Abstract:
An apparatus and method for internally calibrating a direct conversion receiver (DCR) through feeding a calibration signal via ESD protection circuitry is disclosed. The apparatus includes an internal signal generator for generating a calibration signal, a front-end input stage for receiving an RF signal at an input node, an ESD protection unit for protecting against electrostatic discharge, and a switch unit coupled to the ESD protection unit, for selectively passing a calibration signal to the front-end input stage, whereby the connection of the switch unit and the ESD protection unit means that when the DCR is operating in normal mode, the switch unit will not affect the noise performance and matching of the receiver.

Description:
BACKGROUND 
   The present invention relates to calibrating a direct conversion receiver, and more particularly to an apparatus and method for feeding an internal calibration signal through an electro-static discharge protection circuitry for a direct conversion receiver. 
   The basic principle of a superheterodyne receiver is to convert a band of RF signals to an intermediate frequency (IF) by mixing a first tunable local oscillator (LO) signal with the RF signals, and further convert the IF signals to a baseband frequency by mixing a second LO signal with the IF signals. A direct conversion receiver (DCR) is a receiver that directly converts RF signals to the baseband frequency in a single conversion step, without having to first convert the RF signals to an intermediate frequency. In a direct conversion receiver the LO signal is set to the same frequency as the desired RF channel, which means that the intermediate frequency is zero, or Direct Current (DC). Direct conversion receivers therefore require less external circuitry than other receivers and have the added advantages of lightweight and low cost. 
   A DCR has the well-known problem of DC offset associated with the process of down-converting the RF signal, however, which may initially be small but is amplified during signal processing, as the signal will pass through at least one gain stage. This amplified DC offset can cause saturation of the signal, at best decreasing sensitivity of the receiver, and at worst causing the receiver to fail. DC offset can also be created if there is leakage of the local oscillator signal to the front-end RF input. This can affect the linearity of the receiver. The less linear the front-end receiver is, the more the local oscillation signal will be affected by interference. 
   If the DCR is calibrated, the level of DC offset inherent in a received RF signal can be accurately measured, stored, and used to compensate for DC offset in future operations. Some receivers employ a method of external calibration for this purpose, which requires external circuitry, therefore negating the advantages the DCR has of simple circuitry. To solve this problem, a related art DCR uses a process of internal calibration, whereby an internal calibration signal is generated for adjusting DC offset of received signals. 
   Please refer to  FIG. 1 .  FIG. 1  is a diagram of a related art DCR  10  that uses internal calibration. Please note that the diagram is a simplified diagram and only shows the essential parts. The DCR  10  comprises a front-end input stage  20  for receiving an RF signal, further comprising an antenna  22  for receiving the RF signal, a filter  24  for filtering the received RF signal, and a low noise amplifier  26 , for amplifying the filtered RF signal; an internal signal generator  30 , for generating a calibration signal; and a switch unit  40 , selectively coupled to the internal signal generator  30  and the front-end input stage  20 , for allowing the calibration signal to be fed to an input node of a processing unit  52  when the switch unit  40  is in the ‘on’ position and for disconnecting the internal signal generator  30  from the front-end input stage  20  when the DCR  10  is operating in normal mode. 
   When the DCR  10  is in calibration mode, i.e. the switch unit  40  is turned on and the calibration signal is passed to the processing unit  52 , the DC offset can be correctly calculated through following signal processing. When the switch unit  40  is turned off, although the switch unit  40  is not operational, the direct coupling of the switch unit  40  to the front-end input stage  20  will cause some parasitic resistance and parasitic capacitance to be present in the DCR  10 . The parasitic resistance introduces thermal noise into the circuit, which greatly affects the noise performance of the circuit and changes the receiver characteristics. The parasitic capacitance affects the input matching condition of the front-end circuit. The performance of the related art DCR  10  is thus degraded. 
   SUMMARY 
   It is therefore one of the objectives of the present invention to provide a DCR with internal calibration utilizing ESD protection circuitry for eliminating the extra parasitic resistance and parasitic capacitance, to solve the above-mentioned problem. 
   Briefly described, a first embodiment of the present invention comprises a front-end input stage for receiving and amplifying an RF signal at an input node; an electrostatic discharge (ESD) protection circuit, for protecting the Direct Conversion Receiver from ESD pulses, where the ESD protection circuit comprises a plurality of diodes connected in series between two voltage supply rails; an internal signal generator for generating a calibration signal; a processing unit for processing the RF signal and for calculating the DC offset of said signal; and a switch unit comprising a MOS transistor, coupled to the internal signal generator and the ESD protection circuit, for selectively passing the calibration signal to the front-end input stage by means of a control signal connected to the switch unit, so the switch unit is not directly connected to the front-end input stage of the DCR. 
   A second embodiment of the present invention is disclosed. Briefly described, the embodiment comprises a front-end input stage for receiving and amplifying an RF signal at an input node; an electrostatic discharge (ESD) protection circuit, for protecting the Direct Conversion Receiver from ESD pulses, where the ESD protection circuit comprises at least one diode and a switch unit connected in series between two voltage supply rails, and the switch unit is a MOS transistor for passing a calibration signal to the front-end input stage when the DCR is operating in calibration mode and for protecting the DCR from ESD pulses when the DCR is operating in normal mode by means of a control signal input to the switch; a processing unit for processing the received RF signal from the front-end input stage and for calculating the DC offset of said signal; and an internal signal generator, connected to the switch unit, for generating the calibration signal; wherein when the DCR is operating in normal mode, the switch unit functions as a diode, so the connection of the switch to the front-end input stage does not introduce parasitic resistance and parasitic capacitance to the circuit. 
   A method for calibrating a DCR by means of an internal calibration signal is also disclosed. Briefly described, the method comprises generating an internal calibration signal by means of an internal signal generator; selectively passing the calibration signal through an ESD protection circuit to a front-end input stage by means of a switch unit coupled to a control signal; processing the signal such that the DC offset is measured and the value stored, the stored value being used to compensate for DC offset when the receiver is operating in normal mode. The connection of devices so that the signal is passed through the ESD protection circuit before being input to the front end ensures that the presence of the switch will not introduce parasitic resistance and parasitic capacitance to the DCR when the switch is not operational, i.e. when the DCR is not operating in a calibration mode. 
   It is an advantage of the present invention that the presence of the switch unit does not affect the performance of the receiver when the receiver is operating under normal mode. It is a further advantage that a calibration signal can be generated internally through the connection of the switch unit and the ESD protection circuit, without requiring complex circuitry. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of a direct conversion receiver according to the related art. 
       FIG. 2  is a diagram of a direct conversion receiver according to a first embodiment of the present invention. 
       FIG. 3  is a diagram of a direct conversion receiver according to a second embodiment of the present invention. 
       FIG. 4  is a diagram of a direct conversion receiver according to a third embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Please refer to  FIG. 2 .  FIG. 2  is a diagram of a DCR  100  according to a first embodiment of the present invention. The DCR  100  comprises a front-end input stage  120  for receiving an RF signal at an input node; a plurality of electrostatic discharge (ESD) protection units  130   a ,  130   b  collectively comprising three diodes  132 ,  134 ,  136  connected in series between two voltage supply rails, V cc  and ground; an internal circuit  50  including a processing unit  52 ; an internal signal generator  30 ; a control signal generator  190 ; and a switch unit  170 . The front-end input stage  120  has the same functionality as that of the above-mentioned related art front-end input stage  20 . These ESD protection units  130   a ,  130   b  together locate at the input of the front-end input stage  120  to build the ESD protection circuit for the DCR  100  to dissipate ESD pulses which might damage the front-end input stage  120  and the internal circuit  50 . The switch unit  170  is connected to the control signal generator  190  and the internal signal generator  30 . The internal circuit  50  is coupled to the front-end input stage  120 . The processing unit  52  is used to process signal(s) from the front-end input stage  120 . For example, the processing unit  52  is capable of estimating DC offset by processing the calibration signal received from the internal signal generator  30  in calibration mode and processing the RF signal received from the front-end input stage  120  in normal mode. 
   The ESD protection unit  130   b  consists of one n+/p-well diode  132  and one p+/n-well diode  134 , connected in series between ground and the front-end input stage  120 , and the ESD protection unit  130   a  has a p+/n-well diode  136  connected between the front-end input stage  120  and a positive voltage supply rail V cc . Please note that more than three diodes may be used in the ESD protection units  130   a ,  130   b  and the number of diodes in this embodiment is merely meant to be an illustration and not a limitation of the present invention. The switch unit  170  is implemented by an NMOS transistor, having a gate coupled to the control signal generator  190 , a drain coupled to the internal signal generator  30 , a source coupled to a p+region of the second p+/n-well diode  134 , and a body coupled to the ground. The ESD protection units  130   a ,  130   b  are designed to protect the DCR  100  from the effects of electrostatic discharge by utilizing the series of diodes  132 ,  134 ,  136  connected between two voltage supply rails, in order to discharge positive ESD pulses from the input to the positive supply rail via the diode  136  and discharge negative ESD pulses from ground to the input via the diodes  132 ,  134 . The control signal generator  190  is utilized for controlling the switch unit  170  to selectively pass the calibration signal to the front-end input stage  120  through the switch unit  170  and the p+/n-well diode  134 . When a control signal generated from the control signal generator  190  having a high logic level is input to the gate, the switch unit  170  can allow the calibration signal to flow from the drain to the source. However, when the control signal has a low logic level or the control signal generator  190  generates no control signal, there is insufficient voltage built up at the gate for inducing the conductive channel in the switch unit (NMOS transistor)  170 . Therefore, any signals outputted from the internal signal generator  30  are blocked from flowing from the drain to the source, that is, the calibration signal cannot be passed to the front-end input stage  120 . During normal mode, i.e. when the calibration signal is not being passed, the switch unit  170  is not operating. In the related art, the presence of the switch during normal mode causes some parasitic resistance and parasitic capacitance to be present in the DCR, affecting the noise performance and matching of the receiver. However, in the present invention, the switch unit  170  is connected to the front-end input stage  120  through the ESD protection unit  130   b . The parasitics of the diodes in the ESD protection units  130   a ,  130   b  that discharge current are the inherent parasitics and the switch  170  does not contribute additional parasitics to the input node. 
   Please refer to  FIG. 3 .  FIG. 3  is a diagram of a DCR  200  according to a second embodiment of the present invention. The DCR  200  comprises a front-end input stage  120  for receiving an RF signal at an input node; a plurality of electro-static discharge (ESD) protection units  230   a ,  230   b  comprising two diodes  240 ,  260  and a switch unit  250  connected in series between two voltage supply rails, V cc  and ground; an internal circuit  50  including a processing unit  52  used to process incoming signals; a control signal generator  190 ; and an internal signal generator  30 . The internal circuit  50  is coupled to the front-end input stage  120 . 
   The key difference between the DCRs  100  and  200  is the configuration of the ESD protection circuitry. In this embodiment, the ESD protection unit  230   b  comprises the n+/p-well diode  240  and the switch unit  250  connected in series between ground and the internal circuit  50 , and the ESD protection unit  230   a  has a p+/n-well diode  260  connected between the front-end input stage  120  and a positive voltage supply rail V cc . Please note that more than two diodes may be used in each ESD protection unit  230   a ,  230   b  and the number of diodes in this embodiment is merely meant to be an illustration and not a limitation of the present invention. The switch unit  250  in this embodiment is implemented by an NMOS transistor having a gate coupled to the control signal generator  190  for receiving a control signal generated from the control signal generator  190 , a source coupled to the calibration signal, a drain coupled to the front-end input stage  120 , and a body coupled to the source. In this embodiment, the switch unit  250  is part of the ESD protection unit  230   b . When the DCR  200  is in calibration mode, the control signal having a high logic level is passed from the control signal generator  190  to the gate of the NMOS transistor (switch unit  250 ) in order to induce a conductive channel between the drain and the source, allowing current to flow, and the calibration signal to be passed. When the DCR  200  is not in calibration mode, the switch unit  250  functions as a pure diode, with the gate in a low logic level. The NMOS in this embodiment has a lateral p-guard-ring/n+ drain, and it cannot pass the calibration signal when the gate of the NMOS transistor (switch unit  250 ) is in a low logic level. As the switch unit  250  acts as a switch in calibration mode while acts as a diode in normal mode, the presence of the switch unit  250  will not contribute additional parasitics to the DCR  200  under normal mode. 
   Please refer to  FIG. 4 .  FIG. 4  is a diagram of a DCR  300  according to a third embodiment of the present invention. The DCR  300  comprises a front-end input stage  120 ; a plurality of electro-static discharge (ESD) protection units  330   a ,  330   b  comprising two diodes  340 ,  360  and a switch unit  350  connected in series between two voltage supply rails, V cc  and ground; an internal circuit  50  including a processing unit  52  used to process incoming signals; a control signal generator  190 ; and an internal signal generator  30 . The internal circuit  50  is coupled to the front-end input stage  120 . 
   In this embodiment, the ESD protection unit  330   a  comprises the diode  360  and the switch unit  350  connected in series between a positive voltage supply rail V cc  and the front-end input stage  120 , and the ESD protection unit  330   b  has the n+/p-well diode  340  connected between the front-end input stage  120  and ground. Please note that more than two diodes may be used in each of the ESD protection units  330   a ,  330   b  and the number of diodes in this embodiment is merely meant to be an illustration and not a limitation of the present invention. The switch unit  350  in this embodiment is implemented by a PMOS transistor, having a gate coupled to the control signal generator  190  for receiving a control signal, a source coupled to the calibration signal, a drain coupled to the front-end input stage  120 , and a body coupled to the source. In this embodiment, the switch unit  350  is part of the ESD protection unit  330   a . The operation of this embodiment is the same as the previous embodiment except that the NMOS transistor has been replaced with a PMOS transistor, which is connected between the front-end input stage and V cc . The calibration signal is selectively passed to the front-end input stage  120  by utilizing the control signal having a low logic level input to the gate of the PMOS transistor (switch unit  350 ). When the DCR  300  is not in calibration mode, the switch unit  350  functions as a pure diode, with the gate in a high logic level. As detailed in the previous embodiment, the switch unit  350  acts as a switch in calibration mode while acting as a diode in normal mode. Therefore, the presence of the switch unit  350  will not contribute additional parasitics to the DCR  300  under normal mode 
   Please note that the calibration signal can be generated in many ways. A first method involves dividing a VCXO output F VCXO  by 4 and up-converting the output F VCXO /4 and a VCO output to generate a calibration signal F VCO +F VCXO /4. A second method is to up-convert F VCO  and the output from a ring oscillator to generate a calibration signal F VCO +F ring . A third method is to up-convert F VCO  and the output from a filter-calibration-oscillator F cal  to generate a calibration signal F VCO +F cal . A fourth method is simply to take the output of the VCO as the calibration signal. Since these methods are well known to those skilled in the art, further description is omitted for brevity. 
   The connection of the switch unit to the ESD protection circuit negates the negative effects caused by the related art direct connection of the switch unit to the front-end input stage, giving the direct conversion receiver higher performance. In the first embodiment of the present invention, the input node will not suffer from extra parasitic capacitance caused by the presence of the switch unit. In the second and third embodiments of the present invention, the switch unit serves as a switch in calibration while acts as a diode in normal mode, and therefore does not affect the noise performance and matching of the front-end input stage. When the switch unit is in the ‘on’ position, i.e. the receiver is in calibration mode, the calibration signal can be passed to the front-end input stage of the receiver. When the switch unit is in the ‘off’ position, i.e. the receiver is operating in normal mode, the connection of the switch unit and the ESD protection unit allows the receiver to operate as though no switch were present. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.