Global Positioning System Reference
In-Depth Information
TABLE 7.2. Various types of delay lock loop discriminators and a description of them
Type
Discriminator
D
Characteristics
Coherent
I
E
−
I
L
Simplest of all discriminators.
Does not require the
Q
branch
but requires a good carrier
tracking loop for optimal
functionality.
I
E
+
Q
E
)
−
(
I
L
+
Q
L
)
(
Early minus late power. The
discriminator response is nearly
the same as the coherent dis-
criminator inside
1
2
±
chip.
I
E
+
Q
E
)
−
(
I
L
+
Q
L
)
(
Noncoherent
Normalized early minus late
power. The discriminator has
a great property when the chip
error is larger than a
I
E
+
Q
E
)
+
(
I
L
+
Q
L
)
(
1
2
chip;
this will help the DLL to keep
track in noisy signals.
I
P
(
I
E
−
I
L
)
+
Q
P
(
Q
E
−
Q
L
)
Dot product. This is the only
DLL discriminator that uses all
six correlator outputs.
where the phase of the carrier replica drifts compared to the phase of the incoming
signal. The upper plot shows the output of the three correlators in the in-phase
arm, and the lower plot shows the correlation output in the quadrature arm of
the DLL with six correlators. This situation is a result of different frequencies
for the signal and the replica; it results in a constantly changing phase difference
(misalignment). There are a few reasons why this can happen, for example, the
PLL could be not in a lock state.
Figure 7.14 shows a case when the PLL is in a lock state. Because of the precise
carrier replica from the PLL, it is seen in Figure 7.14 that the correlators are
constant over time. This would not be the case if the carrier replica is not adjusted
to match the frequency and phase of the incoming signal.
If the code tracking loop performance has to be independent of the performance
of the phase lock loop, the tracking loop has to use both the in-phase and quadra-
ture arms to track the code.
The DLL now needs a feedback to the PRN code generators if the code phase
has to be adjusted. Some common DLL discriminators used for feedback are listed
in Table 7.2.
The table shows one coherent and three noncoherent discriminators. The re-
quirements of a DLL discriminator is dependent on the type of application and the
noise in the signal. The discriminator function responses are shown in Figure 7.15.
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