers have built and measured in our labs. They can provide a starting point for new RF designs. They are not available as evaluation kits.
Objective: To verify the performance of this dual-band triple-mode CDMA front-end IC using a common, digital, 85MHz IF SAW filter and an 85MHz AMPS FM IF SAW filter.
This project entailed custom-tuning to modify the FM/cellular/PCS mixers all to a common 85MHz IF. This was required because of access to low-cost IF SAW filters. A compromise in _-IF performance at the PCS band resulted, because the RF receive-image filter and the duplexer didn't provide sufficient band-edge rejection. Nonetheless, the design proved surprisingly robust because of the high IP2 of the digital PCS mixer.
The MAX2320 low-noise amplifier (LNA) plus mixer is designed for dual-band CDMA cellular-phone handsets, but it can also be used in dual-band TDMA, GSM, EDGE, or WCDMA applications. It offers two LNA gain states to meet the required CDMA dynamic range, with a switchover hysteresis margin. There are three mixers: one for the analog cellular path and two for the digital modulation paths at the cellular and PCS bands. The digital path mixers have a common IF output, as they can provide sufficient spurious and image rejection with a single IF. This capability eliminates one IF filter. The MAX2320 has separate cellular-band and PCS-band buffered VCO inputs and outputs, eliminating the need for extra transmit upconverter VCO buffers.
Tested Circuit 
