Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
1998-03-16
2002-01-15
Hunter, Daniel (Department: 2684)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S013400, C455S572000, C455S574000, C455S575100
Reexamination Certificate
active
06339711
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a radio apparatus such as a portable radiotelephone apparatus.
2. Description of the Related Art
In recent years, an increasing number of portable radiotelephone apparatuses (hereinafter referred to as portable telephone apparatuses) have been used. In addition, portable telephone apparatuses that are small and that have high performance have been aggressively developed.
Current developing trends of portable telephone apparatus are for example small size for high portability, low power consumption for long time operation, and high linearity for high resistance against disturbing waves.
Currently, studies for solving problems necessary to accomplish such features have been performed.
Next, problems of the current portable telephone apparatus will be described in the order of a receiving portion, a synthesizer (namely, a local oscillator), a transmitting portion, and an antenna.
First, problems of the receiving portion will be described.
The receiving portion has two problems. As a first problem, the current consumption of the portable telephone apparatus is large. As a second problem, when a signal is received, a DC offset takes place, resulting in causing the reception characteristic of the portable telephone apparatus to deteriorate.
Since the reception characteristic of the portable telephone apparatus should always satisfy the required performance, the reception characteristic is designated so that the portable telephone apparatus properly operates in the worst radio wave environment. An example of the worst radio wave environment is a situation of which an unnecessary signal defined as a mutual modulation characteristic or a selectivity of adjacent channels is present. In other words, when an unnecessary signal other than a necessary signal is present in a system band, the level of the unnecessary signal is the maximum value of which a desired bit error rate defined in the system is satisfied.
Generally, to satisfy the standard value of the system in the worst radio wave environment, the radio apparatus should properly operate in the worst condition. Thus, in other than the worst radio wave environment, the portable telephone apparatus operate with the performance that satisfies the worst condition. To satisfy the standard in case of the worst radio wave environment, the receiving portion of the portable telephone apparatus should have linearity. In other words, the distortion of the receiving portion should be decreased so that the standard is satisfied. This problem relates to currents that flow in circuit blocks of the receiving portion (such as a low noise amplifier and a frequency converter).
Generally, to improve the linearity of a circuit, the operating current thereof should be increased. Thus, the power consumption of the portable telephone apparatus considered for the worst radio wave environment excessively increases. This is because the portable telephone apparatus is not always in the worst radio wave environment. In other words, the portable telephone apparatus normally operates in other than the worst radio wave environment.
Next, the second problem of the receiving portion (namely, when a signal is received, a DC offset causes the reception characteristic to deteriorate.
Generally, in an active circuit such as a frequency converter, a low frequency filter, or a low frequency amplifier used in the receiving portion of the portable telephone apparatus, the output signal thereof overlaps with a desired signal, thereby generating a DC component. Such a DC component is generated by a self-mixing operation.
As the simplest technique for removing the DC component, an AC coupling capacitor may be connected to the output stage of the active circuit. In this case, part of the desired signal component is deleted. In other words, a notch takes place.
Thus, a carrier-to-noise (C/N) characteristic may be improved for an FSK signal with a high modulation index of which a desired signal component is small in the vicinity of the DC region.
A technique for removing a DC offset using an AC coupling capacitor has been proposed. This technique can be effectively used for a two-value FSK signal with a high modulation index for pagers. Since a signal component in the vicinity of the DC region is small, the AC coupling capacitor does not largely attenuate a signal component.
However, in an FSK signal and a four-value FSK signal that have been used for high speed data transmission in recent years and that have low modulation indexes, since there are many signal components in the vicinity of the DC region, the second problem cannot be practically solved.
Such a DC offset that takes place in the receiving portion has a problem in the heterodyne system. This problem is much serious in the direct conversion system that has been used in the mobile communication field in recent years. The problem of the DC offset in the direct conversion system has different features from the problem in the heterodyne system. Next, the features of the problem in the direct conversion system will be described.
In the direct conversion system, an external radio signal (RF signal) and a local signal with the same frequency thereof are sent to a mixer so as to directly convert an RF signal into a baseband signal.
When the mixer is mathematically ideal, the isolation between each terminal is infinite. Thus, a signal supplied to a particular terminal does not take place at other terminals.
However, since a mixer used in the direct conversion type portable telephone apparatus does not have an infinite isolation, a local signal of the portable telephone apparatus is radiated from the antenna. The local signal radiated from the antenna is reflected by an external reflector. The reflected signal is received by the antenna and then sent to the mixer. Since the frequency of the signal that is sent to the mixer from the antenna is the same as the frequency of the local signal, a multiplying operation as a mixing function causes a DC component (namely, a DC offset) to take place at a baseband output terminal.
Since the DC offset varies depending on the amount of reflection of the local signal (namely, a reflector in the vicinity of the antenna), this DC offset more adversely affects the reception characteristic than a DC offset of the portable telephone apparatus and a DC offset of an active device.
Since the direct conversion type portable telephone apparatus is small, the user carries it with his/her hand, bag, and pocket, the situation of an external reflector varies time by time. Thus, since the amount of reflection of a local signal varies time by time, the DC offset varies time by time. Since the DC offset cannot be suppressed, the reception sensitivity deteriorates.
To compensate the DC offset, a capacitor may be disposed in a downstream circuit. Since the capacitance of the capacitor is constant, a time-varying transient response of the DC offset largely affects a reception error rate.
Thus, the conventional receiving portion cannot solve the two problems with respect to the low current consumption and the improvement of the reception characteristic. In particular, the problems in the direct conversion system are severer than the problems in the heterodyne system.
Next, the problem of the synthesizer of the conventional portable telephone apparatus will be described.
In the conventional portable telephone apparatus, a frequency synthesizer is used. The frequency synthesizer comprises a reference oscillator, a reference frequency divider, a phase comparator, a loop filter, a VCO, and a comparing frequency divider. The frequency of the comparing frequency divider is varied from N
1
to N
2
so as to switch a frequency. The frequency switching time depends on a natural angular frequency &ohgr;n and a dumping coefficient &zgr; of the loop of the loop filter. When the natural angular frequency and dumping coefficient are selected for a stable oscillation frequency and low noise, the frequency switching time becomes long.
The fre
Kayano Hiroyuki
Maeda Tadahiko
Otaka Shoji
Sekine Syuichi
Tsurumi Hiroshi
Hunter Daniel
Kabushiki Kaisha Toshiba
Woldetatios Yemane
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