Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Reexamination Certificate
1998-10-09
2001-11-27
Maung, Nay (Department: 2681)
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
C455S234100
Reexamination Certificate
active
06324389
ABSTRACT:
The invention relates to a method and a circuit arrangement for receiving a radio frequency signal. The invention can be applied preferably in receivers of digital, time-division data transfer systems, such as in mobile stations.
In a direct conversion receiver, that is in a zero intermediate frequency receiver, a radio frequency signal is converted directly to a baseband without there being intermediate frequency. Since intermediate frequency stages are not needed, only a few components are needed in a receiver, which makes it a preferable solution for use in various applications. In mobile stations, direct conversion receivers are, however, used rarely. On one hand, this is due to the fact that prior known methods are almost impossible to implement in practice in industrial manufacturing or on the other hand, in presented solutions the unique features of digital mobile phone systems have not been taken into consideration. The most important problem in the implementation of a direct conversion receiver is the control of offset voltages. By an offset voltage one means voltage which has become summed up in a signal, essentially direct voltage which is not included in a received useful signal.
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 direct conversion receiver. In it, an RF signal received by an antenna
138
is conducted via a duplex filter
102
to a pre-amplifier
104
. The purpose of the duplex filter is to permit the use of the same antenna both in transmitting and receiving. Instead of a duplex filter, also a synchronous antenna changeover switch can be used in a time-division system. An RF signal which is received from an amplifier
104
is low-pass filtered
106
and demodulated in an I/Q demodulator
108
into an in-phase signal
108
a
and into a quadrature signal
108
b
. A local oscillator signal
114
b
which is needed in demodulation is received from a synthesizer
114
. In block
110
, removal of direct voltage and automatic gain control (AGC) are carried out. Block
110
is controlled by a processing block
116
which may contain, for example, a microprocessor. Automatic gain control is regulated by a signal
110
a
and removal of the offset voltage is regulated by a signal
110
b
. Signals received from block
110
are converted into digital signals in block
112
from which the signals are further led to digital signal processing circuits in the processing block
116
.
The transmitter unit comprises an I/Q modulator
128
which forms from an in-phase signal
128
a
and from a quadrature signal
128
b
, a carrier-frequency signal which is low-pass filtered and/or high-pass filtered by a filter
130
. The carrier-frequency signal is amplified by an RF amplifier
132
and the amplified signal is transferred via a duplex filter
102
to an antenna
138
. A power control unit
134
of the transmitter controls the amplification of the RF amplifier
132
on the basis of the measured output power
136
and of the control
134
a
received from the processor.
FIG. 1
also shows, attached to the processing unit, the memory unit
126
and user interface means which comprise a display
118
, a keyboard
120
, a microphone
122
and an earpiece
124
.
Practical solutions for implementation of a direct conversion receiver have been described more closely, for example, in the following publications:
[1] Microwave Engineering Europe, January 1993, pages 59 . . . 63,
[2] Microwave Engineering Europe, May 1993, pages 53 . . . 59 and
[3] patent application EP 0 594 894 AI.
The solution for a receiver presented in the reference [1] is based on direct voltage coupled baseband parts and in them the DC offset voltage compensations are attained via extremely complicated control systems which contain D/A converters, A/D converters, digital filters and predicting control algorithms. Various correction values are stored into memories, for example, for different channels and for AGC adjustments.
When a DC-coupled baseband part is used, great fluctuations in direct voltage can create a problem. Fluctuations in a direct voltage component of a signal may be due to, for example, temperature changes in which case they are typically slow or, for example, due to variations in frequency or level of a local oscillator signal in which case the fluctuations in direct voltage are rapid. Also a received signal may contain, due to modulation, a direct voltage component which cannot be removed, which makes the determination of the real offset voltage extremely difficult and complicated. When the above mentioned complicated solutions for control of the offset voltage are used, the manufacturing costs of the device become so great that the advantages of the direct conversion receiver are lost.
The advantage of the AC-coupled baseband parts is that slow fluctuations in offset voltage are dissipated and the total amount of the offset voltage can be reduced. The disadvantage is, however, that there may be charges which remain in the separating capacitors of the direct voltage and which discharge slowly.
FIG. 2
shows a prior known solution for reducing the offset voltage of a signal. In a circuit according to
FIG. 2
, a baseband signal Vin is conducted to an amplifier
21
, the output P
21
of which has been conducted via a capacitor C
21
to the input P
22
of an amplifier
22
. In the place of the amplifier
22
, it is also possible to have, for example, an analog/digital converter. A reference voltage has been conducted to the input of the amplifier
22
via a resistor R
21
. Prior to the start of the receiving, a short control pulse DCN connects a switch
23
to an on-state and then the output terminal of the capacitor C
21
becomes set at a reference voltage Vref and at the point P
22
if there is any offset voltage, it is dissipated. If, at the moment of the removal of the offset voltage, there is, for example, a positive baseband useful signal in addition to the offset voltage on the signal path, the DCN control pulse removes the offset voltage occurring at the point P
22
but, as the DCN control pulse ends, a negative offset voltage is generated at the point P
22
. In this situation the offset voltage is thus not dissipated but the operation of the removing circuit of the offset voltage results in a new offset voltage which falls slowly towards the reference voltage Vref. The offset voltage caused by the removing circuit of the offset voltage depends on the extent of the instantaneous value of the baseband signal occurring in the capacitor C
21
at the moment when the DCN control pulse ends.
In order to circumvent the new offset voltage resulting from the removing circuit of the offset voltage, in the solution described above, the zeroing of the offset voltage should be performed at such an instant when the received signal is essentially noise, which means in practice before the onset of the signal reception. In many time-division systems for mobile communications, such as GSM (Global System for Mobile Communications) and PCN (Personal Communications Network) systems, the power transmitted by the base station is not disconnected before the start of a new time interval. Thus there are baseband signals present in a receiver also before the start of the actual moment of reception, that is during the time when the offset voltage should be removed. This is why the earlier mentioned solution for reducing the offset voltage is not applicable in the above mentioned systems.
The aim of the invention is to devise a simple solution for implementation of a direct conversion receiver so that the above mentioned problems connected to the solutions according to prior art can be avoided.
One idea of the invention is that the direct voltage component of the signal which has been received and converted to a baseband is separated by the first separating means of direct voltage in a signal line and, in addition, the direct voltage components of both the signal preceding the separating mean
Maung Nay
Nokia Mobile Phones Ltd.
Perman & Green LLP
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