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GP2021/IG/GQ1Q 参数 Datasheet PDF下载

GP2021/IG/GQ1Q图片预览
型号: GP2021/IG/GQ1Q
PDF下载: 下载PDF文件 查看货源
内容描述: [Correlator, 16-Bit, CMOS, PQFP80, 14 X 14 MM, 2 MM HEIGHT, MS-022BC, MQFP-80]
分类和应用: 时钟外围集成电路
文件页数/大小: 63 页 / 511 K
品牌: ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
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GP2021  
of the embedded real-time clock on the chip to give a good  
estimate of GPS time for the next fix; the navigation  
solutioncan be used to measure clock drift and calculate a  
correction for the clock to overcome ageing. The user’s  
location (or a good estimate of it) along with the Almanac  
and the correct time will indicate which satellites should  
be searched for. These may be used to find an estimate of  
Doppler effects, while the previous clock error is the best  
available estimate of the present clock error. If this  
information is not available then the receiver must scan a  
much wider range of values, which will greatly increase  
the time to lock. The satellite Clock Correction and  
Ephemeris are needed for the navigation solution, so if a  
recent set is held in memory the calculations may begin  
as soon as lock is achieved and not need to wait for the  
Satellite Navigation message re-transmission (18 to 36  
seconds).  
situations) the signal-to-noise ratio (SNR) is poor for some  
satellites. As a result, the software must be ‘wideband’ to  
find the signal and also ‘narrow band’ to reduce noise,  
leading to very different programs in different applications.  
For all tracking channels, the signal processing software  
needs the following sequence of activities:  
1. Program CHx_SATCNTLregister to select the desired  
GPS Gold code (PRN number) for the selected satellite  
and also select the code type for the mode of the  
correlator tracking arm. It is often best, when in  
acquisition mode, to fix the tracking arm to Dithering  
Mode (alternate ‘Early’ and ‘Late’) and do a search in  
two phases at once and then switch to a tracking mode  
once a satellite is found.  
2. Program  
CHx_CARR_INCR_LO  
and  
CHx_CARR_INCR_HI registers. The values  
programmed into these two registers are concatenated  
and are used to set the local oscillator frequency for the  
digital mixing performed in the 12-channel correlator to  
bring the incoming 2-bit digitised signal from the RF  
front end down to baseband. The value to be  
programmed is equal to the nominal local oscillator  
frequency plus the estimated Doppler shift  
compensation plus the estimated user clock frequency  
drift compensation.  
The 12-channel correlator on the GP2021 contains four  
different types of registers:  
G Control Registers that are used to program functions  
of the device.  
G Status Registersthat provide a status indication of the  
process taking place in the device.  
G Accumulated Data Registers that provide the results  
of correlation with the C/Acode every millisecond. This  
is the raw data used to acquire and track satellite signals.  
G Measurement Data Registers which latch the carrier  
DCO phase, carrier cycle count, code DCO phase,  
1millisecond epoch, and the 20 millisecond epoch count  
at every 9·09 or 100 milliseconds interval. This is the  
raw data used to compute pseudorange.  
3. Program  
CHx_CODE_INCR_LO  
and  
CHx_CODE_INCR_HI The value to be programmed  
in these registers represents twice the nominal chipping  
rate of the C/A code (2·046 MHz) plus, if desired, a  
small compensation for the Doppler shift and user clock  
frequency drift.  
4. Release the tracking channel reset by programming  
the RESET_CONTROL register with the proper value.  
This will cause the correlation process to start.  
Software Sequence For Acquisition  
The spectrum of each Navstar GPS satellite signal is  
spread-spectrum modulated using 1023 chip Gold codes.  
This causes the satellite signals seen by a GPS receiver  
to be so weak that they are buried in the noise and can  
only be detected by correlation. The GPS signal power at  
a GPS receiver antenna is typically approximately  
2130dBm, whereas the noise in the GPS signal band  
(2·046MHz wide) is approximately 2111dBm at room  
temperature. To correlate the received signals therefore,  
a locally generated code must be chosen to precisely match  
the spreading code type, rate, and phase.  
5. Obtain accumulated data from Accumulated Data  
Register readings. Several consecutive readings on the  
same tracking channel can be added to increase, at  
will, the integration period of the correlation.  
6. Decide if the GPS signal has been found by  
comparing the correlation result with a threshold. If found  
then jump to a signal pull-in algorithm. Note that both  
in-phase and quadrature phase accumulated data have  
to be considered since at this time, the carrier DCO  
local oscillator phase is not necessarily in phase with  
the incoming GPS signal.  
7. If the GPS signal has not been found, a new trial has  
to be made with different carrier DCO, code DCO, or  
Gold code phase programming. Typically, both DCOs  
would be held constant while the Gold code phase is  
varied to try all of the 2046 half chip positions possible,  
then the carrier DCO would be programmed with slightly  
different values and the Gold code phase positions  
would again be scanned. The Gold code phase is varied  
by programming the CHx_CODE_SLEW register and  
can be varied by increments of half a code chip.  
This pattern is then multiplied bit-by-bit with the incoming  
data stream and the results integrated over the code length  
to recover the signal.  
The process of signal acquisition is simply the matching of  
receiver settings to the actual signal values. To make  
matters more complicated the satellite carrier frequency is  
shifted a little by the Doppler effect due to the motion of  
the satellite, the user clock will drift randomly, and (in most  
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