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A GNSS signal is captured through the receiver's [[Antennas|antenna]], and is fed to the front end section. The front end is then responsible for "preparing" the received signals for [[Digital Signal Processing|signal processing]] tasks, and many different implementations can achieve the desired results. As allways, some requirement and trade-off analysis is needed when designing a front-end for GNSS receivers, depending on the application at hand. Figure 1 illustrates a typical front end structure in GNSS receivers. | |||
[[File:front_end.png|center|thumb|650px|'''''Figure 1:''''' Example of GNSS receiver's front end structure.]] | |||
Within a front end, the ''frequency synthesizer'' (shown in Figure 1) is the heart, by providing the receiver time and frequency reference for all the components. Such components, at front end architecture level, gather typical interconnected steps to process and convert a RF signal to a [[Baseband Processing|baseband]] digital signal: | |||
*'''Filtering and amplification''': these stages are necessary to ensure low noise and out-of-band rejection in the received signals, as well as amplification stages to compensate for transmission losses. | |||
*'''Down-conversion''': The front end is responsible for down-converting the signal, whether using ''direct conversion''<ref>http://www.gpsworld.com/gnss-system/galileo/pulling-wideband-7053</ref> techniques, to convert the RF spectrum directly to baseband, or ''heterodyning''<ref>[[wikipedia:Heterodyne receiver]]</ref> approaches, where typically a multi-stage approach is used to shift the spectrum to intermediate frequencies (IF), with the appropriate band filtering, ultimately converting the IF signals to baseband<ref>[[wikipedia:Digital down converter]]</ref>. | |||
*'''Quantization''': The incoming signals are ditized through ''analog to digital converters''<ref>[[wikipedia:Analog-to-digital converter]]</ref> (ADC), ensuring that quantization errors and dynamic ranges are appropriate to accomodate the signal's characteristics. | |||
*'''Automatic Gain Control''': The ''automatic gain control''<ref>[[wikipedia:Automatic gain control]]</ref> (AGC) stage is closely related to the down-conversion and quantization steps, and is responsible for adjusting the gain of the front end section to an appropriate level, in terms of the signal's range of input and output levels. | |||
These operations and different stages in front end signal processing are described in the following sections. | |||
==Filtering and Amplification== | |||
Due to the GNSS signal's low power upon reception, there is usually a set of filtering and low-noise amplification stages after the antenna. Although the implementation itself varies between architectures and hardware realizations, the principle is the same: the signal is amplified, keeping the noise figure low and rejecting possible out-of-band interference. This can be achieved by interleaving Low-Noise Amplifiers (LNA) and filter stages. For more details on amplification, filtering and losses see the [[Antennas|antenna]] section. | |||
==Down-conversion== | |||
==Quantization== | |||
==Automatic Gain Control== | |||
==From RF to Baseband== | |||
==Related articles== | |||
*[[Generic Receiver Description]] | |||
*[[System Design Details]] | |||
==References== | |||
<references/> | |||
[[Category:Receivers]] | [[Category:Receivers]] | ||
Revision as of 11:15, 8 April 2011
| Receivers | |
|---|---|
| Title | Front End |
| Author(s) | GMV |
| Level | Advanced |
| Year of Publication | 2011 |
A GNSS signal is captured through the receiver's antenna, and is fed to the front end section. The front end is then responsible for "preparing" the received signals for signal processing tasks, and many different implementations can achieve the desired results. As allways, some requirement and trade-off analysis is needed when designing a front-end for GNSS receivers, depending on the application at hand. Figure 1 illustrates a typical front end structure in GNSS receivers.
Within a front end, the frequency synthesizer (shown in Figure 1) is the heart, by providing the receiver time and frequency reference for all the components. Such components, at front end architecture level, gather typical interconnected steps to process and convert a RF signal to a baseband digital signal:
- Filtering and amplification: these stages are necessary to ensure low noise and out-of-band rejection in the received signals, as well as amplification stages to compensate for transmission losses.
- Down-conversion: The front end is responsible for down-converting the signal, whether using direct conversion[1] techniques, to convert the RF spectrum directly to baseband, or heterodyning[2] approaches, where typically a multi-stage approach is used to shift the spectrum to intermediate frequencies (IF), with the appropriate band filtering, ultimately converting the IF signals to baseband[3].
- Quantization: The incoming signals are ditized through analog to digital converters[4] (ADC), ensuring that quantization errors and dynamic ranges are appropriate to accomodate the signal's characteristics.
- Automatic Gain Control: The automatic gain control[5] (AGC) stage is closely related to the down-conversion and quantization steps, and is responsible for adjusting the gain of the front end section to an appropriate level, in terms of the signal's range of input and output levels.
These operations and different stages in front end signal processing are described in the following sections.
Filtering and Amplification
Due to the GNSS signal's low power upon reception, there is usually a set of filtering and low-noise amplification stages after the antenna. Although the implementation itself varies between architectures and hardware realizations, the principle is the same: the signal is amplified, keeping the noise figure low and rejecting possible out-of-band interference. This can be achieved by interleaving Low-Noise Amplifiers (LNA) and filter stages. For more details on amplification, filtering and losses see the antenna section.