LINEAR LT5515 Data Sheet & Board-Level Design Issues & Considerations

February 24, 2005

LT5515 board level design issues and considerations. The LT5515 Direct Conversion Receiver is a high performance radio frequency integrated circuit. The actual product design and implementation requires optimum placement of the LT5515 and its external components, as well as good RF layout for the printed circuit board (PCB) in order to maximize the receiver performance. In general, direct conversion receivers are sensitive to leakage of the Local Oscillator signal to the Baseband (BB) and the RF ports, and also leakage of the RF signal to the BB and LO ports. Such leakages result in degradation of important receiver parameters.

Board level issues with direct conversion receivers.

1. LO leakage to the Baseband outputs.

LO leakage to the BB outputs (as well as 2nd harmonic of LO at the BB outputs) may degrade the phase balance of the baseband I/Q signals. Suggested actions:

• Provide high frequency terminations to ground with small value shunt capacitors at the BB outputs, placed close to the BB pins. • The BB output signal lines should be routed on the bottom side of the PCB for improved isolation. • Use good conservative layout with solid ground plane on the 1st and 2nd PCB layers. Use lots of ground vias, especially in the areas separating the LO signal lines from the BB signal lines, as well as in areas separating RF signal lines from BB signal lines and RF signal lines from LO signal lines.

2. RF leakage to BB outputs.

RF, as well as LO leakage to Baseband, produces static and dynamic DC offset at the BB outputs. This may lead to IP2 performance degradation. Suggested actions: Refer to suggested actions in item 1.

3. LO leakage to the RF input.

LO leakage to the RF input may result in a LO “self-mixing” which produces static DC offset at the BB outputs. Suggested actions: Refer to suggested actions in item 1.