Application of Low Noise Amplifier in Mobile Phone GPS

As early as after the 911 terrorist incident in 2001, the United States, for security reasons, required mobile phones to have GPS positioning functions to determine the real-time location of the phone. In recent years, with the rapid development of my country's infrastructure, road construction has changed with each passing day; people's work and life rhythms have accelerated, and GPS navigation and positioning systems have become increasingly important and demand has increased. Recently, there was news that Nokia has announced a new version of Ovi Maps for its smartphones, which will include high-end walking and driving navigation. This move is seen as a huge change in the GPS mobile navigation industry and GPS navigation-related industries. With the general improvement of mobile phone performance and the solution of unique problems of mobile phone GPS receivers, GPS functions are no longer exclusive to high-end mobile phones, and are developing into standard features of ordinary mobile phones.

GPS Function Introduction

GPS is the full name of Global Positioning System, which consists of 24 satellites distributed in 6 orbits at different altitudes. It is divided into two functions: navigation and positioning; and divided into military and civilian applications. The military frequency is 1.227GHz, which is exclusively used by the US military. It is mainly used for military and war purposes such as weapon guidance and reconnaissance, with high accuracy. The civilian frequency is 1.575GHz, which is free for global use. It is mainly used in naval aviation, natural sciences, and more widely in the field of car navigation. Only when valid signals from 3 stars are received at the same time can its position be determined.

The GPS signal is weak and the receiver sensitivity is usually around -136dBm.

Challenges of Mobile GPS

Features of mobile phone applications: small size, low power consumption, good productivity, cheap components, short design time, and fast product launch. The GPS function based on the mobile phone platform has higher requirements and is more challenging than that of an independent GPS receiver. The following aspects should be considered: electronic environment, power consumption, size, price, convenience in design, and easy realization of performance indicators.

Common single-chip GPS receivers use CMOS technology and cannot achieve high receiver sensitivity. Compared with cars driving on a relatively wide road, handheld GPS is more flexible than vehicle-mounted ones, and the signal is weaker at the location, and multipath reception is more common. In order to achieve stronger receiving capabilities, it is necessary to add filters and low-noise amplifiers at the front end of the receiver.

GPS design considerations for mobile phones

The electronic environment is even worse. GPS receiver and mobile phone amplifier are on the same board. For 1.575GHz GPS signal, the out-of-band signals are mainly: 900MHz, 1800MHz GSM amplifier; 1800-1900MHz PCS amplifier; 2.4GHz WLAN, Bluetooth; and 2.5-2.7GHz WiMAX. Although these signals will not exist at the same time, each is very strong and has a significant impact on GPS reception performance. Experiments show that: -15dBm out-of-band signal will cause the gain of GPS low noise amplifier to drop by 1dB. Therefore, when choosing a mobile phone GPS low noise amplifier, we should not only look at its own noise coefficient and gain, but also pay attention to its out-of-band attenuation index, which is crucial to the capture ability, tracking performance and positioning accuracy of the GPS receiver, and even whether it can work normally.

CDMA and WCDMA mobile phones work in duplex mode, and the transmission signal and noise of their power amplifiers will interfere with or even block the GPS signal. Therefore, the GPS receiver must be equipped with an LNA and a high-performance filter. The LNA increases its receiving sensitivity, while the filter enhances the out-of-band attenuation. Another benefit in design and wiring is that the LNA can be placed close to the antenna end, reducing the requirements for the location of the GPS receiving module and improving the noise figure of the GPS receiver by about 1.5dB. However, since the GSM mobile phone transceiver works in time division, the out-of-band attenuation requirements for the GPS signal are relatively not very stringent, and the high-performance filter can be omitted, and a simple filtering plus low noise amplifier solution can be used to implement the GPS receiving front end to save costs.

There are currently several solutions for mobile phone GPS receiving front-ends for different systems and platforms:

--In the GMS system, active antennas are selected, that is, the low noise amplifier and GPS receiving antenna are integrated together;
--In the CDMA and WCDMA systems, there are discrete low noise amplifiers plus filters, and the materials and performance of the two devices can be selected. The above solutions are challenges to design time, design difficulty, performance indicators, PCB size and number of components.

As market demand changes, GPS receiver front ends that integrate filters and low noise amplifiers have been widely used, and their benefits are obvious. Like discrete devices, the two have different combinations: LNA+ceramic filter or LNA+SAW (surface acoustic wave filter). In the market, the CaAs E-pHEMT+FBAR solution has the best performance: low noise, better linearity, and better out-of-band attenuation.

Avago is committed to GPS receiving front-end module

Avago combines the industry-leading CaAs E-pHEMT process and FBAR filter technology to provide a series of GPS RF front-end modules, providing a variety of options for GPS plus low-noise amplifier front-ends, as shown in Figures 1, 3, 4, and 5. The structures are: Filter + LNA + Filter; Filter + LNA; LNA + Filter; or pure LNA. When planning and designing these products, Avago fully considered their application environment, and they are the first choice for mobile phone GPS reception design.

Features of CaAs E-pHEMT low noise amplifier

Compared with other materials and processes, the advantages of CaAs E-pHEMT are: low noise figure, making it the first choice for low noise amplifiers; high linearity at low operating voltage, which improves the dynamic range of the receiver and is beneficial to the subsequent signal mixing and demodulation; high gain is the value of the amplifier. Therefore, CaAs E-pHEMT is the perfect combination of the three important indicators of low noise amplifiers: noise figure, linearity and gain, which is difficult to achieve for Si material low noise amplifiers.

Characteristics of FBAR filter

Ceramic filters can withstand large power and have good temperature characteristics, but they are large in size and their Q value is not as good as FBAR.
SAW filters can withstand small power, have poor temperature characteristics, have low Q value, and are cheap. They are also used in some mobile phones with low performance requirements
. FBAR filters: good performance, small size, and easy to integrate. With the increasing number of duplexers and filters replacing SAW in mobile phones, their cost-effectiveness has been recognized by everyone.

ALM-1712 GPS Receiver Front-end Example

Figure 1 below shows Avago's ALM-1712 product, which consists of two FBAR filters and an LNA, with a total noise figure of 1.65 dB for the three stages. The RF-side ESD is >3KV, eliminating the need for external ESD protection devices, saving space and cost. It works as low as 1V, and the shutdown function meets the low power consumption requirements of mobile phone applications, while the variable bias provides an ideal balance among various performance indicators. The package is 4.5 (L) x 2.2 (W) x 1 (H) mm3, and the peripheral devices are only 8 resistors, capacitors and inductors.

Figure 1: Avago ALM-1712 product

Figure 2 below shows that the gain at 1.575GHz is 12.8 dB under Vdd = 2.7V, Idd = 8.0mA bias; linear IIP3 = +7 dBm, IP1dB = +3 dBm, and the out-of-band attenuation at 900MHz is as high as 95dBc, and the out-of-band attenuation at 1800MHz is as high as 90dBc, which effectively ensures that the GPS receiver can function well in harsh electronic environments. ALM-1712 meets the special requirements of mobile phone GPS: electronic environment, power consumption, volume and cost requirements, as well as design time and performance indicators, which are self-evident for mobile phone manufacturers.

Figure 2

Figures 3, 4, and 5

Below are the main indicators of two common models in the market. In the case that both low-noise amplifiers use CaAs E-pHEMT technology, it is not difficult to see that the FBAR filter's suppression of the PCS band is 21dB higher than that of the SAW (63 vs. 42)

Figure 1 is the development roadmap of Avago's GPS receiver front-end product line. It is not difficult to find that Avago attaches great importance to the GPS receiver front-end module and low noise amplifier market. With the wide application of products with different functional performance in GPS receivers, Avago will continue to launch more products that meet the market. For details, please consult Avago's important agent in the Asia-Pacific region, Shiqiang Telecom .

Figure 1: Avago Technology GPS Receiver Front-end Product Development Roadmap

Aiming at GSP, ISM and WiMAX markets, Avago recently launched the MGA-24106 low noise amplifier, which inherits the advantages of noise figure, gain and linearity indicators of Avago low noise amplifier products. Under 3.3V6mA bias, 1.57GHz, the noise figure is 0.87, the gain is 19.6dB, the IP1dB is -8.7dBm, and the IIP3 is -0.2dBm. It can work normally at a voltage as low as 1.8V. It is packaged in 1.5x1.2x0.5mm3 and only requires a few peripheral components. It is superior in size, power consumption and low voltage operation, and its price is more competitive in the market. It has broad application prospects in handheld and mobile terminal wireless devices.

Figure 2 shows the performance comparison of FBAR, SAW and ceramic filters for mobile phones. FBAR technology is another leading technology product line of Avago, and occupies a large share in duplexers, multiplexers and various filters in mobile phones.

Figure 2: Performance comparison of FBAR, SAW and ceramic filters