Cell phones must now integrate Global Positioning System (GPS) functionality to satisfy the E911 mandate. Although many GPS receiver/processor integrated circuits (ICs) incorporate on-board low-noise-amplifier (LNA) front ends, the noise performance and resultant system sensitivity of these integrated LNAs are not always adequate. To reduce trace losses, discrete LNAs can be located near the antenna. When coupled with tuning and filtering, this approach can improve noise performance by more than 1.5 dB over onchip, integrated LNAs. The UPC8232T5N MMIC silicon-germanium (SiGe)-Carbon LNA from California Eastern Laboratories (Santa Clara, CA), for example, delivers a noise figure of 0.9 dB with 17.5 dB gain. At 2.7 to 3.3 V, its power consumption is 3.2 mA.
According to measurements taken at California Eastern Laboratories, an out-of-band (OOB) signal strength of –15 dBm can decrease the gain in an LNA by as much as 1 dB. In doing so, it will reduce GPS system sensitivity. These measurements were made by injecting OOB signals into test LNAs and monitoring the gain (at 1.575 GHz) until it was decreased by 1 dB. The input power level that caused the desensitization was then recorded. The OOB signal strength is especially important, as 1.575-GHz GPS LNAs are subject to a variety of OOB signals like 2.4-GHz WLAN. If the strength of these signals is sufficiently high, it can reduce the LNA gain in the GPS band.
This desensitization can be mitigated through tuning and filtering. The tuning that yields the most superior low-noise performance employs a low-loss inputseries L-C network. Essentially a bandpass filter, it enables the UPC8232T5N to deliver 17.46 dB gain. In distributed filtering, a low-loss surface-acoustic-wave (SAW) bandpass filter is placed ahead of the LNA. A high-rejection SAW bandpass filter is placed just after it. In this "real-world" configuration, interfering signals are rejected prior to the LNA. Desensitization is improved by more than 20 dB. This pre-filter also reduces any intermodulation of out-of-band signals in the LNA. These benefits are achieved with a noise penalty of less than 0.5 dB.
The high-rejection post-LNA SAW filter is a higher-loss device. Yet it eliminates any strong signals outside of the GPS band that might be amplified by the LNA. Some designs put this high-rejection filter ahead of the LNA. But this configuration can add 1.5 dB to the overall noise figure. In an example of a distributed line-up, the overall noise figure was maintained at 1.5 dB including all of the associated filter losses (see figure). The improved desensitization was above +5 dBm.
The implementation of a distributed filtering circuit requires close attention to component specification and interaction. The LNA should be tuned for best noise figure and input match. A poor input match will induce ripple loss in the SAW filter ahead of the LNA. The LNA is particularly sensitive to the Q of the input inductor. As a result, the components used to construct the input matching circuit should include high-Q devices.
ACKNOWLEDGEMENTS
CEL Applications Engineering Team members Mouqun Dong, Bernard Urborg, and Lydia Tong contributed to this article.