Pulse or digital communications – Receivers – Angle modulation
Patent
1997-10-06
1999-11-23
Le, Amanda T.
Pulse or digital communications
Receivers
Angle modulation
329306, 329124, 4552261, 375344, H04L 2714
Patent
active
059913428
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a frequency error detector that determines the frequency deviation between a carrier frequency generated in a receiver and the carrier frequency of a received signal, with a first differential power meter that forms the power difference between spectral components of the spectrum of the received signal that are symmetrical with the receiver carrier frequency.
BACKGROUND INFORMATION
Frequency error detectors are used for automatic frequency control (AFC) in control loops that are in radio transmission receivers, for example, for coherently modulated signals (e.g., quad amplitude modulation, QAM) to adjust the carrier frequency of the receiver to the carrier frequency of the transmitter, i.e., the carrier frequency of the received signal. In addition, these receivers for coherently modulated signals often have another closed loop for carrier phase synchronization which is capable of synchronization in the case of relatively small frequency deviations. The frequency error allowed in synchronization of the carrier phase is called the lock-in range. Since the transmission carrier frequency is not known at the start of a transmission or after a system failure, the receiver must be capable of reliably estimating this frequency. The accuracy of this estimation must be within the lock-in range of the carrier phase synchronization. The size of this lock-in range depends on several system parameters of the broadcast system. Narrow-band broadcast systems with a high number of modulation levels have a smaller lock-in range, but broad-band systems with fewer modulation levels have a larger lock-in range.
A conventional frequency error detector includes a suitable device that synchronizes the carrier frequency of the receiver at the transmission carrier frequency. This conventional device measures the differential power of the received signal spectrum symmetrically with the center of the band. An error signal generated in this way will disappear at the correct carrier frequency in the receiver. A quadricorrelator as descrived in German Patent No. 37 07 762 is one such frequency error detector. This quadricorrelator reacts very sensitively to signal distortions; i.e., it supplies false information about the frequency deviation when the received signal spectrum is skewed because of channel distortion. Straight-line broadcast systems usually have great channel distortion. Therefore, the background art frequency error detector would not be suitable for a receiver in a transmission system where channel distortion is expected.
The object of the present invention is therefore to provide a frequency error detector that will supply the most accurate possible information about the deviation in the carrier frequency in the receiver in comparison with the transmitted carrier frequency regardless of channel distortions.
SUMMARY OF THE INVENTION
According to the present invention, in addition to a first differential power meter, a second differential power meter is provided to form the power difference between spectral components that are symmetrical with the received carrier frequency and belong to the spectrum formed by undersampling of the received signal, and a summation circuit is provided that generates, from the output signals of the two differential power meters, a summation signal that provides information on the frequency deviation.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a block schematic of a receiver according to the present invention.
FIG. 2 shows a frequency error detector used in the receiver according to the present invention.
FIG. 3a shows a first example of a signal spectra emitted by a first differential power meter.
FIG. 3b shows a second example of the signal spectra emitted by the first differential power meter.
FIG. 3c shows a third example of the signal spectra emitted by the first differential power meter.
FIG. 4a shows a first example of the signal spectra emitted by a second differential power meter.
FIG. 4b shows a second example of t
REFERENCES:
patent: 4470145 (1984-09-01), Williams
patent: 4721924 (1988-01-01), Masdea et al.
patent: 5014352 (1991-05-01), Chung
Ha Dac V.
Le Amanda T.
Robert & Bosch GmbH
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