Chip eases direct conversion receiver designs

Chip eases direct conversion receiver designs

Linear Technology won a 2007 Product of the Year award for its LT5575 direct-conversion I/Q demodulator. We recently spoke to Design Manager Min Zou about the product’s development.

“The concept for the LT5575 was to design a direct conversion receiver with a flexible architecture, capable of covering multiple RF standards while simplifying system design and reducing overall costs. Although direct conversion receiver designs had been widely used in cordless phones and cellular handsets, they had not proliferated in base stations, even with their lower costs, due to performance limitations,” Min Zou told us.

The design team

“In spite of technical challenges we knew what we needed to do to achieve a good IIP3 and noise figure before initiating the LT5575 design. To achieve good IIP2 and low dc offset at baseband outputs, however, the design was very tricky. We had to pay special attention, not only to the chip’s layout to avoid device mismatches and thermal gradient issues, but also to high-frequency isolation between RF and LO input ports.

“With the design of the LT5575, we were looking at integrating two transformers on the same die to simplify board layout and reduce receiver system costs. To attain broadband operating frequency range, these two on-chip transformers would require more interconnect metal windings and therefore a larger die area.

“The coupling between the transformers was then a big concern, particularly through the conductive substrate coupling, which was difficult to characterize and model. The 3-D electromagnetic simulation was very time consuming and impractical for achieving the accuracy needed. Based on a good understanding of the wafer fabrication process, we built a special isolation structure around and underneath the transformers to minimize coupling in-between. When the assembled part was back and ready to test, the results were very encouraging, including IIP2 and dc offset performance.”

Min Zou concluded, “Our field applications and marketing engineers confirmed our customers’ demand for a high-performance product. Customer feedback presented another design challenge: to achieve even higher input 1-dB compression (P1dB) and to ensure that the part would survive even under +20 dBm (6.32 Vp-p at 50-Ω impedance) RF input power. We put our efforts into resolving this design challenge quickly.”

Christina Nickolas