Abstract:
A wireless user terminal ( 42 ) and system ( 40 ) implementing a mixed signal CODEC ( 100 ) including an improved sigma-delta ADC ( 18 ) which limits input signals into a switched capacitor configuration and avoids adding circuit overhead in the signal path is disclosed herein. This sigma-delta analog-to-digital converter ( 18 ), having an input signal and an output signal, includes a switch (sw 1 ), a clipping circuit ( 20 ), and a known sigma-delta ADC ( 34 ). It solves the clipping signal problem by limiting the signal right at the input of the sigma-delta ADC ( 34 ). The clipping circuit ( 20 ) couples to the switch (sw 1 ) and the sigma-delta ADC ( 34 ) for switching the voltage applied to the sigma-delta ADC between the input signal (v in ) and at least one threshold voltage (V n  and V p ).

Description:
FILED OF THE INVENTION 
   This invention relates generally to the field of electronic systems and, in particular, to a wireless user terminal and system having signal clipping circuits for switched capacitor sigma delta analog-to-digital converters included within audio codec systems. 
   BACKGROUND OF THE INVENTION 
   The codifier/decodifier (CODEC) is the algorithm that handles the coding and decoding of audio signals within an electronic system. Specifically, an audio CODEC is a custom mixed-signal core providing analog-to-digital (A/D) and digital-to-analog (D/A) conversion. A simple serial interface is used to exchange digital data (D/A input and A/D output) between the application specific integrated circuit (ASIC) and CODEC core. Prior art CODEC features delta-sigma A/D and D/A oversampled converters and low power dissipation. 
   A typical uplink channel for a mobile phone voiceband or audio CODEC includes a microphone, amplifier, sigma-delta analog-to-digital converter (ADC) and a digital filter coupled together on one chip. This first chip couples to a digital signal processor for processing the digital signal received. Another chip includes a radio frequency (RF) modulator which is coupled to a last component that includes a RF power amplifier. The signal is transmitted over an antenna to a downlink channel for the mobile phone voiceband CODEC. 
   Initially, the audio CODEC receives an analog voice signal through the microphone and converts it to a digital signal. The digital signal is forwarded to a digital signal processor for processing. This signal is transmitted to a receiver. In the receiver, the digital signal is processed through the digital signal processor and forwarded to a D/A converter. The analog signal is fed to a speaker. 
   In most prior art CODECs, the sigma-delta ADC is scaled for a maximum output corresponding to the +3 dbm0 code of the pulse code modulation (PCM) data. The analog signal corresponding to this digital upscale value is far less than the maximum allowable dynamic range, which usually is limited by the supply range. This fact could potentially overload the A/D and consequently the digital filter. An FCC test, mandatory in the U.S., falls under this category. Once the digital filter overloads, internal clipping mechanisms prevent wrap around of the digital signal, thus creating a digital representation of a trapezoidal signal that contains harmonics with sufficient power to increase the FM modulation depth. 
   First and second order sigma delta analog modulators are inherently stable under large input level variations. Higher order modulators, however, can become unstable during the overload condition. Clipping the input signal to a pre-determined safe operation level, prevents the modulator from going unstable, without having the need to recover stability after the overloading condition is removed. In other cases, even inherently stable sigma-delta structures have to be protected by a clipping mechanism to prevent post digital filtering from generation of a rail-to-rail digital representation of a quasi-square wave which can over-modulate the RF channel in a typical transmit CODEC channel for wireless applications. 
   Several implementations have been proposed to solve this problem. Most of them deal with clipping the signal in a previous analog amplifier stage. One solution is provided in U.S. patent application Ser. No. 09/351,610, which discloses a multiplexer amplifier having an analog output signal, a sigma-delta ADC having an input coupled to the analog output signal and a clipping circuit coupled to the input of the ADC for clipping the analog output signal. While this analog solution avoids saturation and provides an effective clipping mechanism to prevent wrap around of the digital signal, it is prone to overshoot and settling issues. 
   In present systems, however, the signal is fed to the A/D directly from external sources, such as a microphone or an RF mixer. Accordingly, many audio CODECs no longer include the microphone and amplifier. Thus, there is a need for a wireless user terminal and system that incorporate a sigma-delta analog-to-digital converter (ADC) that is free of overshoot and settling issues. 
   SUMMARY OF THE INVENTION 
   A wireless user terminal and system that implement a mixed signal CODEC including an improved sigma-delta ADC limits input signals into a switched capacitor configuration and avoids adding circuit overhead in the signal path. Additionally, the improved sigma-delta ADC substantially reduces overshoot and settling problems common in user terminals. This improved sigma-delta ADC, having an input signal and an output signal, includes a switch, a clipping circuit, and a sigma-delta ADC. It solves the clipping signal problem in wireless user terminals by limiting the signal right at the input of the sigma-delta ADC. The clipping circuit couples to the switch and the sigma-delta ADC for switching the voltage applied to the sigma-delta ADC between the input signal and at least one threshold voltage. When the input signal goes above a prescribed upper threshold, the fixed threshold voltage is applied to the sigma-delta ADC, which converts fixed threshold voltage into a digital signal. Moreover, when the input signal goes below that prescribed threshold, the incoming signal is applied to the sigma-delta ADC, which converts the incoming signal. In the alternative, when the input signal goes below a prescribed lower threshold, the fixed threshold voltage is applied to the sigma-delta ADC, which converts fixed threshold voltage. Furthermore, when the input signal goes above that prescribed threshold, the incoming signal is applied to the sigma-delta ADC, which converts the incoming signal. Given this solution, minimum power and area overhead exist. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numbers indicate like features and wherein: 
       FIG. 1  is a schematic of a known simplified input stage of a sigma delta modulator; 
       FIG. 2  is a schematic of a signal clipping circuit in accordance with the present invention; 
       FIG. 3   a  is a diagram of the input voltage applied with respect to time; and 
       FIG. 3   b  is a diagram of the clipped input voltage in accordance with the present invention. 
       FIG. 4  illustrates a communications system that implements the signal clipping circuit of one embodiment of the present invention; 
       FIG. 5  illustrates a block diagram of a wireless user terminal implemented in an embodiment of the present invention; 
       FIG. 6  illustrates a wireless user terminal block diagram that implements the signal clipping circuit according to an embodiment of the present invention; 
       FIG. 7  illustrates a wireless user terminal receiver block diagram that implements the signal clipping circuit according to an embodiment of the present invention; 
       FIG. 8  illustrates the transmitted spectra for TDMA (GSM) and CDMA (IS-95) systems; and 
       FIG. 9  illustrates a spectral definition of 2G and 3G cellular regulations. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   A circuit is presented here, that clips the incoming signal to predetermined levels without disturbing the signal path and adding little overhead to the power and area requirements. In  FIG. 1 , analog clipping circuits  40  and  42  are coupled to the differential inputs of the sigma-delta A/D  26 , to avoid overdriving the sigma-delta A/D  26 . The analog clipping circuits  40  and  42  add minimum overhead in area and power. For the preferred embodiment, the maximum allowable dynamic range at the input of the sigma delta A/D  26  is a minimum of 0.625 volts and a maximum 2.375 volts. The fully differential signal is 3.5 volts (+1.75 volts to −1.75 volts). Each single ended signal is clipped at a low of 0.625 volts (V RL ) and a high of 2.375 volts (V RH ). This clipping problem solution adds a pre-amp to the signal path. The amplifier then, has to perform better than the noise specification of the channel which implies high current consumption and silicon area utilization. This solution adds a constraint to the external driving source since now the input to the chip is not capacitively coupled anymore but rather has low resistance. 
   A voiceband CODEC  18  having an improved sigma-delta A/D converter in accordance with the present invention is shown in FIG.  2 . The incoming signal v in  is connected to the comparators C p    24  and C n    22  (which can be designed for very low current since speed and offset are not a primary concern), as well. Threshold voltages, V p  and V n  (which can be generated from the bandgap or derived from the supply voltage through a resistor/diode division), are coupled to comparators, C p    24  and C n    22 , respectively. The incoming signal is sensed by comparators C p  and C n , comparing the incoming signal with voltages V p  and V n . Comparators C p  and C n  are connected to switches, sw p  and sw n , respectively for switching in voltage levels, V p  and V n , respectively. All three switches, sw 1 , sw p  and sw n , couple to a sigma-delta ADC  34 . Switch sw 1  couples to receive the incoming signal V in . 
   In operation, when incoming signal v in  rises above the threshold voltage V p , switch sw 1  opens and comparator C p  turns on, closing switch sw p . Accordingly, the fixed voltage V p  is supplied to the sigma delta ADC  34 . When the value of the signal goes below the threshold voltage, comparator C p  shuts off, opening switch sw p . Simultaneously, switch sw 1  closes and incoming signal v in  is supplied directly to sigma-DAC  34 . 
   When incoming signal v in  goes below threshold voltage V n , switch sw 1  opens and comparator C n  turns on, closing switch sw n . Accordingly, the fixed voltage V n  is supplied to the sigma delta ADC  34 . When the value of signal v in  rises above the threshold voltage V n , the comparator C n  shuts off, opening switch sw n . Simultaneously, switch sw 1  closes and incoming signal v in  is supplied directly to the sigma delta ADC. 
     FIG. 3   a  displays the input signal v in , while  FIG. 3   b  shows the clipped input signal v clip  seen by the sigma-delta ADC  34 . As shown in  FIG. 3   b , when incoming signal v in  rises above the threshold voltage V p , switch sw 1  opens and comparator C p  turns on, closing switch sw p . As a result, the voltage V clip  is equal to the threshold voltage V p , When the value of the signal v in  goes below the threshold voltage V p , the comparator C p  shuts off, opening switch sw p . Switch sw 1  closes and, as a result, voltage v clip  equals the incoming signal v in . When the incoming signal v in  goes below threshold voltage V n , switch sw 1  opens and the comparator C n  turns on, closing switch sw n . Accordingly, voltage v clip  equals the fixed voltage V n . 
   The signal clipping circuit for switched capacitor sigma delta analog-to-digital converter (ADC) of the present invention can be used in a variety of telecommunication and other applications. Conveniently, the signal clipping circuit for improved sigma delta analog-to-digital converters can be implemented in wireless user terminals and base stations operating according to international standards, such as for example CDMA (Code Division Multiple Access) and GSM (Global System for Mobile Communication). 
     FIG. 4  illustrates a wireless communication system in which the signal clipping circuit for improved sigma delta analog-to-digital converters of the present invention may be implemented. Wireless communication system  40  comprises a wireless user terminal (a cellular handset being illustrated)  42  that communicates with a base station (a cellular base station being illustrated)  44  over an uplink channel  46  and downlink channel  48 . The base station and the wireless user terminal unit operate in a similar manner. 
   Cellular communication in system  40  can be facilitated in Time Domain Duplex (TDD) or in Frequency Domain Duplex (FDD). In Time Domain Duplex (TDD) the communication between wireless user terminal  42  and base station  44  is on a single channel. Much like a walky-talky, the channel is shared in time by the mobile station transmitter and the base station transmitter. A time slot is dedicated to the uplink and another timeslot is dedicated to a downlink. The relative length of the uplink and downlink time slots can be adjusted to accommodate asymmetric data traffic. If it is found that downlink data traffic is on average twice that of uplink, then the downlink time slot is twice as long as the uplink time slot. In Frequency Domain Duplex (FDD) the wireless user terminal  42  and the base station  44  communicate over a pair of radio frequencies. The lower frequency is the uplink during which the mobile station sends information to the base station. Both uplink and downlink are each composed of a signal source, a transmitter, the propagation path, a receiver and a method of presenting the information. Both wireless user terminal and base station embody the invention with transmitters, which convert digital data to analog signals at high speed and with high resolution. The base station could convert the entire multi-carrier downlink signal to analog for use in a single RF transmitter. The wireless user terminal is explained in the following. 
     FIG. 5  presents a top-level block diagram  50  of the wireless user terminal  42 . In wireless user terminal  42 , radio frequency (RF) signals are received and transmitted by the RF section  52 . In the embodiment illustrated, RF section  52  comprises a duplexer  76  coupling an antenna  78  to a receiver  68  and a power amplifier  74 . A modulator  72  is coupled to power amplifier  74  and to a synthesizer  70 . Synthesizer  70  is further coupled to receiver  68 . RF section  52  is further coupled to an analog baseband  54 . In the embodiment illustrated, analog baseband  54  comprises an RF interface  56  and an audio interface  58 . A speaker  88  and a microphone  90  are coupled to audio interface  58 . RF interface  56  is coupled to both receiver  68  and modulator  72  of RF section  52 . The analog RF interface  56  includes I and Q analog-to-digital converters (ADCs), according to the present invention, and digital-to-analog converters (DACs), for conversion between the analog and digital domains. Audio interface  58  may also include I and Q analog-to-digital converters (ADCs), according to an embodiment of the present invention, and digital-to-analog converters (DACs), for conversion between the digital and analog domains. Analog baseband  54  is further coupled to a digital baseband  60 . 
   In the illustrated embodiment, digital baseband  60  comprises three elements: digital signal processor (DSP)  62 , microcontroller unit (MCU)  64  and application specific integrated circuit (ASIC)  66 . DSP  62  couples audio interface  54  to RF interface  56  and to microcontroller unit (MCU)  64 . Digital signal processor (DSP)  62  and microcontroller unit (MCU)  64  are further coupled to ASIC backplane  66 . Microcontroller unit (MCU)  64  is further coupled to a user interface  80 , which comprises at least a user display  82  and a keyboard  84  (an optional SIM card  86  is also disclosed). 
   The digital signal processor (DSP)  62 , provides programmable speech coding and decoding (vocoder), channel coding and decoding, equalization, demodulation and encryption. The microcontroller unit (MCU) handles level 2 &amp; 3 protocol, radio resource management, short message services, man-machine interface and the real-time operating system. The ASIC backplane  66  performs all chip-rate processing. While top level diagram  50  illustrates RF section  52 , analog baseband  54  and digital baseband  60  as being separate packages or chips, the invention contemplates substitution of any of the above with an equivalent function, such as an RF function, and/or an analog baseband function and/or a digital baseband function. The functions will remain the same even if the actual implementation varies. The invention further contemplates that RF section  52 , analog baseband  54  and digital baseband  60  may be selectively combined and/or integrated into one or two packages or chips. 
   An uplink voice processing chain  46  for a wireless user terminal  42  is illustrated in FIG.  6 . This channel includes a CODEC  100  coupling a microphone  90  to a vocoder  98 , a baseband modulator  96  coupling vocoder  98  to a digital-to-analog converter  92  at high speed and high resolution. An RF transmitter  94  (part of RF section  52 ) couples an antenna  78  to digital-to-analog converter  92 . Within RF transmitter  94 , modulator  72  is implemented as two RF mixers, I and Q driven by the synthesizer, implemented as an RF local oscillator. RF transmitter CODEC  100  includes an audio amplifier (not shown), sigma-delta analog-to-digital converter (ADC) (not shown) and a digital filter (not shown) coupled together on one chip. The CODEC receives an analog voice signal through the microphone and converts it to a digital signal. While CODEC  100  is shown as being separate from digital baseband  60 , it may also be internal to digital baseband  60 . CODEC  100  transcodes audio signals into digital words using the algorithms contained in the VOCODER. This signal is then complex modulated, converted to analog (I&amp;Q) and applied to the transmitter. The transmitter is complex modulated at the radio frequency assigned to the handset. It uses a power amplifier coupled to the antenna  78  to transmit the digital signal, effectively communicating the (digital) voice information to the base station receiver. 
   A downlink voice channel  48  for wireless user terminal  42  is illustrated in FIG.  7 . This channel includes an RF receiver  102  (part of RF section  52 ) coupling antenna  78  to an analog-to-digital converter (ADC)  104 , according to the invention, a vocoder  98  coupling a demodulator  96  to a CODEC  100 , and a speaker  88  coupled to CODEC  100 . While CODEC  100  is shown as being separate from digital baseband  60 , it may also be internal to digital baseband  60 . CODEC  100  transcodes the digital words into analog signals using the algorithms contained in the VOCODER. CODEC  100  includes a digital filter, DAC and audio amplifier coupled together on one chip. The RF receiver uses an AGC circuit which varies the IF amplifier gain as a function of the received signal. The goal is to present the analog-to-digital converters (ADCs) with a full-scale analog signal without distortion and with minimal noise. 
   The band structure of the cellular system in which the communication system of the present invention operates is composed of tightly packed RF carriers with very high spectral density. As illustrated in  FIG. 8 , the world&#39;s most widely deployed TDMA system is GSM, where the GMSK-modulated carriers are placed on a 200-Khz raster  106  with adjacent channel signal interference suppressed to −30 dBc at the first adjacent channel and −60 dBc at the second. The 2-G CDMA system used in America (IS-95) uses QPSK-modulated (at 1.2288 Msps) carriers spaced at 1.25 Mhz  108  with very little guard band. Each carrier can be modulated with up to 32 Walsh codes, which are used to separate the users. 
     FIG. 9  illustrates the spectral definition of the 2G and 3G cellular regulations. The base station transmitter operates on the upper frequency band. For example, in Europe the base station receives from 1900 to 1980 Mhz and transmits from 2110 to 2170 Mhz. 
   The signal analog-to-digital converter of the present invention can be use in other applications, such as data communication systems, hard disk drives, cd players, video displays, and any other application where there is a large amount of data that must be converted quickly. 
   Those skilled in the art to which the invention relates will appreciate that various substitutions, modifications and additions can be made to the described embodiments, without departing from the spirit and scope of the invention as defined by the claims.