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7: 12-Channel Correlator
GP4020 GPS Baseband Processor Design Manual 55
7.2.5 Carrier Mixers
The Carrier Mixers multiply the digital input signal by the Carrier DCO digital local oscillator to generate a signal at
baseband. Both the I and Q Carrier DCO phases are directed to the appropriate mixers. The mixing of the Carrier
DCO outputs with the input signal that produces a baseband signal, which can have the values ±1, ±2, ±3 and ±6.
7.2.6 Code Mixers
The Code Mixers multiply the I and Q baseband signals from the Carrier Mixers with both the PROMPT and
TRACKING local replica codes to produce four separate correlation results. The correlation results are passed to
the Accumulate and Dump blocks for integration.
7.2.7 Accumulate and Dump
The Accumulate and Dump blocks integrate the Mixer outputs over a complete code period of nominally 1ms.
There are four separate 16-bit accumulators for each channel. These represent the correlation of the I and Q
signals with the PROMPT and TRACKING codes, over the integration period. There is no overwrite protection
mechanism on these registers so the data must be read before the next DUMP.
7.2.8 Code Phase Counter
The Code Phase Counter counts the number of half–chips of generated code and stores this value in the
CHx_CODE_PHASE register on each TIC.
7.2.9 Code Slew Counter
The Code Slew Counter is used to slew the generated code by a number of half chips in the range 0 to 2047. In
Update mode, the slew occurs following the next DUMP. In PRESET mode, it occurs at the next TIC. All slew
operations are relative to the current code phase. The Code Slew counter must be written to, each time a slew is
required. During the slewing process, the accumulators for the channel being slewed are inhibited so that the first
result is valid. If a slew is written while a channel is disabled, the slew will occur as soon as the channel is enabled.
7.2.10 Epoch Counter
The Epoch Counters keep track of the number of code periods over a 1-second interval. This is represented by a 5-
bit word for the number of 1 ms integration periods (0 to 19), plus a 6-bit word containing the number of 20 ms
counts (0 to 49). The Epoch Counters can be pre-loaded to synchronise them to the data stream coming from the
satellite. This value will be transferred immediately to the counter when in Update Mode, or after the next TIC if in
PRESET Mode.
The Epoch Counter values are latched on each TIC into the CHx_EPOCH register. In addition, the instantaneous
values are available from the CHx_EPOCH_CHECK register.
7.3 Software Requirements
The GP4020 12-channel correlator can be operated in different ways dependent upon the GPS Receiver system
required by the user. So to accommodate this and to allow dynamic adjustment of loop parameters, the GP4020
12-channel correlator has been designed to use software for as many functions as possible. This flexibility means
that the device cannot be used without a microprocessor closely linked to it. Since a processor is always needed to
convert the output of the 12-channel correlator into useful information, this is not a significant limitation.
The software associated with the 12-channel correlator can be divided into two separate modules:
1) Acquire and track satellite signals to give pseudo-ranges;