Circuit for extracting carrier signals

DETAILED DESCRIPTION OF THE INVENTION With the receiver comprising a carrier-reproducing circuit 10, shown in FIG.
1, it is possible to extract in a highly reliable way a continuous phase-modulated carrier signal (CPM-signal) applied to input-terminal 1 and having a rational modulation index h such that the resulting carrier has a small phase jitter.
Before embarking on the explanation of the carrier reproducing circuit, the structure of such a CPM-signal is further elucidated.
An information signal to be transferred digitally, represented by a series (a.
sub.
m) of m-nary digital symbols, (a.
sub.
m)=.
.
.
a.
sub.
-2, a.
sub.
-1, a.
sub.
0, a.
sub.
1, a.
sub.
2, .
.
.
a.
sub.
n (1) with a.
sub.
m =.
+-.
(2n+1) and n=0, 1, 2, .
.
.
etc.
can be represented as a function of time by a(t)=.
SIGMA.
a.
sub.
m p(t-mT) (2) with T representing the duration of the digital symbols and p(t) representing a rectangular pulse of a length of T.
Before transferring such signals they are preferably modulated in that the modulated carrier signal has a constant amplitude.
The major advantage of such a carrier signal is the fact that non-linear signal processing such as non-linear amplification for obtaining a high efficiency, can be used without affecting the information contained in the signal.
Each digital modulation having a constant amplitude can be written as: u(t)=cos (.
omega.
.
sub.
c t+.
phi.
(t)) (3) wherein .
omega.
.
sub.
c represents the angular frequency of the carrier signal and .
phi.
(t) represents the phase as a function of time.
If .
phi.
(t) is varied as a function of a (t) the following equation will be obtained: .
phi.
(t)=.
phi.
(alt)) (4) In Phase Shift Keying (PSK) .
phi.
(t) is kept constant over the symbol time T and changed abruptly at the symbol transitions.
However, there are also types of modulation such as Continuous Phase Frequency Shift Keying (CPFSK), for which the following condition holds: d .
phi.
(t)/d(t)=K.
a(t) (5) wherein K is a constant.
With Continuous Phase Frequency Shift Keying the detection process for obtaining a better S/N ratio can be spread out over more than one symbol interval