GLOBAL POSITIONING SYSTEM (GPS) functionality is being widely added to cellular phones and other handheld devices. Yet cell-phone manufacturers are still placing restrictive requirements on device size and power consumption. To meet these conflicting demands, GPS receiver/processor integrated circuits (ICs) have been developed that incorporate onboard, low-noise-amplifier (LNA) front ends. The problem is that the noise performance and resultant system sensitivity with these LNAs have not always been sufficient. As a result, some manufacturers are opting for external LNA devices. California Eastern Laboratories details such an approach in its three-page application note titled, "Using the UPC8232T5N Discrete LNA to Improve GPS Signal Performance in Mobile Handsets."
To reduce trace losses, discrete LNAs can be located near the antenna. When coupled with tuning and filtering, such an approach can improve noise performance by more than 1.5 dB over onchip LNAs. But if the LNA is subjected to high enough out-of-band signals, such as 2.7-GHz WiMAX, desensitization will occur. Desensitization can be mitigated via tuning and filtering.
Distributed filtering places a low-loss surfaceacoustic- wave (SAW) bandpass filter ahead of the LNA and a high-rejection SAW bandpass filter just after it. Interfering signals are then rejected prior to the LNA. The LNA should be tuned for best noise figure and input impedance match. The components used to construct the input matching circuit also should include high-Q devices.
California Eastern Laboratories, 4590 Patrick Henry Dr., Santa Clara, CA 95054-1817; (408) 919-2500, FAX: (408) 988-0279, Internet: www.cel.com.
