Coded data generation or conversion – Analog to or from digital conversion – Differential encoder and/or decoder
Reexamination Certificate
1999-12-09
2001-11-27
Young, Brian (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Differential encoder and/or decoder
C341S144000, 36
Reexamination Certificate
active
06323795
ABSTRACT:
The present invention relates to an electronic digital-to-analog converter circuit for a baseband transmission system.
BACKGROUND OF THE INVENTION
More particularly, the invention relates to an electronic digital-to-analog converter circuit for transforming a digital signal into an analog signal without phase distortion, and in particular a circuit comprising a sigma-delta type modulator and a filter having a finite impulse response.
A preferred field of application of the invention is the field of mobile telephony. The invention is described essentially in the context of this field. Nevertheless, the scope of the invention should not be limited to the field of mobile telephony, but should be understood as extending to any other field in which the converter circuit and the method of conversion of the invention are applied.
In general, a mobile telephone or a cordless telephone can be structurally subdivided into four distinct blocks: the user block or user interface; the control block; the audio block; and the radio frequency block. The last three blocks to be mentioned constitute the radio unit of the mobile telephone.
FIG. 1
is a block diagram showing the general structure
100
of a mobile telephone. The general structure
100
has four main blocks as mentioned above. The user of a mobile telephone has access to a user interface
120
. The user interface
120
generally comprises a loudspeaker
121
, a microphone
122
, a keypad
123
, and display means
124
. The user interface
120
can optionally include other elements, for example a modem for transferring data.
A radio unit
130
comprises the blocks that are required for baseband transmission: speech encoding means; means for compressing information into data blocks; and means for decompressing such data blocks into a continuous signal. These elements, and others, are shared between a control block
140
, an audio block
150
, and a radio frequency block
160
.
The control block
140
comprises a microprocessor
141
which acts as a central processor unit. The microprocessor
141
performs the procedures necessary for setting up a call. It also controls the various operations of the mobile telephone by means of various programs. These programs include, for example, programs for managing the user interface
120
, monitoring programs (in particular for monitoring battery level), and test programs in order to facilitate maintenance of the mobile telephone. Other programs manage the connection between the mobile telephone and the nearest transmission relay. The programs associated with the user interface
120
manage, in particular, interactions between the user and the other programs, in particular by interpreting the information provided by the user to the microprocessor
141
via the digital keypad
123
and a link
101
, and by controlling the display means
124
via a link
102
.
The control block
140
also has memories
142
which are used in mobile telephones for storing, in particular, the operating system, the serial number, and the telephone number associated with the mobile telephone, or indeed rights to use various services. These memories
142
can also be used when setting up a call. Data information, memory addresses, and commands are interchanged between the memories
142
and the microprocessors
141
over a bidirectional bus
103
.
The audio block
150
is essentially constituted by a signal processor unit
151
which makes use of numerous programs. The signal processor unit
151
receives information from the microphone
122
over a link
104
. A link
105
serves to transmit signals between the signal processor unit
151
and the loudspeaker
121
. Information is also interchanged between the signal processor unit
151
and the microprocessor
141
over a bidirectional link
106
.
Communication with the radio unit
130
of the mobile telephone takes place by means of a special radio frequency interface
160
in which analog-to-digital and digital-to-analog conversions are performed. The radiofrequency block
160
comprises, in particular: an antenna
161
connected to a duplexer
162
; a transmitter
163
; a receiver
164
; and a frequency generator unit
165
. The microprocessor
141
manages the operation of the transmitter
163
, the receiver
164
, and the frequency generator
165
over respective connections
107
,
108
, and
109
. The signal processor unit
151
can send signals to the transmitter
163
and can receive signals from the receiver
164
over respective links
110
and
111
. The frequency generator
165
is connected to the transmitter
163
and to the receiver
164
over respective connections
112
and
113
. The duplexer
162
receives signals from the transmitter
163
over a link
114
and sends signals to the receiver
164
over a link
115
.
In the transmitter
163
, signals carrying speech information and other information as required for telecommunication are modulated for transmission by means of a radiofrequency carrier wave. Modulation operations are usually performed at an intermediate frequency which is mixed with the desired transmission frequency. Various methods of modulation are known. They depend on the type of signal and on the equipment available for transmission. To transmit analog information, it is possible to use frequency modulation or frequency shift keying (FSK). To transfer digital information, it is possible to use phase shift keying (PSK) e.g. of the &pgr;/4 PSK type, or to use Gaussian minimum shift keying (GMSK).
FIG. 2
is a simplified block diagram of a transmission system for the transmitter
163
, and more particularly of a prior art digital-to-analog converter circuit. The signal coming from the signal processor unit
151
is sent in the form of digital data to an interface
210
of the transmitter
163
. The signal received by the transmitter
163
is processed in succession by the interface
210
, by a modulator
220
performing GMSK type modulation, by a digital-to-analog converter
240
(DAC), by a sample-and-hold circuit
230
, and by an analog filter
250
.
In the prior art, for reasons to do with energy consumption and with ease of implementation, the DAC
240
is usually a switched-capacitor DAC. Thus, for example, each group of k bits from the modulator
220
produces a voltage that is proportional to the value encoded on the k bits directly at the output from the DAC
240
. That method gives rise to problems of linearity between the signal from the DAC
240
and the signal input to the DAC
240
. These problems of linearity are made worse by the fact that there is a sample-and-hold circuit
230
in the above-described transmission system. The DAC
240
is a switched-capacitor digital-to-analog converter. This means that for a half-period of a cyclic clock signal controlling the transfer of data along the above-described transmission system, the values of the bits processed by the DAC
240
are not available. The sample-and-hold circuit
230
is therefore required to maintain each processed bit value for at least one half-period of the clock signal. The presence of the sample-and-hold circuit
230
nevertheless increases problems of non-linearity in the transmission of the signal. In certain transmitters, and in particular in transmitters used in GSM type mobile telephone systems, the distortion of the transmitted signal can be very detrimental to the quality of the call.
The prior art, in particular patent EP A-0642221, describes an electronic digital-to-analog converter circuit for a baseband transmission system. The circuit includes an interpolator for increasing a sampling frequency of the digital signal, a sigma-delta type modulator, a digital-to-analog converter block comprising a finite impulse response filter (FIR), and finally an analog filter of lowpass filter type.
However, the FIR described in that document is a conventional FIR, i.e. having a transfer function of the type h(z)=1+a
1
z
−1
+a
2
z
−2
+a
3
z
−3
+ . . . . Such a filter is not adapted to an application in the GSM fi
Maecker Arnaud
Yang Fuji
Alcatel
Nguyen John
Sughrue Mion Zinn Macpeak & Seas, PLLC
Young Brian
LandOfFree
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