2.7  ANALOG/DIGITAL RECEIVERS

The majority of early GPS receiver designs made extensive use of analog signal

processing techniques, however, most modern receivers incorporate digital signal

processing to replace analog receiver functions wherever possible.  The following

examples are provided to give a description of the differences between these two

design techniques.  Figure 2 1 shows a multi channel GPS receiver in which code

correlation is performed using analog mixing techniques at the intermediate

frequency (IF).  Each satellite signal to be tracked requires a separate hardware

processing channel which consists of an analog correlator, code translator, IF

stage, and base band converter.  The bandwidth of the IF stage is designed to

accommodate the GPS data rate and maximum carrier doppler shifted frequency.

ANALOG

DIGITAL

PRESELECTOR

GAIN

FINAL

DOWN

CORRELATOR

A/D

IF

CONVERSION

CHANNEL 1

TRANSLATOR

SIGNAL

AND

DATA

PROCESSING

CHANNEL 2

OTHER CHANNELS

Figure 2 1.  Analog GPS Receiver Architecture

Figure 2 2 illustrates a GPS receiver using a largely digital architecture.  Analog

signal processing is limited to preselection and gain applied to the GPS signals

during down conversion with fixed translation frequencies.  The down converted

signals are digitized through sampling and are then ready for further digital

processing.  The digital signal processor (DSP) functions shown in Figure 2 2

include correlation, code and carrier acquisition, and data recovery.

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