Demodulators – Phase shift keying or quadrature amplitude demodulator
Patent
1995-12-04
1997-01-28
Mis, David
Demodulators
Phase shift keying or quadrature amplitude demodulator
329300, 329302, 329306, 375324, 375328, 375329, 375334, H03D 300, H04L 2714, H04L 2722
Patent
active
055981259
DESCRIPTION:
BRIEF SUMMARY
This application claims benefit of international application PCT/F194/00230 filed Jun. 2, 1994.
The invention relates to a method for a quadrature demodulation of a digitally modulated signal. In the method a signal to be received is mixed essentially to quadrature related I and Q baseband signals. The invention also relates to a demodulator comprising mixing means for mixing a signal to be received into I and Q signals, which are essentially in quadrature with each other.
The method of the invention suits best for constant amplitude modulations, e.g. for FSK (Frequency Shift Keying) signals and MSK (Minimum Shift Keying) signals, but the principle may also be applied to non-constant amplitude modulations.
A quadrature demodulator is typically used e.g. in connection with QAM (Quadrature Amplitude Modulation), PSK (Phase Shift Keying) and CPM (Continuous Phase Modulation). FIG. 1 shows a typical demodulator using quadrature demodulation for demodulating a digitally modulated signal. The input of a receiver comprises a power splitter 11 splitting an RF or intermediate frequency signal S to be received in-phase to the signal input of a mixer 12 of branch I and to the signal input of a mixer 13 of branch Q. Via a second power splitter 14, a local oscillator signal LO is applied in-phase to the local oscillator input of the mixer 12 of branch I and, phase-shifted by 90 degrees, to the local oscillator input of the mixer 13 of branch Q. The base-band signals I and Q obtained from the outputs of the mixers are each applied via a dedicated baseband amplifier 15 and 16, respectively, to a processing unit 17, in which sampling and decision-making are performed. The baseband amplifiers possibly also comprise low-pass filtering and equalizers.
The contents of the processing unit 17 depends on which modulation method is used each time. For example, in the case of a QPSK (Quadrature Phase Shift Keying) signal, the processing unit comprises decision-making comparators (comparing whether the signal is positive or negative, i.e. whether a logic one or a logic zero has been received) as well as clock and carrier wave regenerating circuits. The clock regenerating circuits set a correct sampling instance and the carrier wave regenerating circuits keep the local oscillator signal LO phase-locked with respect to an incoming signal.
When using quadrature demodulation according to the above, various problems occur in practice, which will be described in the following. oscillators and of phase hits of the local oscillator signal LO, the hits being caused by mechanical and electrical reasons and causing rotations of a constellation diagram as well as jitter. occurring in the output voltage of the mixers (12 and 13) and in the subsequent amplifiers (15 and 16) and leading to a displacement of decision-making limits. baseband amplifiers, signal distortion is generated, because the lowest frequencies of the signal are cut off. This appears as distortion especially in long trains of ones and zeroes. (microphonism) of the quadrature mixer (12 and 13), causing interference voltages in baseband signals. These are especially disadvantageous at so-called direct reception (incoming signal is an RF signal), as the amplification of the baseband amplifiers should be high. where a wide frequency band comes to the mixers (12 and 13). Thereby, even signals located far from the reception frequency and having varying amplitudes cause rectification in the mixers. This appears in a baseband signal as an interference voltage corresponding to amplitude changes. Alternatively, a mixing together of two interference signals having frequencies close to each other may cause intereferences in a baseband signal.
All phenomena mentioned above cause errors in the reception.
Attempts have been made earlier to solve the above-mentioned problems by following means. attenuated in some cases by using differential detection. This has been implemented in such a way that the phase of a reference carrier wave generated by a separate oscillator and phase-locked (at narro
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patent: 5128966 (1992-07-01), Bang
patent: 5159710 (1992-10-01), Cusdin
patent: 5398002 (1995-03-01), Bang
Mis David
Nokia Telecommunications Oy
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