Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Patent
1998-10-09
2000-11-07
Zweizig, Jeffrey
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
H03L 500
Patent
active
061442437
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a method and a circuit arrangement for reducing the offset voltage of a signal. The invention can be applied preferably in receivers of digital, time-division systems for data transfer such as in mobile stations. The offset voltage refers to voltage which has become summed up into a signal, essentially direct voltage which is not included in the received useful signal.
BACKGROUND ART
One general problem is that the offset voltage becomes summed up into baseband signals. This can happen, for example, in I/Q receivers which are used in mobile communications, that is receivers in which a baseband in-phase signal and quadrature signal are formed. The problem is a particular problem in so-called direct conversion receivers in which the received signal is converted directly to a baseband and the baseband extends to near the zero frequency. Then the offset voltage may be considerably higher than the pre stage noise in which case the signal-to-noise ratio deteriorates largely due to this effect of the offset voltage.
The formation of the offset voltage can be due to many different factors:
The local oscillator of a receiver may itself become linked to the front of the receiver that is to high-frequency parts in which case the local oscillator signal obtains access to the radio frequency gate that is to the RF gate of the I/Q modulator and becomes mixed with the actual local oscillator signal which is entering the local oscillator gate, that is the LO gate at the actual mixer and forms an offset signal at the output of the mixer. The strength of this signal will depend on the phase and level of the local oscillator signal. This phenomenon has been described in more detail, for example, in U.S. Pat. No. 5,212,826.
Secondly, the harmonic frequency of the clock oscillator which impacts on the channel frequency can become mixed with the local oscillator signal and form direct voltage at the output of the mixers in the I/Q demodulator.
At the receivers using intermediate frequency, the second local oscillator signal can itself become linked to the first local oscillator signal, in which case the signals get mixed with each other in the first mixer and as a result they form a mixing result which impacts on the intermediate frequency and generates offset voltage at the output of the I/Q demodulator.
In addition, offset voltage can be generated into baseband signals from switching on the supply voltages of the receiver.
The above mentioned phenomena which lead to the generation of offset voltage are not described in more detail in this context because these phenomena are prior known to a person skilled in the art. Also several methods are prior known for removing the offset voltage:
If the offset voltage stayed constant during the entire reception time, it would be possible to remove the offset voltage digitally. In that case, the dynamics of the analog/digital converter, that is the A/D converter should, however, be extended by the highest possible offset voltage, which would raise considerably the price of the A/D converters. In addition, extra digital signal processing would require higher clock frequencies, which in turn would lead to increased current consumption by the device.
Problems which are due to leakage and detrimental connection of signals can be reduced by increasing RF shielding but this will lead to an increase in the size and weight of the device, and especially with respect to direct conversion receivers it is not possible to adequately resolve the problem with this approach.
By using two intermediate frequencies, the problem can be overcome almost entirely but, due to large amount of components and intermediate frequency filters needed, the receiver will be expensive and large in size.
FIG. 1 shows a prior known block diagram of a transmitter-receiver of a mobile station and in this block diagram, the receiver is a so-called direct conversion receiver. An RF signal received by an antenna 138 is conducted via a duplex filter 102 to a pre-amplifier 104.
REFERENCES:
patent: 4514685 (1985-04-01), Gilker
patent: 4713563 (1987-12-01), Marshall et al.
patent: 4745594 (1988-05-01), Takahashi
patent: 5212826 (1993-05-01), Rabe et al.
patent: 5748681 (1998-05-01), Comino et al.
patent: 5945864 (1999-08-01), Weber et al.
Nokia Mobile Phones Ltd.
Zweizig Jeffrey
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