**4. GNSS receiver front-end**

GNSS receivers generally utilize RF down-conversion to an intermediate frequency (IF), using one or two conversions, followed by analog-to-digital conversion (ADC). The output of ADC is interfaced to a general purpose processor (GPP) for digital signal processing operations such as correlation, acquisition, tracking, and PNT extraction. The section where the signal remains analog, i.e. up to ADC, is usually termed as front-end. Receiver architectures based on how they process signal can be classified as superheterodyne, low IF, zero IF (homodyne), and direct-digital (bandpass sampling). Receiver performance metrics can be quite detailed but the most important ones are sensitivity, selectivity, inter-modulation characteristics, nonlinearities, and spur-free dynamic range.

Superheterodyne receiver is the most classical architecture, and has been utilized in many communication systems due to its excellent sensitivity, selectivity, and dynamic range. Typical configuration is shown in **Figure 11**. However, the architecture is not flexible for

**Figure 11.** Antenna unit and superheterodyne receiver front-end.

is inevitable. Filter after the first-stage LNA does not degrade the noise figure much and provides good selectivity and rejection for out-of-band signals. However, in the presence of a strong interferer, antenna LNA can be overloaded and signal acquisition can be lost.

**Figure 10.** Antenna LNA: (a) filter after first stage and (b) filter before first stage.

GNSS receivers generally utilize RF down-conversion to an intermediate frequency (IF), using one or two conversions, followed by analog-to-digital conversion (ADC). The output of ADC is interfaced to a general purpose processor (GPP) for digital signal processing operations such as correlation, acquisition, tracking, and PNT extraction. The section where the signal remains analog, i.e. up to ADC, is usually termed as front-end. Receiver architectures based on how they process signal can be classified as superheterodyne, low IF, zero IF (homodyne), and direct-digital (bandpass sampling). Receiver performance metrics can be quite detailed but the most important ones are sensitivity, selectivity, inter-modulation characteristics, non-

Superheterodyne receiver is the most classical architecture, and has been utilized in many communication systems due to its excellent sensitivity, selectivity, and dynamic range. Typical configuration is shown in **Figure 11**. However, the architecture is not flexible for

**4. GNSS receiver front-end**

176 Multifunctional Operation and Application of GPS

linearities, and spur-free dynamic range.

multi-standard systems, not well suited for integrated circuits due to filter requirements for image-reject and non-linear products, and consumes substantial power.

To overcome the drawbacks of superheterodyne receiver, low-IF and zero-IF receiver configurations are proposed. Typical architecture of these receivers is displayed in **Figure 12**. In zero-IF configuration, image problem is completely removed, hence sharp and IC unfriendly image reject filters are not needed. Since gain is shared between RF and baseband amplifiers, requirements on these amplifiers become complicated to have sufficient SFDR. But, the most troubling problem of zero-IF is that the leakage of LO mixes with itself, putting severe requirements on second-order intermodulation products of the receiver. This leakage causes DC offsets at the baseband, and high baseband gain amplifies these offsets together with flicker noise to degrade receiver performance. Also, Doppler shifts of received signals can be lost. Low-IF configuration overcomes these problems but image issue comes up again and usually resolved by image-reject mixer design, which is not so easy in IC topology. Despite severe requirements on gain, noise, and linearity, low power budget and flexibility in DSP made these architectures very popular, especially among receiver ICs.

Today's multi-band and multi-constellation receivers mostly utilize software-defined radio architecture to process digital I and Q signals. When two or more frequencies of GNSS are targeted, the GPP unit of the receiver becomes critical for signal quality and high data throughput, which may require parallel processing of correlations [53]. In contrast to communication systems

**Figure 12.** Zero-IF and low-IF receiver front-end. IF filters are either low-pass (zero-IF) or bandpass (low-IF).

where received signal SNR is high, GNSS receivers rely on long coherent integration (>1 ms) to exploit processing gain. Clock stability becomes an issue especially during carrier phase tracking. Instead of quartz oscillators, temperature-compensated crystal oscillators (TCXO) with an accuracy better than one part per million with very low phase noise are used in high-end receivers.

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