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| 型号： | GP2000 |
| 厂商： | Mitel Networks Corporation |
| 英文描述： | () |
| 中文描述： | （） |
| 文件页数： | 7/25页 |
| 文件大小： | 256K |
| 代理商： | GP2000 |
15
GP2000 – GPS CHIPSET DESIGNER’S GUIDE
~ !
~
!
Expressing this in the time domain gives:
Over a sample interval
D
T :
therefore:
T13(yi2yi21) = T23(xi2xi21)1XiDT
or more conveniently:
yi = yi211
3 (xi2xi21)1
3 xi
The loop is designed so that the factors
(T2/T1) and
(DT/T1) are powers of 2 to reduce processor loading.
T1 and T2 are related to the loop characteristics as follows:
Loop natural frequency,
vn =
Damping factor,
z =
vn
Pull-in range
Dvp =
4
= 2zv
nKf K02vn
2
Pull-in time from
Dv0, Tp =
3
Pull-out range,
Dvp0 = 183vn3(z11)
Closed loop bandwidth,
W = vn
Hz
K
f and K
0 are the phase error detector gain (the EML dis-
criminator gain, units per radian of code phase error) and the
code DCO conversion gain (rad/s per control unit) respectively.
Carrier Acquisition & Tracking
Carrier Tracking Loop
The carrier tracking loop tracks the incoming carrier to pro-
duce carrier cycle and carrier phase measurements to smooth
the code pseudo-ranges. It also aids the code tracking loop since
the ratio of the code to carrier frequency is a constant. Carrier
acquisition and tracking is initially achieved using a 4-quadrant
frequency discriminator to reduce the frequency error from a
few hundred Hertz to a few Hertz.
This is followed by a 2nd order Frequency-Locked Loop (FLL)
which has zero steady-state error for a constant rate of change
in frequency (acceleration).
FLLs offer superior dynamic performance, robustness and
insensitivity to interference over PLLs.
Four-Quadrant Frequency Discriminator
The discriminator correction term is derived by comparing
I and Q correlations between successive readings (assuming a
data bit transition has not occurred):
DI = Ik2Ik21
DQ = Qk2Qk21
The choice of correction and sign of application is determined
from the current correlations and their respective magnitudes
(see Fig. 15).
For
_Ik_._Qk_: IFIk.0 correction = DQ,ELSEcorrection = 2DQ
For
_Ik_<_Qk_: IF Qk.0 correction =2DI, ELSE correction= DI
Kf3K0
T1
=
T2
2
(xi2xi21)
DT
dx
dt
(yi2yi21)
DT
dy
dt
=
;
=
dx
dt
dy
dt
T13
=
T23
1x
T2
T1
DT
T1
~ !
p
114
z 2
8
z
p 2
16
Dv0
2
zvn
3
Frequency-Locked Loop
The discriminator correction term is derived by comparing
I and Q correlations between successive readings (assuming a
data bit transition has not occurred) using the cross product:
fk = QkIk21 2 IkQk21
For
Ik
2 1 Q
k
2 ≈ I
k21
2 1 Q
k21
2 :
fk = (Ik
21Q
k
2) sin(f
k2fk21)
where
fk and fk21 are the carrier phases at successive
readings.
Therefore, for small
fk2fk21,
fk ≈ (Ik
21Q
k
2)Df,
where
Df is the carrier phase change over 1 millisecond.
Using a discretised second order Jaffe-Rechtin filter of
bandwidth
BLF, and normalising to the correlation power, the
following frequency correction terms can be derived for the
estimated carrier frequency:
Dv = vk2vk21 = Tvk1 =2vnF fk
vnF = 189BLF
T = sampling interval of 1ms.
The optimum setting for
BLF for a given rate of change of
acceleration (jerk) is given by:
=
Carrier Lock Indicator
Carrier lock can be monitored by averaging the dot product
between correlations:
Lock indicator =
IkIk211QkQk21
This quantity averages to zero until the loop locks which
in turn drives the cross product,
QkIk212IkQk21, to zero.
Once locked, the average value of the dot product can be
used to estimate the signal power.
025
T
Fig. 15 Four-quadrant frequency discriminator
v
2
nF
..
.
D
Q2VE
D
Q1VE
D
Q1VE
D
Q2VE
D
Q1VE
D
Q2VE
D
Q2VE
D
Q1VE
D
I2VE
D
I1VE
D
I1VE
D
I2VE
D
I1VE
D
I2VE
D
I2VE
D
I1VE
Q
I
Qk>0
Qk<0
Ik>0
Ik<0
_
Ik_>_Qk_
_
Qk_>_Ik_
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