Using External LNAs to Improve GPS Signal Performance in Mobile Handsets
By John Allan, Product Marketing Manager, California Eastern Laboratories

Global positioning technology is finding its way into more and more wireless products. This is especially true of cell phones. E911 has mandated the integration of GPS functionality into these handsets and as consumers become better acquainted with the technology, their demand for GPS data and services is sure to increase.

While cell phone manufacturers are eager to add GPS, they’ve also placed strict requirements on device size and power consumption. This has led to the development of GPS receiver/processors ICs that integrate a low noise amplifier front end, or LNA. Unfortunately, the noise performance and resultant system sensitivity of these onboard LNAs are not always adequate. To reduce front-end noise and improve sensitivity, manufacturers are turning to additional, external LNA devices. These external LNAs can be located near the antenna so trace losses are reduced. This, coupled with tuning and filtering, can improve noise performance in the GPS circuit by more than 1.5dB.

Key desirable features for external GPS LNAs are low noise, high gain, small size, and low power consumption. A programmable power-save function that shuts down the device to conserve power is also essential. NEC’s UPC8232T5N MMIC LNA is engineered to meet all these requirements. Designed specifically for the implementation of GPS in battery-powered handsets, the SiGe:C UPC8232T5N also features integrated matching circuits, ESD protection, and a robust bandgap regulator. The UPC8232T5N delivers a noise figure of 0.9dB with 17.5dB gain (Figure 1), and power consumption of just 3.2mA at 2.7 – 3.3V. Its miniature size — 1.5 x 1.5 x 0.37 mm — and the lack of need for a complex external active bias network make the UPC8232T5N ideal for today’s ever-smaller phones.

Out-of-Band Signals and Desensitization
1.575GHz GPS LNAs, especially in highly-integrated phones, are subject to a variety of out-of-band (OOB) signals: 900MHz phone transmission, 1800-1900MHz PCS signals, 2.4GHz WLAN and Bluetooth, and 2.5-2.7GHz WiMAX . If the strength of these signals is sufficiently high, it can have the undesirable effect of reducing the LNA gain in the GPS band, an effect called desensitization. Measurements taken at California Eastern Laboratories (CEL) show that an OOB signal strength of -15dBm can decrease the gain in an LNA by as much as 1dB, thereby reducing GPS system sensitivity. These measurements were made by injecting OOB signals into test LNAs and monitoring the gain (at 1.575GHz) until it was decreased by 1dB — then recording the input power level that caused the desensitization.

Tuning to Mitigate Desensitization
Desensitization can be mitigated through tuning and filtering. The tuning that yields the best performance employs a low-loss input series L-C network as shown in Figure 2. Essentially a band-pass filter, it enables the UPC8232T5N to deliver 17.46dB gain (Figure 3). By adding a second capacitor in a “T” network, Figure 4, a high-pass filter is created. This circuit will decrease the sensitivity of the LNA to 900MHz signals, but has no favorable effect on signals above the 1575MHz GPS band. It also has a slightly negative effect on the noise figure. The table in Figure 5 compares the sensitivity to out-of-band signals for the two configurations.

Distributed Filtering: The Optimal Solution
The most common and effective way of dealing with desensitization is filtering. The most optimal is distributed filtering. In distributed filtering, a low-loss SAW band-pass filter is placed ahead of the LNA, and a high-rejection SAW band-pass filter just after it. In this “real world” configuration, Figure 6, interfering signals are rejected prior to the LNA, improving desensitization by more than 20dB. This pre-filter also reduces any intermodulation of out-of-band signals in the LNA. All this is achieved with a noise penalty of less than 0.5dB — which in turn can be mitigated by employing an extremely low noise LNA like the UPC8232T5N.

The high-rejection post-LNA SAW filter is a higher-loss device, but it eliminates any strong signals outside the GPS band which might be amplified by the LNA. Some designs put this high-rejection filter ahead of the LNA, but this configuration can add 1.5dB to the overall noise figure, performance that is not acceptable in most situations.

The performance of a distributed line-up is illustrated in the tables in Figures 7 and 8. Note the overall noise figure, which is maintained at 1.5dB including all the associated filter losses, and the improvement in desensitization, which is now above +5dBm.

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, which in turn will degrade overall noise performance. Since the LNA is particularly sensitive to the Q of the input inductor, the components used to construct the input matching circuit should include high-Q devices. Traces should also be adequately sized, as excessively narrow traces will add to the loss. Figure 9 illustrates a UPC8232T5N evaluation board that employs distributed filtering.

Conclusion
With the right components and careful circuit design, engineers implementing GPS functionality can address the challenges posed by out-of-band signals. With the UPC8232T5N MMIC LNA and distributed filtering, they can do it without sacrificing real estate or power consumption.

CEL Applications Engineering Team members Mouqun Dong, Bernard Urborg and Lydia Tong contributed to this article.

California Eastern Laboratories
www.cel.com
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