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Reduce interference in GPS-enabled applications

Posted: 16 Jul 2007 ?? ?Print Version ?Bookmark and Share

Keywords:GPS? GPS-enabled applications? reduce interference?

By Chew Ean Tan
Avago Technologies

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, the GPS chipset can be prevented from being overloaded by the strong interfering power, and the chipset is able to provide a linear amplification to the received signal.

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).

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.

Integrating the LNA with the filter also causes the module's input impedance to look more concentrated (small impedance spreading on Smith chart) due to the filter's narrow bandwidth. This will make the impedance matching between the antenna and the input LNA module much easier as compared to a discrete LNA without a post-filter. A single IC solution also ensures a more reliable and consistent receiver performance.

Netgear WNR854T

3. Measurement setup for out-of-band blocking ability
Click for larger image

Analysis and discussion
Figure 3 shows the test set up for gain compression measurement on a GPS signal due to the out-of-band interferer. The power level of the PCS/cellular band signal is set according to the values in the table in Figure 3 to represent the range of isolation between GPS path and PCS/cellular path. The input power level of the 1.575GHz GPS signal is fixed at -35dBm while the output power from the power amplifier at the PCS and cellular is taken at +24dBm. The isolation level is varied by applying different input power level at the input of the LNA module (or GPS antenna). The input power contributed by the interferer at the input of the module can be calculated by using the following formula:

Input Power at GPS antenna = Interferer Power Level " Isolation

For example, when the isolation between the GPS path and PCS/cellular path is 15dB, the interferer's input power level at the GPS antenna is calculated to be +9dBm. The GPS and interferer (PCS/cellular) signals are combined together using a power combiner.

4. Gain compression vs. isolation

The measured gain compressions for GPS signal over various isolation levels are shown in Figure 4. From the measurement result, in order to avoid the GPS signal from being compressed by the interferer power, the GPS path and PCS/cellular path need to have 40dB of isolation. This will mean that by having 40dB isolation between the GPS and the PCS or cellular path, the filter-LNA-filter solution can be replaced by this LNA module.

Referring to Figure 1 and Figure 3, while the PCS power amplifier is transmitting a +24dBm output power and the isolation between the GPS-PCS paths is 40dB, the power level from the interferer signal that leaks into the GPS receiver chipset:-

Interferer Power Level to GPS Chipset :
= PCS PA Output Power " GPS-PCS Isolation " LNA Module's PCS Band Rejection
= +24dBm - 40dB " (54dBc - 13dB) = -57dBm

Conclusion and implementation
The LNA module is effectively blocking the PCS signal from leaking into the GPS chipset. The interferer power is as low as -57dBm while having 40dB isolation between GPS path and PCS path. With proper arrangement and design, the single LNA module solution is able to replace the filter-LNA-filter solution which has higher noise figure and more complicated architecture. This single chipset solution offers a superior system's noise figure and great linearity while having excellent out of band rejection. In addition, this LNA module provides a simple, compact and low manufacturing cost solution to the designer while maintaining the good performance of the GPS receiver. The design cycle is shorter as well due to the simple matching network design.

In order to achieve 40dB isolation, a dual-antenna solution can be used. By using a dual-antenna solution, the GPS signal will have a separate path or chain from the PCS/cellular signal to meet the 40dB isolation. Figure 5 shows the block diagram of the dual-antenna design.

5. The latest RFICs are adding enough logic to be considered SoCs

References
1. What is GPS?
2. S. Spiegel et al. "Improving the Isolation of GPS Receivers for Integration with Wireless Communication Systems," Proc IEEE RFIC Symposium, pp563-566, 2003.
3. Yut H. Chow et al, "A 1V, 0.9dB Noise-Figure High Linearity LNA MMIC for Concurrent GPS Handset application," Proc Asia Pacific Microwave Conference, 2006.

About the Author
Chew Ean Tan
is an application engineer with Avago Technologies' Wireless Semiconductor Division. Chew Ean holds a BEng (HOns) in Electrical and Electronic Engineering from University of Technologies Malaysia. She can be reached at chew-ean.tan@avagotech.com.




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