GP2021 datasheet(10/62 Pages) MITEL

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GP2021

be a 0. To avoid having an unused LSB in the accumulators,

the values in EARLY–MINUS–LATE mode are halved from

the +2, 0, –2 that results from the calculation (+1 or –1) – (+1

or –1) to +1, 0, –1. This must be considered when choosing

thresholds in the software, as the correlation results will be

exactly half of the values otherwise expected.

At the end of every code sequence (1023 chips in GPS

mode or 511 chips in GLONASS mode) a DUMP signal is

generated to latch the Accumulated Data for use by the signal

tracking software. Each channel is latched separately, as the

satellite signals are not received in phase with each other.

Source Selector

In Real_Input mode the Source Selector selects which

input signal pair to use (SIGN0/MAG0 or SIGN1/MAG1). In

Complex_Input mode SIGN0/MAG0 are passed to the In–

phase arm and SIGN1/MAG1 to the Quadrature arm. The data

is treated as having the values shown in Table 2 below (in both

modes):

Carrier Mixers

The Carrier Mixers multiply the digital input signal by the

Carrier DCO digital local oscillator to generate a signal at

baseband. In Real_Input mode both I and Q Carrier DCO

phases are directed to the appropriate mixers. In

Complex_Input mode a single In–Phase Carrier DCO output

is used in both mixers since the input signal is already in I and

16 BIT ACCUMULATE

AND DUMP – Q_TRACKING

16 BIT ACCUMULATE

AND DUMP – Q_PROMPT

CODE

SLEW

C/A CODE

GENERATOR

CODE

PHASE

COUNTER

CODE

DCO

EPOCH

COUNTERS

DUMP

ACCUMS, CODE PHASE, ETC

16 BIT ACCUMULATE

AND DUMP – I_PROMPT

16 BIT ACCUMULATE

AND DUMP – I_TRACKING

CARRIER

CYCLE

COUNTER

CARRIER

DCO

SOURCE

SELECTOR

SIGN 0

AND

MAG 0

SIGN 1

AND

MAG 1

SELECT

SOURCE

AND

SELECT

MODE

IN AND OUT

DATA

BUSSES

CODE

MIXER

CARRIER

MIXER

1, 0

Fig.5 Tracking Module block diagram

Sig

Mag

Value

-3

-1

Table 2 SIGN/MAG values

Q form. The mixing of the Carrier DCO outputs with the input

signal produces a baseband signal which can have the values

±1, ±2, ±3 and ±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 4 separate

correlation results. The correlation results are passed to the

Accumulate and Dump blocks for integration.

Accumulate and Dump

The Accumulate and Dump blocks integrate the Mixer

outputs over a complete code period of nominally 1ms.

There are 4 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.

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.

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 it will occur

as soon as the channel is enabled.

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