Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Patent
1994-09-22
1998-12-15
Urban, Edward F.
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
455307, 455310, H04B 130
Patent
active
058505989
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a homodyne receiver and to a process for direct conversion (Direct Conversion Receiver), especially of angle-modulated carrier signals, specifically including those in which the converted signal (IF) exhibits a direct voltage component (DC component).
2. Description of the Related Art
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 intended to correspond to the demodulated signal. The frequency of the local oscillator is intended to be set in a Phase-Locked Loop (PLL) to the carrier frequency. The PLL for frequency and phase locking is required because when using customary reference signal sources there is in principle a relatively large or small frequency offset in relation to the carrier frequency. The synchronization of the local oscillator with the carrier frequency or respectively the frequency If 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 used 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 mixed-in frequency of 90.degree. is set for one branch. The mixed signal in the branch without any 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 be inserted if required 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 relation, 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 more economic and faster AD converters is substantially ruled out.
For heterodyne receivers, the use of a bandpass filter for the IF signal was proposed for example in WO 88/10033. By this means, all DC components are separated. However, along with the separated DC components, DC components of the converted signal are also separated 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, information on the modulating signal is also accordingly lost, consequently a considerable disturbance of the demodulated signal takes place. In particular, the nonlinear distortion factor is increased.
EP 0 437 373 A2 relates to a calibration device for homodyne receivers. In this case it is provided, inter alia, to pr
REFERENCES:
patent: 4653117 (1987-03-01), Heck
patent: 4736390 (1988-04-01), Ward et al.
patent: 4766392 (1988-08-01), Moore
patent: 4944025 (1990-07-01), Gehring et al.
Nguyen Lee
Sican Gesellschaft fur Silizium-Anwendungen und Cad/Cat
Urban Edward F.
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