Patent Publication Number: US-2023164007-A1

Title: Receiver for receiving multilevel signal

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority under 35 U.S.C. §119(a) to Korean Patent Application No. 10-2021-0160802, filed on Nov. 19, 2021, which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     Various embodiments generally relate to a receiver for receiving a multilevel signal, and more particularly, to a receiver capable of minimizing an offset of a sampler. 
     2. Related Art 
       FIG.  1    is a block diagram showing a conventional receiver  1  for receiving a Pulse Amplitude Modulation 4-level (PAM4) signal. 
     The conventional receiver  1  includes a linear equalizer  10 , a first sampler  21 , a second sampler  22 , and a third sampler  23 . 
     The linear equalizer  10  receives an input signal IN, performs an equalization operation, and outputs an equalization signal OUT. 
     The first sampler  21  compares the equalization signal OUT with a first reference voltage VREFH and samples a result thereof to generate the first signal OUTH. 
     The second sampler  22  compares the equalization signal OUT with a second reference voltage VREFM and samples a result thereof to generate a second signal OUTM. The second reference voltage VREFM is smaller than the first reference voltage VREFH. 
     The third sampler  23  compares the equalization signal OUT with a third reference voltage VREFL and samples a result thereof to generate a third signal OUTL. The third reference voltage VREFL is smaller than the second reference voltage VREFM. 
     The first to third reference voltages VREFH, VREFM, and VREFL are used to distinguish four levels of the PAM4 signal. 
     For example, the first reference voltage VREFH distinguishes a fourth level from a third level, the second reference voltage VREFM distinguishes the third level from a second level, and the third reference voltage VREFL distinguishes the second level from a first level. 
     Each of the first, second, and third samplers  21 ,  22 , and  23  includes input transistors for receiving differential signals. 
     In general, an input offset exists in the input transistors due to threshold voltage mismatch or beta mismatch. 
     An error may occur when the sampler determines a level due to the input offset, and accordingly, bit error rate (BER) may increase. 
     As shown in  FIG.  1   , the conventional receiver  1  includes the linear equalizer  10  and a plurality of samplers  21 ,  22 , and  23 , and an input offset exists in each of the plurality of samplers  21 ,  22 , and  23 . Accordingly, the bit error rate may be further increased. 
     SUMMARY 
     In accordance with an embodiment of the present disclosure, a receiver may include a plurality of linear equalizers receiving an input signal; and a plurality of samplers configured to sample a plurality of equalization signals output from the plurality of linear equalizers according to a clock signal. Each of the plurality of linear equalizers compares the input signal with a reference voltage among a plurality of reference voltages to determine a level of the input signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views, together with the detailed description below, are incorporated in and form part of the specification, and serve to further illustrate various embodiments, and explain various principles and beneficial aspects of those embodiments. 
         FIG.  1    illustrates a conventional receiver. 
         FIG.  2    illustrates a receiver according to an embodiment of the present disclosure. 
         FIG.  3    illustrates a first linear equalizer according to an embodiment of the present disclosure. 
         FIG.  4    illustrates a first sampler according to an embodiment of the present disclosure. 
         FIG.  5    illustrates a receiver according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description references the accompanying figures in describing illustrative embodiments consistent with this disclosure. The embodiments are provided for illustrative purposes and are not exhaustive. Additional embodiments not explicitly illustrated or described are possible. Further, modifications can be made to the presented embodiments within the scope of teachings of the present disclosure. The detailed description is not meant to limit embodiments of this disclosure. Rather, the scope of the present disclosure is defined in accordance with claims and equivalents thereof. Also, throughout the specification, reference to “an embodiment” or the like is not necessarily to only one embodiment, and different references to any such phrase are not necessarily to the same embodiment(s). 
       FIG.  2    illustrates a receiver  100  receiving a multilevel signal (e.g., a PAM4 signal) according to an embodiment of the present disclosure. 
     The receiver  100  includes a first linear equalizer  110 , a second linear equalizer  120 , a third linear equalizer  130 , a first sampler  210 , a second sampler  220 , and a third sampler  230 . 
     The first linear equalizer  110  compares an input signal IN with a first reference signal (e.g., a first reference voltage) VREFH, performs an equalization operation, and outputs first equalization signals OUTHP and OUTHN. The first equalization signals OUTHP and OUTHN are differential signals. 
     The first sampler  210  samples the first equalization signals OUTHP and OUTHN according to a clock signal CLK and outputs a first signal OUTH. 
     The second linear equalizer  120  compares the input signal IN with a second reference signal (e.g., a second reference voltage) VREFM, performs an equalization operation, and outputs second equalization signals OUTMP and OUTMN. The second equalization signals OUTMP and OUTMN are differential signals. The second reference voltage VREFM is smaller than the first reference voltage VREFH. 
     The second sampler  220  samples the second equalization signals OUTMP and OUTMN according to the clock signal CLK and outputs a second signal OUTM. 
     The third linear equalizer  130  compares the input signal IN with a third reference signal (e.g., a third reference voltage) VREFL, performs an equalization operation, and outputs third equalization signals OUTLP and OUTLN. The third equalization signals OUTLP and OUTLN are differential signals. The third reference voltage VREFL is smaller than the second reference voltage VREFM. 
     The third sampler  230  samples the third equalization signals OUTLP and OUTLN according to the clock signal CLK and outputs a third signal OUTL. 
     The first to third reference voltages VREFH, VREFM, and VREFL are used to distinguish first to fourth levels of the PAM4 signal. 
     For example, the first reference voltage VREFH distinguishes the fourth level from the third level, the second reference voltage VREFM distinguishes the third level from the second level, and the third reference voltage VREFL distinguishes the second level from the first level. 
       FIG.  3    is a circuit diagram illustrating the first linear equalizer  110  of  FIG.  2    according to an embodiment. 
     The first linear equalizer  110  includes a first transistor (e.g., a first PMOS transistor) MP 1  having a first terminal (e.g., a source) and a second terminal (e.g., a drain) coupled between a first power source VDD and a first node N 1 , and a second transistor (e.g., a second PMOS transistor) MP 2  having a first terminal (e.g., a source) and a second terminal (e.g., a drain) coupled between the first power source VDD and a second node N 2 . 
     A bias signal BIAS is provided to control terminals (e.g., the gates) of the first PMOS transistor MP 1  and the second PMOS transistor MP 2 . 
     The bias signal BIAS may be provided as a substantially fixed value after an optimal operating condition is found. 
     The first linear equalizer  110  includes a variable capacitor CC and a variable resistor RC coupled between the first node Ni and the second node N 2 . For example, the variable capacitor CC and the variable resistor RC may be coupled in parallel between the first node N 1  and the second node N 2 . 
     The variable capacitor CC and the variable resistor RC may be adjusted according to a Nyquist frequency and an amplification ratio. 
     The first linear equalizer  110  includes a third transistor (e.g., a third PMOS transistor) MP 3  having a first terminal (e.g., a source) and a second terminal (e.g., a drain) coupled between the first node Ni and a third node N 3  and a fourth transistor (e.g., a fourth PMOS transistor) MP 4  having a first terminal (e.g., a source) and a second terminal (e.g., a drain) coupled between the second node N 2  and the fourth node N 4 . 
     The input signal IN is provided to a control terminal (e.g., a gate) of the third PMOS transistor MP 3 , and the first reference voltage VREFH is provided to a control terminal (e.g., a gate) of the fourth PMOS transistor MP 4 . 
     The first linear equalizer  110  includes load resistors RL coupled between the third node N 3  and a second power source VSS and between the fourth node N 4  and the second power source VSS. 
     The third node N 3  outputs a second differential equalization signal (e.g., a negative equalization signal) OUTHN, and the fourth node N 4  outputs a first differential equalization signal (e.g., a positive equalization signal) OUTHP. 
     The first linear equalizer  110  performs an equalization operation for amplifying signal attenuation in the Nyquist frequency region of a channel. For example, the first linear equalizer  110  may perform an equalization operation for attenuating low-frequency signal components and amplifying components in the Nyquist frequency region of a signal transmitted through a channel. 
     Unlike the conventional linear equalizer that receives only the input signal IN, the first linear equalizer  110  of this embodiment compares and amplifies the input signal IN and the first reference voltage VREFH. 
     For example, when the input signal IN is greater than the first reference voltage VREFH, the first differential equalization signal OUTHP has a voltage greater than that of the second differential equalization signal OUTHN, and when the input signal IN is smaller than the first reference voltage VREFH, the first differential equalization signal OUTHP has a voltage smaller than that of the second differential equalization signal OUTHN. 
     The configuration and operation method of the second linear equalizer  120  and the third linear equalizer  130  are substantially the same as those of the first linear equalizer  110  except signals used therein. Accordingly, detailed descriptions of the configurations and operation methods of the second and third linear equalizers  120  and  130  may be omitted for the interest of brevity. 
       FIG.  4    is a circuit diagram illustrating the first sampler  210  of  FIG.  2    according to an embodiment of the present disclosure. 
     The first sampler  210  includes a fifth transistor (e.g., a fifth PMOS transistor) MP 5  having a first terminal (e.g., a source) and a second terminal (e.g., a drain) coupled between the first power source VDD and a fifth node N 5  and a control terminal (e.g., a gate) to which a clock signal CLK is applied. 
     The first sampler  210  includes a sixth transistor (e.g., a sixth PMOS transistor) MP 6  having a first terminal (e.g., a source) and a second terminal (e.g., a drain) coupled between the fifth node N 5  and a sixth node N 6  and a control terminal (e.g., a gate) to which a first differential equalization signal (e.g., a positive first equalization signal) OUTHP is applied, and a seventh transistor (e.g., a seventh PMOS transistor) MP 7  having a first terminal (e.g., a source) and a second terminal (e.g., a drain) coupled between the fifth node N 5  and a seventh node N 7  and a control terminal (e.g., a gate) to which a second differential equalization signal (e.g., a negative first equalization signal) OUTHN is applied. 
     The first sampler  210  includes an eighth transistor (e.g., an eighth PMOS transistor) MP 8  having a first terminal (e.g., a source) and a second terminal (e.g., a drain) coupled between the sixth node N 6  and the seventh node N 7  and a control terminal (e.g., a gate) to which the second power voltage VSS is applied. 
     The eighth PMOS transistor MP 8  provides a virtual ground in the process of comparing and amplifying the positive first equalization signal OUTHP and the negative first equalization signal OUTHN by the operation of a latch and improves an amplification characteristics of the latch. 
     The first sampler  210  includes a ninth transistor (e.g., a ninth PMOS transistor) MP 9  having a first terminal (e.g., a source) and a second terminal (e.g., a drain) coupled between the sixth node N 6  and an eighth node N 8  and a control terminal (e.g., a gate) coupled to a ninth node N 9  and a tenth transistor (e.g., a tenth PMOS transistor) MP 10  having a first terminal (e.g., a source) and a second terminal (e.g., a drain) coupled between the seventh node N 7  and the ninth node N 9  and a control terminal (e.g., a gate) coupled with the eighth node N 8 . 
     The first sampler  210  includes an eleventh transistor (e.g., a first NMOS transistor) MN 1  having a first terminal (e.g., a source) and a second terminal (e.g., a drain) coupled between the eighth node N 8  and the second power source VSS and a control terminal (e.g., a gate) to which a clock signal CLK is applied and a twelfth transistor (e.g., a second NMOS transistor) MN 2  having a first terminal (e.g., a source) and a second terminal (e.g., a drain) coupled between the ninth node N 9  and the second power source VSS and a control terminal (e.g., a gate) to which a clock signal CLK is applied. 
     The first sampler  210  includes a thirteenth transistor (e.g., a third NMOS transistor) MN 3  having a first terminal (e.g., a source) and a second terminal (e.g., a drain) coupled between the eighth node N 8  and the second power source VSS and a control terminal (e.g., a gate) coupled to the ninth node N 9 , a fourteenth transistor (e.g., a fourth NMOS transistor) MN 4  having a first terminal (e.g., a source) and a second terminal (e.g., a drain) coupled between the ninth node N 9  and the second power source VSS and a control terminal (e.g., a gate) coupled to the eighth node N 8 , and a fifteenth transistor (e.g., a fifth NMOS transistor) MN 5  having a first terminal (e.g., a source) and a second terminal (e.g., a drain) coupled between the eight node N 8  and the ninth node N 9  and a control terminal (e.g., a gate) to which a clock signal CLK is applied. 
     The first sampler  210  includes an SR latch  211  that outputs the first signal OUTH according to a set signal S and a reset signal R. 
     The first sampler  210  includes an inverter  212  for inverting a voltage SB of the eighth node N 8  to generate the set signal S, and an inverter  213  for inverting a voltage RB of the ninth node N 9  to generate the reset signal R. 
     When the clock signal CLK is at a high level, the voltages SB and RB of the eighth node N 8  and the ninth node N 9  are precharged to a first value (e.g., a low level). 
     At this time, both the set signal S and the reset signal R are at a second value (e.g., the high level), and the first signal OUTH maintains an existing value by the operation of the SR latch  211 . 
     When the clock signal CLK is at the low level, the voltages SB and RB of the eighth node N 8  and the ninth node N 9  are amplified differentially according to a voltage difference between the positive first equalization signal OUTHP and the negative first equalization signal OUTHN. 
     For example, when the positive first equalization signal OUTHP is greater than the negative first equalization signal OUTHN, the voltage of the sixth node N 6  becomes greater than the voltage of the seventh node N 7 , and the voltage SB of the eighth node N 8  is amplified to the low level and the voltage RB of the ninth node N 9  is amplified to the high level. 
     Accordingly, the set signal S becomes the high level, the reset signal R becomes the low level, and the first signal OUTH becomes the high level. 
     Conversely, when the positive first equalization signal OUTHP is smaller than the negative first equalization signal OUTHN, the voltage of the seventh node N 7  becomes greater than the voltage of the sixth node N 6 , and the voltage of the seventh node N 7  becomes higher than that of the sixth node N 6 , and the voltage SB of the node N 8  is amplified to the high level and the voltage RB of the ninth node N 9  is amplified to the low level by the latch operation. 
     Accordingly, the set signal S becomes the low level, the reset signal R becomes the high level, and the first signal OUTH becomes the low level. 
     The first sampler  210  may have an offset in the input transistors. However, the input offset of the first sampler  210  is reduced in inverse proportion to the amplification ratio of the first linear equalizer  110  of  FIG.  3   . 
     Accordingly, the first sampler  210  does not require a separate circuit for compensating for the input offset. 
     Since the second sampler  220  and the third sampler  230  differ only in signals and have substantially the same structure as the first sampler  210 , descriptions of the configurations and operation methods of the second and third linear samplers  220  and  230  will be omitted for the interest of brevity. As described above, the first, second, and third liner equalizers  110 ,  120 , and  130  are coupled to the first, second, and third samplers  210 ,  220 , and  230 , respectively. Each of the first, second, and third linear equalizers  110 ,  120 , and  130  may provide a pair of differential equalization signals with a given amplification ratio to each of the first, second, and third samplers  210 ,  220 , and  230 , and the input offset of each of the first, second, and third samplers  210 ,  220 , and  230  may be in inverse proportion to the amplification ratio. As a result, a receiver (e.g., the receiver  100  in  FIG.  2   ) according to an embodiment of the present disclosure including the first, second, and third linear equalizers  110 ,  120 , and  130  respectively coupled to the first, second, and third samplers  210 ,  220 , and  230  may have a reduced input offset compared to that of a conventional receiver (e.g., the receiver  1  in  FIG.  1   ). 
       FIG.  5    is a block diagram illustrating a receiver  1000  according to an embodiment of the present disclosure. 
     The receiver  1000  includes a decoder  300  that generates a data signal DO corresponding to the multilevel input signal IN by decoding the outputs (or output signals) OUTL, OUTM, and OUTH of the first, second, and third samplers  210 ,  220 , and  230 . 
     Since generating the data signal DO from the first signal OUTH, the second signal OUTM, and the third signal OUTL may be known in the art, a detailed configuration and operation of the decoder  300  will be omitted for the interest of brevity. 
     The receiver  1000  may further includes a reference voltage generator  400  that generates the first reference voltage VREFH, the second reference voltage VREFM, and the third reference voltage VREFL being provided to the first linear equalizer  110 , the second linear equalizer  120  and the third linear equalizer  130 , respectively. 
     Although various embodiments have been illustrated and described, various changes and modifications may be made to the described embodiments without departing from the spirit and scope of the invention as defined by the following claims.