Satellite Signal Acquisition, Tracking, and Data Demodulation - Understanding GPS: Principles and Applications - page 234

K·DT

DT

K

(·) 2 I P

Σ (·)

+

Σ

LPF

C

+

DT

K·DT

K

(·) 2

Q P

Σ (·)

÷

C/N 0 EST.

N 0

DT K·DT

K

1
K

Scale
Factor

(·) 2

Q N LPF

Σ (·)

Figure 5.42 Basic carrier-to-noise power ratio meter.

a basic phase lock indicator design, which provides an optimistic phase lock indica-
tor that decides quickly and changes its mind slowly but is not as reliable as the pes-
simistic phase lock indicator, which decides slowly and changes its mind quickly.
The phase lock indicator concept is simple: if the loop is in phase lock, then I PS
will be maximum and Q PS will be minimum. The phase of the envelope tends to stay
near the I -axis. As the phase jitter increases in the loop due to noise, dynamic stress,
and so forth, the phase of the envelope jitters around the I -axis. Eventually, the jitter
will reach a level where a cycle will be slipped or complete loss of phase lock occurs.
In Figure 5.43 the absolute values of the prompt I and Q signals and passes these
through a LPF. The in-phase filtered result, I P , is divided by a scale factor, K 2 , and
this result is compared to the unscaled quadraphase filtered result, Q P . The decision
is made that phase lock has been achieved if the scaled average absolute amplitude
of I P is greater than the average absolute amplitude of Q P . This decision is based on
the averaged phase being less than about 15º one sigma.
Observe in Figure 5.43 that the first positive decision results in an optimistic
decision. The pessimistic decision that phase lock has been achieved does not occur
until several optimistic decisions in a row are made. The first time that the threshold
criteria is not met, the pessimistic phase lock indicator is set false. The optimistic

DT

NO

N Optimistic

Phase lock = true
Pcount(2)=0

A

Pcount(1)=
Pcount(1)+1

Pcount(1)
>Lp?

|·| LPF YES

I P ÷

D

YES

Pessimistic phase
lock = true

K 2

A>B?

DT NO

+ Pessimistic

Phase lock = false
Pcount(1)=0

+

B

Pcount(2)=
Pcount(2)+1

|·| K 1 Pcount(2)

> Lo?

Q P

Σ Σ NO

− +

YES

Z − 1

Optimistic phase
Lock = false

Lowpass filter

Figure 5.43 Basic phase lock detector with optimistic and pessimistic indicators.