Demodulators – Phase modulation demodulator – Input signal combined with local oscillator or carrier...
Patent
1989-04-11
1991-05-14
Blum, Theodore M.
Demodulators
Phase modulation demodulator
Input signal combined with local oscillator or carrier...
H03D 306, H04L 2722
Patent
active
050159623
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
An object of the present invention is a phase demodulator of the DPSK (differential phase shift keying) type.
Another object of the invention is the application of this device to a landing aid system of the MLS (microwave landing system) type.
2. Description of the Prior Art
Increasing use is being made, in data transmission, of the modulation of the phase of a radioelectrical carrier wave. In general the signal is written as: that corresponds to the carrier frequency and O(t), is the modulation frequency as a function of time. For biphase shift keying (BPSK) modulation, the signal .theta.(t) will be equal alternately to 0 or to .pi. depending on whether the bit sent is 0 or 1. This type of modulation can be extended to multiple-phase shift keying (MPSK) modulation.
At reception, it is therefore necessary to demodulate (or decode) the signal received to extract the useful signal .theta.(t) in the composite signal s(t). The demodulator according to the invention aims at achieving a demodulation of this type when neither the frequency nor the phase of the carrier wave is known with precision.
To resolve this problem, various approaches are known and are especially described in Spilker Jr., Digital Communications by Satellite, Prentice Hall. One of these approaches, described on page 304 and called the Costa loop, uses a voltage controlled oscillator (VCO) and an automatic control loop. Another approach, described on page 302, also uses a VCO type oscillator and automatic control loops, It further comprises frequency doubling devices. The main drawback of these devices is their complicated nature and the fact that they require a VCO-type oscillator, which makes them expensive.
An object of the present invention is a phase demodulator which, in particular, does not require any oscillator of the VCO type.
To this end, it uses an oscillator feeding a phase demodulator. The output signal of the phase demodulator is mixed with the demodulated signal and is applied to a loop, the output signal of which:
controls the demodulator;
constitutes the output signal of the demodulator.
SUMMARY OF THE INVENTION
More precisely, an object of the invention is:
A phase demodulator comprising:
means (2) for modulating the phase of the first signal (S.sub.1) of fixed frequency (w.sub.1) neighbouring the frequency (w.sub.o) of a second signal (s(t)), to be demodulated;
means (5;51) enabling the mixing of the above two signals;
means (3) forming a loop filter receiving the signal (S.sub.5 ; S.sub.51) coming from the mixing means, the output signal of said loop filter means (3) providing for the control of the modulation means (2);
processing means (4) providing for filtering operation, and receiving the output signal of the loop filter means and forming the output signal of the device.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, special features and results of the invention will emerge from the following description, given as a non-restrictive example and illustrated by the appended drawings, of which:
FIG. 1 is a schematic diagram showing the working of the device according to the invention;
FIG. 2 shows an embodiment of device represented in the preceding figure;
FIGS. 3, and 4a-4c are explanatory drawings showing the operation of the device according to the invention.
In these different figures, the same references refer to the same elements.
DESCRIPTION OF A PREFERRED EMBODIMENT
FIG. 1 shows a schematic diagram of the device according to the invention.
It has an oscillator 1 giving a signal S.sub.1 at a constant and stable frequency which is written as follows:
where A.sub.1 is its amplitude and .omega..sub.1 is its pulsation.
This signal is directed towards a phase modulator 2. The output signal S.sub.2 of the modulator 2 is written: modulation achieved by the modulator 2.
The signal S.sub.2 is then mixed with the input signal to be demodulated s(t) given by the expression (1). The output signal of the mixer 5 is written: t+.theta.(t)].multidot.Sin [.omega..s
REFERENCES:
patent: 3938052 (1976-02-01), Glasson et al.
patent: 4042884 (1977-08-01), Querry
Brault Daniel
Grousseau Alain
"Thomson-CSF"
Blum Theodore M.
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