Homodyne receiver and process for correcting the converted recei

Details Homodyne receiver and process for correcting the converted recei Homodyne receiver and process for correcting the converted recei

1994-09-22

1999-04-20

Mis, David

Demodulators

Phase shift keying or quadrature amplitude demodulator

Input signal combined with local oscillator or carrier...

329306, 375325, 375327, 375329, H03D 300, H04L 27227

Patent

active

058960619

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a homodyne receiver (Direct Conversion Receiver), especially for angle-modulated carrier signals, specifically including those in which the converted signal (IF) exhibits a DC voltage component (DC component), and to a process for correcting the converted input signal.
Direct conversion receivers are known for example from DE 29 02 952 C2. According to this, it is theoretically sufficient to mix the received signal, if required after a preamplification, with the carrier frequency generated by a local oscillator and to separate off any high sum frequencies which arise in this case using a lowpass filter. The filtered signal is to correspond to the demodulated signal. The frequency of the local oscillator in a phase locked loop (PLL) is intended to be set to the carrier frequency. The PLL for frequency and phase locking is necessary because, when using customary reference signal sources, there is in principle a relatively large or small frequency deviation in relation to the carrier frequency. The synchronization of the local oscillator with the carrier frequency or respectively the frequency of the emitting oscillator must therefore first be forced. To this end, the received carrier is utilized as reference signal for the locking. The error signal employed for controlling the local oscillator cannot, if the received signal is very weak and is disturbed on the transmission path, be distinguished from the DC offset arising in the case of the direct conversion of the modulated carrier signal. The PLL locking then fails.
According to GB 2 192 506 the angle-modulated input signal is split into two branches and the frequency of a local oscillator is mixed with the two branches; in this case, a phase shift of the frequency mixed in of 90.degree. is set for one branch. The mixed signal in the branch without phase shift is named the in-phase signal (I), and the mixed signal in the branch with phase shift is named the quadrature signal (Q). Lowpass filters and analog/digital converters are provided. The digital signals from both branches are passed to a digital signal processor (DSP). In the latter, the demodulated signal is calculated from the I and Q signals. The I and Q signals also exhibit a DC offset originating from the direct conversion of the modulated carrier signal. Further undesired DC offsets may arise on account of the crosstalk of the local oscillator onto the signal inputs of the mixer and on account of the relative phase relation of the local oscillator to the carrier of the received signal. Any amplifiers which may possibly be inserted likewise lead to a further DC offset. The total sum of the DC offset voltages (resulting from operating temperature, the aging of the components, phase angle, crosstalk, amplifier offset) may be, in the case of very small input signals, several tens of thousands of times larger than the useful signal, so that an AD converter must have a large dynamic range in order to be able still to resolve the useful signal. Thus, taking in account the required resolution of the useful signal, the use of economic and fast AD converters is substantially ruled out.
For heterodyne receivers, the use of a bandpass filter for the IF signal was proposed for example in WO88/10033. By this means, a DC offset is separated off. However, along with the DC offset, DC components of the converted signal are also separated off by the bandpass filters. In the case of many types of modulation, the (short-term) DC component of the converted signal contains information on the modulating signal. Along with the separating of the DC component of the converted signal, accordingly information on the modulating signal is also lost, consequently a considerable disturbance of the demodulated signal takes place. In particular, the nonlinear distortion factor is increased.
Further more substantial components which lead to disturbing distortions of the orthogonal quadrature signals in the baseband in the case of the homodyne receiver are, besides the DC offset, the amplitude difference a

REFERENCES:
patent: 4484143 (1984-11-01), French et al.
patent: 5280538 (1994-01-01), Kataoka et al.
patent: 5438591 (1995-08-01), Oie et al.

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