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Reduce interference in GPS-enabled applications

Global Positioning System (GPS) is a navigation system formed by 24-satellites placed in the Earth's six orbits and enables users to determine their position accurately from any location. The system was initially used by the military but was introduced to civilians in the 1980s [1]. Since then, GPS has become popular as a survival and navigation tool. Manufacturers have integrated GPS receivers into various consumer products which are portable with mobile connectivity such as vehicles or wireless devices. A handset is an ideal product to be GPS-enabled. The integration of a GPS receiver into a handset can create a simultaneous-GPS (S-GPS) application where the GPS receiver is used together with other wireless communication systems from various frequency bands such as PCS and cellular. Consumers expect a handset with GPS capability to be reliable in receiving and amplifying the signal from the satellites because any error on the reception would cause inaccurate information on the location. Unfortunately, the quality of the GPS signals are often times compromised by interfering RF signals.

Intra-System Interference
The integration of a GPS receiver on the same board with other wireless mobile communication transmitters exposes the receiver to the intra-system interference which may degrade the GPS receiver's sensitivity and linearity. While the transmitter is in transmit mode, part of the transmitting signal will leak to the GPS receiver path. Consequently, the receiver would experience a high total input power that may saturate the receiver's back end. This would generate a non-linear signal at the receiver's back end and create errors to the receiving signal. In order to avoid this phenomenon, the out-of-band transmitting signal needs to be blocked from going into the GPS receiver path. Therefore, the GPS receiver path is required to have a good rejection on the out-of-band transmitting signal (interferer). By having a good rejection to the interferer, it will prevent the GPS chipset from being overloaded by the strong interfering power, and the chipset is able to provide a linear amplification to the received signal.

Reduce interference in GPS-enabled applications 1


1. Rx Front End Simplified Block Diagram.


GPS Filter to Preserve Receiver's Sensitivity and Linearity
Typically, the designer will put filters at both sides of the GPS LNA. A filter in front of the LNA helps to reject the out-of-band signal and prevent the LNA from being saturated. This filter should have a very low insertion loss. Putting a high insertion loss filter before the LNA should be avoided because this will increase the system's noise figure. According to Friis equation, the total noise figure is dominated by the noise figure or loss of the first stage. A second filter at the back of the LNA can be used to further improve the out-of-band rejection to prevent the later stage from being overloaded.

However, refer to the noise calculation shown in Figure 2, a front filter with insertion loss as low as 0.5dB in front of the LNA will still degrade the cascaded noise figure although the LNA has an exceptional good noise figure of 0.8dB. The cascaded noise figure is dominated by the first stage just when the gain is adequately high. The negative gain of the first stage filter causes the cascaded noise figure to degrade to 1.35dB. Besides, this solution involves three components (filter-LNA-filter).

Reduce interference in GPS-enabled applications 2
2. Noise Calculation for Filter-LNA-Filter GPS Receiver.


LNA-Filter Module Simplifies S-GPS Design
The solution explained in the previous section can be simplified to an LNA-filter solution by using an LNA with very good linearity as the first stage and a very good out-of-band rejection filter as the second stage. This section explains an LNA-filter module which is suitable to be used at the front end of a GPS receiver. The module is an integration of a low noise high linearity Enhancement Pseudomorphic HEMT (E-pHEMT) LNA and a low insertion loss superior-out-of-band rejection FBAR filter. This combination will create a front-end with excellent noise figure while maintaining the linearity.

E-pHEMT is Avago Technologies' proprietary technology which can produce highly linear LNAs. FBAR is a resonator technology developed by Avago Technologies that can produce small size filters with excellent quality factor (Q), which translates into a very steep filter roll off or superior out-of-band rejection. With the integration of FBAR filter, the LNA module offers sufficient rejection to the cellular and PCS bands which helps the receiver's performance in concurrent or simultaneous GPS (S-GPS) operation[3][4].

An LNA-filter module with high linearity enables it to handle higher input power without compressing the received signal. Ultimately, the filter in front of the LNA module can be omitted as long as there is enough isolation between the GPS path and the PCS or cellular paths. Without a front filter, the system's noise figure is now dominated by the LNA, where the noise figure can be as low as 0.8dB. This implementation would greatly improve the sensitivity of the receiver.

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