Patent Publication Number: US-7907676-B2

Title: Adjustable dual-band link

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 12/268,986, filed Nov. 11, 2008, now U.S. Pat. No. 7,599,422, issued Oct. 6, 2009, which is a continuation of U.S. patent application Ser. No. 12/030,700, filed Feb. 13, 2008, now U.S. Pat. No. 7,450,629, issued Nov. 11, 2008, which is a divisional of U.S. patent application Ser. No. 11/022,469, filed Dec. 22, 2004, now U.S. Pat. No. 7,349,484, issued Mar. 25, 2008, which applications are incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the communication of data. More specifically, the present invention relates to the communication of data over frequency-selective channels having one or more notches. 
     BACKGROUND 
     In typical baseband transmission systems that employ equalization techniques for equalizing channels with low-pass characteristics, a usable bandwidth extends from near DC up to a maximum frequency that is determined by a high-frequency roll-off of a communications channel and signal-to-noise (SNR) requirements of a receiver. Referring to  FIG. 4 , when a magnitude  310  of a channel response  324  is essentially a monotonically decreasing function of frequency  312 , this limitation is straight forward and baseband signaling is able to utilize the full, usable bandwidth of the channel. For example, if a system has adequate SNR with up to 50 dB of channel attenuation then the channel whose magnitude  310  decreases monotonically at 41.6 dB/decade starting at 1 GHz could support baseband signaling over a 0-12 GHz band of frequencies. 
     It is not unusual, however, for a channel response  314  to include one or more significant notches, such as first notch  316 , that result in a local minimum in the magnitude  310 . Notches may be associated with reflections (due to differences in impedance, parasitic capacitance and manufacturing tolerances) and other non-idealities. At higher frequencies, the channel response  314  recovers substantially before finally dropping again due to the ultimately low-pass nature of the channel. For such channels, the use of baseband signaling, with a usable signaling bandwidth limited from near DC up to the first notch  316 , does not take advantage of all of the usable transmission bandwidth. Additional unutilized bandwidth is available at higher frequencies where the channel response  314  recovers from the first notch  316 . Reconsidering the previously described example with the channel response  314  having the first notch  316  in a notch band of frequencies  322  between 4 and 4.5 GHz, the system could only support baseband signaling over a first band of frequencies  318  between 0-4 GHz. A second band of frequencies  320  between 4.5 and 12 GHz, which has less than 50 dB of attenuation, cannot be used with baseband signaling due to the first notch  316 . As a consequence, this usable transmission bandwidth is not used in the system. There is a need, therefore, for a signaling system that more effectively utilizes the available bandwidth for channels having low-pass characteristics with one or more significant notches. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram illustrating a system with an adjustable link. 
         FIG. 2   a  is a block diagram illustrating a transmission communication link circuit. 
         FIG. 2   b  is a block diagram illustrating a transmission communication link circuit. 
         FIG. 3   a  is a block diagram illustrating a receiving communication link circuit. 
         FIG. 3   b  is a block diagram illustrating a receiving communication link circuit. 
         FIG. 4  is a schematic diagram illustrating two channel responses. 
         FIG. 5  is a flow diagram illustrating a method of operating an adjustable link. 
         FIG. 6  is a flow diagram illustrating a method of operating an adjustable link. 
         FIG. 7  is a block diagram illustrating a circuit having the function of an oscilloscope, herein called an escope, in a receiving communication link circuit. 
     
    
    
     Like reference numerals refer to corresponding parts throughout the drawings. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In one embodiment of an adjustable link, a first transmission communication link circuit has a transmission baseband circuit and a transmission passband circuit. The transmission baseband circuit corresponds to a baseband sub-channel and the transmission passband circuit corresponds to a passband sub-channel. The first transmission communication link circuit also includes a circuit that distributes a first subset of a data stream having a first symbol rate to the transmission baseband circuit and a second subset of the data stream having a second symbol rate to the transmission passband circuit. The first symbol rate and the second symbol rate are each less than a symbol rate of the data stream. The baseband sub-channel and the passband sub-channel are separated by an adjacent guardband of frequencies. The passband carrier frequency is adjusted to define the guardband and the guardband corresponds to a first notch in a channel response of a first communications channel. 
     In some embodiments, the first communications channel is used for communication between first and second integrated circuits. In some embodiments, the first communication channel is a data bus. 
     In another embodiment, the link includes a first data receiving communication link circuit. The first receiving communication link circuit has a receiving baseband circuit and at least a receiving passband circuit. The receiving baseband circuit corresponds to a baseband sub-channel and the receiving passband circuit corresponds to a passband sub-channel. The first receiving communication link circuit includes a circuit that combines the first subset of a data stream having the first symbol rate from the baseband receiving circuit and the second subset of the data stream having the second symbol rate from the passband receiving circuit into the data stream. The first symbol rate and the second symbol rate are each less than the symbol rate of the data stream. The baseband sub-channel and the passband sub-channel are separated by an adjacent guardband of frequencies. The passband carrier frequency is adjusted to define the guardband and the guardband corresponds to the first notch in the channel response of the first communications channel. 
     Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the embodiments. 
       FIG. 1  illustrates an embodiment of a system  50  having a plurality of adjustable links. A first integrated circuit  60  is coupled to a second integrated circuit  62  via a first communication channel. In the system  50 , the first communication channel is illustrated as a data bus having a plurality of signal lines  66 . In some embodiments, a length of the signal lines  66  is less than 1 meter. In some embodiments, each signal line, such as signal line  66   a , has a respective channel response, which may be different from channel responses of the other signal lines  66 . The first integrated circuit  60  has a plurality of data transmission and/or receiving communication link circuits  64 , henceforth denoted by transmission/receiving communication link circuits  64 , for transmitting and receiving data to and from the second integrated circuit  62 . The second integrated circuit  62  also has a plurality of data transmission and/or receiving communication link circuits  68 , henceforth denoted by transmission/receiving communication link circuits  68 , for transmitting and receiving data to and from the first integrated circuit  60 . 
     Each data transmission/receiving communication link circuit, such as data transmission/receiving communication link circuit  64   a , has a respective baseband circuit, such as baseband circuit  116   a  in  FIG. 2   a  or  212   a  in  FIG. 3   a , and at least a respective passband circuit, such as passband circuit  116   b  in  FIG. 2   a  or  212   b  in  FIG. 3   a . The respective baseband circuit corresponds to a baseband sub-channel in the first communications channel. In some embodiments, such as those where the first communications channel is ac-coupled, the respective baseband sub-channel does not contain DC. The passband circuit of a respective data transmission/receiving communication link circuit  64  corresponds to a passband sub-channel in the first communications channel. For a respective data transmission/receiving communication link circuit, a respective band of frequencies corresponding to the baseband sub-channel, such as band of frequencies  318  in  FIG. 4 , and a respective band of frequencies corresponding to the passband sub-channel, such as band of frequencies  320  in  FIG. 4 , are separated by a respective adjacent guardband of frequencies. The respective guardband of frequencies, such as the notch band of frequencies  322  in  FIG. 4 , correspond to a respective notch, such as the first notch  316  in  FIG. 4 , in a respective channel response of the first communications channel. 
     In some embodiments, the channel response is a transfer function of the first communication channel. In some embodiments, the channel response is a step response of the first communication channel. In some embodiments, the channel response is an impulse or pulse response of the first communication channel. 
     The respective baseband sub-channel circuit and the respective passband sub-channel circuit in the respective data transmission/receiving communication link circuit, such as data transmission/receiving communication link circuit  64   a , may be adjusted based on one or more performance characteristics of the first communication channel corresponding to one or more respective signal lines, such as signal line  66   a . In particular, the respective band of frequencies corresponding to the baseband sub-channel and/or the respective band of frequencies corresponding to the passband sub-channel may be adjusted so as to define the respective guardband of frequencies around a respective notch in the respective channel response. Control logic  78  in the first integrated circuit  60  determines the sub-channel settings for the respective data transmission/receiving communication link circuit. The sub-channel settings for the respective baseband circuit may include one or more low-pass filter corner frequencies and/or a respective clock rate. The sub-channel settings for the respective passband circuit may include one or more respective bandpass filter bandwidths, a respective carrier frequency, a respective fundamental frequency and/or a respective clock rate. The sub-channel settings may be stored in a memory  76   a  in the first integrated circuit  60 . In some embodiments, the memory  76   a  is separate from the control logic  78 , while in other embodiments the memory  76   a  is embedded within the control logic  78 . 
     The system  50  may include at least a second communications channel for communicating information, including communications channel circuit  70 , communications channel circuit  74  and signal line  72 . In some embodiments, the information may include sub-channel settings for one or more data transmission/receiving communication link circuits  68  in the second integrated circuit  62 . In other embodiments, the information may include data used to train at least one of the data transmission/receiving communication link circuits  64  or  68 , such as data transmission/receiving circuit  64   a  or  68   a , during a training mode of operation. 
     In some embodiments, the sub-channel settings are stored in a memory  76   b  in the second integrated circuit  62 . In the system  50 , the second communication channel includes a signal line  72 . In some embodiments, the second communication channel may include two or more signal lines. In some embodiments, each pairing of data transmission/receiving communication link circuits in the first integrated circuit  60  and the second integrated circuit  62  may have a separate additional signal line in the second communications channel for communicating respective sub-channel circuit settings. 
     In other embodiments, sub-channel circuit settings and/or data used to train at least one of the data transmission/receiving communication link circuits  64  or  68  may be communicated using one or more of the signal lines  66  in the first communication channel. For example, the sub-channel circuit settings may be transmitted from the first integrated circuit  60  at a slow data rate that is easily received by the second integrated circuit  62 . Alternatively, the sub-channel circuit settings may be transmitted from the first integrated circuit  60  to the second integrated circuit  62  using a dedicated small-bandwidth passband sub-channel. 
     In some embodiments, the first communication channel may include one or more additional notches in the channel response. In some embodiments, the data transmission/receiving communication link circuits  64  in the first integrated circuit  60  and the data transmission/receiving communication link circuits  68  in the second integrated circuit  62  may include one or more additional passband circuits corresponding to additional passband sub-channels. The band of frequencies corresponding to each additional passband sub-channel is separated from the band of frequencies of a lower passband sub-channel by a respective guardband of frequencies. A respective guardband of frequencies corresponds to a respective notch in the respective channel response of the first communications channel. 
     The system  50  in  FIG. 1  shows 3 data transmission/receiving communication link circuits  64  and 3 data transmission/receiving communication link circuits  68 . In other embodiments, the system  50  may have 1, 2 or more than 3 pairs of data transmission/-receiving communication link circuits  64  and  68 . 
     The system  50  in  FIG. 1  illustrates a set of adjustable links for inter-chip communication. In some embodiments, the adjustable link may be used for intra-chip communication, such as between modules in an integrated circuit, such as the first integrated circuit  60 . 
     In some embodiments, the combined bandwidth of the respective baseband sub-channel and the one or more passband sub-channels in the first communication channel for each pair of data transmission/receiving communication link circuits  64  and  68  is more than 1 GHz, 2 GHz, 10 GHz or 12 GHz. In some embodiments, the band of frequencies corresponding to the guardband is at least 0.25 GHz wide, 0.5 GHz wide, 1.0 GHz wide or 2.0 GHz. 
       FIG. 2   a  illustrates an embodiment of data transmission communication link circuit  100 . The data transmission link circuit  100  uses multi-tone communication. A data stream  110  having a symbol rate is distributed by demultiplexer  112  into a first subset  118   a  of the data stream  110  and a second subset  118   b  of the data stream  110  based on clock signals  114 . The first subset  118   a  of the data stream  110  and the second subset  118   b  of the data stream  110  each have a symbol rate that is less than the symbol rate of data stream  110 . The first subset  118   a  of the data stream  110  is coupled to the baseband circuit  116   a . The second subset  118   b  of the data stream  110  is coupled to the passband circuit  116   b . In some embodiments, the data transmission link circuit  100  may include one or more additional passband circuits. 
     The baseband circuit  116   a  and the passband circuit  116   b  each include a digital-to-analog converter  120  and a transmit buffer  122 . In some embodiments, the digital-to-analog converter  120  may also include a serializer. The transmit buffers  122  are coupled to adjustable clock signals  138  that gate an output from the transmit buffers  122 . The clock signals  114  and the clock signals  138  may be generated from a common signal generator, such as a phase lock loop or a delay lock loop (for example, using a voltage divider), or from separate signal generators. 
     The output from the transmit buffer  122   b  is mixed in mixer  126   b  with carrier signal  130   b  generated by oscillator  128 , thereby shifting signals to the band of frequencies corresponding to the passband sub-channel. In some embodiments, the mixer  126   b  is a multiplier. In some embodiments, more than one mixer may be used in a passband circuit, such as passband circuit  116   b . In some embodiments, the carrier signal  130   b  may be a sinusoidal or harmonic signal having an adjustable carrier frequency. In other embodiments, the carrier signal  130   b  may be a square-wave signal having an adjustable fundamental frequency. Outputs from the baseband circuit  116   a  and the passband circuit  116   b  are combined in adder  134  prior to the transmission of signal  136  in the first communication channel. 
     In some embodiments, the baseband circuit  116   a  and the passband circuit  116   b  may modulate the first subset  118   a  of the data stream  110  and/or the second subset  118   b  of the data stream  110 , respectively. In some embodiments, the modulation in the baseband circuit  116   a  is different from that used in the passband circuit  116   b , which is also referred to as bit-loading. Suitable modulation in the baseband circuit  116   a  includes 2 or more level pulse amplitude modulation (PAM), such as two-level PAM or four-level PAM. Suitable modulation in the passband circuit  116   b  includes 2 or more level pulse amplitude modulation (PAM), also referred to as on-off keying, and, as discussed below, 2 or more level quadrature amplitude modulation (QAM) for passbands that are in quadrature with one another. Other suitable modulations include pulse position modulation (PPM) and pulse width modulation (PWM). In some embodiments, the modulation in one or more respective sub-channels of one of the data transmission/receiving link circuits, such as data transmission/receiving link circuit  64   a  in  FIG. 1 , may be different from that used in the other data transmission/receiving link circuits (e.g., link circuits  64   b  and  64   c ,  FIG. 1 ). 
     The data transmission link circuit  100  does not include filters to limit the band of frequencies corresponding to the baseband sub-channel and the passband sub-channel. Instead, use is made of the fact that a rectangular function corresponding to a bit cell in the time domain corresponds to a sinc function in the frequency domain, and that a magnitude of a first sideband of the sinc function is 20 dB less than the magnitude of its peak. In some embodiments, the respective band of frequencies corresponding to the respective guardband may therefore be adjusted by appropriately setting one or more of the clock signals  138  (and thus the corresponding bit cell times) and/or the carrier or fundamental frequency of the carrier signal  130   b.    
     In the absence of band limiting associated with filters, however, the data transmission link circuit  100 , will have constraints on how hard the transmit buffers  122  may be driven. In particular, in embodiments with additional passband circuits, transmit power may be reduced in order to ensure that there is sufficient voltage swing available for the transmit buffers  122 . This may result in poorer performance, for example, a higher error rate. 
       FIG. 2   b  illustrates an embodiment of data transmission communication link circuit  150 . The baseband circuit  116   a  and the passband circuit  116   b  contain an adjustable low-pass filter  124 . In some embodiments, one or more low-pass filters  124  may have fixed characteristics that cannot be dynamically adjusted during normal operation of the link circuit  150 . The passband circuit  116   b  also includes an adjustable bandpass filter  132   b . Therefore, in addition to setting one or more clock signals  138  and/or the carrier or fundament frequency of the carrier signal  130   b , the respective band of frequencies corresponding to the respective guardband may be adjusted by setting a corner frequency of the low-pass filter  124   a , a corner frequency of the low-pass filter  124   b  and/or a bandwidth of the bandpass filter  132   b . In addition to the added degrees of freedom in adjusting the respective guardband, the low-pass filters  124  and the bandpass filter  132   b  also reduce the transmit power constraints associated with additional sub-channels described previously. 
       FIG. 2   b  also illustrates an optional third subset  118   c  of the datastream  110  and an optional passband circuit  116   c  with a corresponding passband sub-channel that is in quadrature with that associated with passband circuit  116   b . The oscillator  128  generates a carrier signal  130   c  that is 90° out of phase with the carrier signal  130   b . The carrier signal  130   b  and the carrier signal  130   c  can also be described as a vector having an in-phase component and an out-of-phase component. Thus, the passband corresponding to the passband circuit  116   b  may be described as an in-phase passband and the passband corresponding to the passband circuit  116   c  may be described as an out-of-phase passband. Other components ( 120   c ,  122   c ,  124   c ,  126   c ,  132   c ) in the passband circuit  116   c  have functions corresponding to those in the passband circuit  116   b . Note that the use of an additional passband sub-channel that is in quadrature also reduces the power constraint associated with additional sub-channels described previously. Also note that in some embodiments the data transmission link circuit  150  may include one or more additional passband circuits and/or additional pairs of passband circuits whose passband sub-channels are in quadrature. 
       FIGS. 2   a  and  2   b  illustrate embodiments of data transmission link circuits  100  and  200  that use so-called direct conversion. Other embodiments may use so-called heterodyne conversion, where signals are converted to one or more intermediate frequencies before conversion to baseband. In these embodiments, more than one mixer, such as the mixer  126   b , in a passband circuit, such as passband circuit  116   b , may be used. In addition, in some embodiments the low-pass  124  and the bandpass filters  132  in  FIG. 2   b , as well as in other embodiments below, may be excluded. 
       FIG. 3   a  illustrates an embodiment of data receiving communication link circuit  200 . The data receiving link circuit  200  uses multi-tone communication. An input  210  received from the first communication channel is coupled to the baseband circuit  212   a  and the passband circuit  212   b . In some embodiments, the data receiving link circuit  200  may include one or more additional passband circuits. 
     The input is mixed in mixer  216   b  with carrier signal  220   b  generated by oscillator  218 , thereby shifting signals from the band of frequencies corresponding to the passband sub-channel. In some embodiments, the mixer  216   b  is a multiplier. In some embodiments, the passband circuit  212   b  includes more than one mixer, such as the mixer  216   b . In some embodiments, the carrier signal  220   b  may be a sinusoidal or harmonic signal having an adjustable carrier frequency. In other embodiments, the carrier signal  220   b  may be a square-wave signal having an adjustable fundamental frequency. 
     An output of the mixer  216   b  in the passband circuit  212   b  and the input in the baseband circuit  212   a  are coupled to receive buffers  224 . The receive buffers  224  are coupled to adjustable clock signals  226  that gate an output from the receive buffers  224 . The baseband circuit  212   a  and the passband circuit  212   b  also include respective analog-to-digital converters  228   a ,  228   b.    
     A first subset  230   a  of a data stream is output by analog-to-digital converter  228   a , and a second subset  230   b  of the data stream is output by analog-to-digital converter  228   b . The first and second subsets  230   a ,  230   b  of the data stream are coupled to multiplexer  232  and are combined into a data stream  236  using clock signals  234 . 
     The first and second subsets  230   a ,  230   b  of the data stream each have a symbol rate that is less than the symbol rate of data stream  236 . The clock signals  226  and  234  may be generated from a common signal generator, such as a phase lock loop or a delay lock loop (for example, using a divider), or from separate signal generators. 
     In some embodiments, the baseband circuit  212   a  and the passband circuit  212   b  may demodulate the first subset  230   a  of the data stream and/or the second subset  230   b  of the data stream, respectively. The demodulation reverses the modulation used in the corresponding data transmission link circuit, such as data transmission link circuit  100  ( FIG. 2   a ) on the other end of the communication channel. The information necessary for accomplishing the demodulation may be provided to the data receiving link circuit  200  using the second communication channel. In some embodiments, the modulation in the baseband circuit  212   a  is different from that used in the passband circuit  212   b , which is also referred to as bit-loading. Suitable demodulation in the baseband circuit  212   a  includes two or more level pulse amplitude modulation (PAM), such as two-level PAM or four-level PAM. Suitable demodulation in the passband circuit  212   b  includes two or more level pulse amplitude modulation (PAM), also referred to as on-off keying, and two or more level quadrature amplitude modulation (QAM) for passbands that are in quadrature with one another. Other suitable modulation include pulse position modulation (PPM) and pulse width modulation (PWM). In some embodiments, the demodulation in one or more sub-channels of one of the data transmission/receiving link circuits, such as data transmission/receiving link circuit  64   a  in  FIG. 1 , may be different from that used in other data transmission/receiving link circuits. 
     The data receiving link circuit  200  does not include filters to limit the band of frequencies corresponding to the baseband sub-channel and the passband sub-channel. Instead, use is made of the fact that a rectangular function corresponding to the bit cell in the time domain corresponds to the sinc function in the frequency domain and that the magnitude of the first sideband of the sinc function is 20 dB less than the magnitude of its peak. In some embodiments, the band of frequencies corresponding to the guardband may therefore be adjusted by appropriately setting one or more of the clock signals  226  (and thus the corresponding bit cell times) and/or the carrier or fundamental frequency of the carrier signal  220   b . Values of the clock signals  226  and/or the carrier or fundamental frequency of the carrier signal  220   b  correspond to those used in the corresponding data transmission link circuit, such as data transmission link circuit  100  ( FIG. 1 ), on the other end of the communication channel. The information necessary for setting these configuration values in the data receiving link circuit  200  may be provided to the data receiving link circuit  200  using the second communication channel, a baseband sub-channel or a passband sub-channel. 
       FIG. 3   b  illustrates an embodiment of data receiving communication link circuit  250 . The baseband circuit  212   a  and the passband circuit  212   b  contain respective adjustable low-pass filters  222   a ,  222   b . In some embodiments, one or more low-pass filters  222  may have fixed characteristics. The passband circuit  212   b  also includes an adjustable bandpass filter  214   b . Therefore, in addition to setting one or more clock signals  226  and/or the carrier or fundament frequency of the carrier signal  220   b , the respective band of frequencies corresponding to the respective guardband may be adjusted by setting a corner frequency of a low-pass filter  222   a , a corner frequency of a low-pass filter  222   b  and/or a bandwidth of the bandpass filter  214   b.    
       FIG. 3   b  also illustrates an optional passband circuit  212   c  with a corresponding passband sub-channel that is in quadrature with that associated with passband circuit  212   b . The oscillator  218  generates a carrier signal  220   c  that is 90° out of phase with the carrier signal  220   b . The carrier signal  220   b  and the carrier signal  220   c  can also be described as a vector having an in-phase component and an out-of-phase component. Thus, the passband corresponding to the passband circuit  212   b  may be described as an in-phase passband and the passband corresponding to the passband circuit  212   c  may be described as an out-of-phase passband. The other components ( 214   c ,  216   c ,  222   c ,  224   c ,  228   c ) in the passband circuit  212   c  have functions corresponding to those in the passband circuit  212   b . The optional passband circuit  212   c  outputs a third subset  230   c  of the datastream. The values of the settings for the corner frequencies of the low-pass filters  222   a ,  222   b ,  222   c  and the bandwidths of the bandpass filters  214   b ,  214   c  correspond to those used in the corresponding data transmission link circuit, such as data transmission link circuit  150  ( FIG. 2   b ), on the other end of the communication channel. These may be provided to the data receiving link circuit  200  using the second communication channel. Also note that in some embodiments the data receiving link circuit  250  may include one or more additional passband circuits and/or additional pairs of passband circuits whose passband sub-channels are in quadrature. 
       FIGS. 3   a  and  3   b  illustrate embodiments of data receiving link circuits  200  and  250  that use so-called direct conversion. Other embodiments may use so-called heterodyne conversion, where signals are converted to one or more intermediate frequencies before conversion to baseband. In these embodiments, more than one mixer, such as the mixer  216   b , in a passband circuit, such as passband circuit  212   b , may be used. In addition, in some embodiments the low-pass  214  and the bandpass filters  222  in  FIG. 3   b  may be excluded. 
     The data transmission link circuit  150  and the data receiving link circuit  250  illustrate adjustable analog filters. In other embodiments, the adjustable filters may be implemented in a digital domain after analog-to-digital conversion, for example, as an FIR filter. 
       FIG. 5  is a flow diagram illustrating an embodiment of a method or process for transmission of data using an adjustable link. A plurality of subsets of the data stream are received  512 . These are converted into respective analog signals  514 . In some embodiments, the respective analog signals are low-pass filtered using respective filters having respective adaptive corner frequencies  516 . For sub-channels other than baseband, the respective analog signals are mixed with respective vectors having respective adaptive carrier or fundamental frequencies to produce respective sub-channel signals  518 . In some embodiments, mixing is accomplished using signal multiplication. In some embodiments, the respective sub-channel signals are bandpass filtered using respective filters having respective adaptive bandwidths  520 . The respective sub-channel signals are combined prior to transmission  522  to produce a composite signal for transmission across a communication channel. Tasks  512  through  522  may be performed continuously, in pipeline fashion, on a continuing data stream. By adjusting one or more bands of frequencies, such as the first band of frequencies  318  ( FIG. 4 ) or the second band of frequencies  320  ( FIG. 4 ), at least one adjacent guardband of frequencies corresponding to one or more notches, such as notch band of frequencies  322 , may be defined. 
       FIG. 6  is a flow diagram illustrating an embodiment of a method or process for receiving data using the adjustable link. An input signal is received  612 . In some embodiments, the signal is bandpass filtered using respective bandpass filters having respective adaptive bandwidths  614  to produce a set of sub-channel signals. The sub-channel signals, other than the baseband sub-channel signal, are mixed with respective vectors having respective adaptive carrier or fundamental frequencies to down convert the respective sub-channel signals  616 . In some embodiments, the mixing is accomplished using signal multiplication. In some embodiments, the resulting sub-channel signals are low-pass filtered using respective filters having respective adaptive corner frequencies  618 . The sub-channel signals are converted into digital values corresponding to respective subsets of the data stream  620 . The sub-sets of the data stream are combined  622  to produce a recovered data stream. Tasks  612  through  522  may be performed continuously, in pipeline fashion, on a successive portions of a received signal so as to produce a continuing data stream. Once again, by adjusting one or more bands of frequencies, such as the first band of frequencies  318  ( FIG. 4 ) or the second band of frequencies  320  ( FIG. 4 ), at least one adjacent guardband of frequencies corresponding to one or more notches, such as notch band of frequencies  322 , may be defined. 
     Referring to  FIG. 1 , for one or more respective data transmission/receiving communication link circuits  64  ( FIG. 1 ) and/or one or more data transmission/receiving communication link circuits  68  ( FIG. 2 ), characteristics of the communication channel, such as signal line  66   a , may be determined jointly or independently. In addition, for the respective data transmission/receiving communication link, the communication channel may be characterized for one or more respective sub-channels. In some embodiments, such channel characterization may use a circuit having the function of an oscilloscope, herein called an escope  700 , as illustrated in  FIG. 7 . The escope  700  is coupled to respective sub-channel signals in one or more data receiving communication link circuits, such as data receiving communication link circuit  200  ( FIG. 3   a ) and data receiving communication link circuit  250  ( FIG. 3   b ), between the receive buffer  224  ( FIGS. 3   a  and  3   b ) and the analog-to-digital converter  228  ( FIGS. 3   a  and  3   b ). Analog signals  710  corresponding to a respective sub-channel are coupled to comparators  712 . The escope  700  is intended for use with 2-PAM modulation. By adding additional comparators  712  it may be extended to an arbitrary multi-level modulation. Each comparator has a respective reference voltage  714 . In some embodiments, one reference voltage, such as reference voltage  714   a , corresponds to a logical 1 or high voltage state and another, such as reference voltage  714   b , to a logical 0 or a low voltage state. In other embodiments, one reference voltage may be fixed and another reference voltage may be varied. For example, one reference voltage may be at threshold, i.e., a data slicer, and one may be anywhere in the eye pattern. In embodiments with multi-level modulation, multiple samples may be taken. Outputs from the comparators  712  are coupled to an XOR gate or logical comparator  716 , which generates an output  718 . The XOR gate or logical comparator  716  may be implemented in hardware or software. By adjusting one or the reference voltages  714  (for example, using the control logic  78  in  FIG. 1 ), the output  718  corresponds to a cross-section of a portion of an eye pattern. The portion of the eye pattern corresponds to a logical 0 or 1 decision at a respective sample time. In this way, a voltage margin may be determined. 
     By further adjusting one or more of the respective clock signals  226  ( FIGS. 3   a  and  3   b ), cross-sections of the eye pattern at different sample times may be determined using the escope  700 . In this way, a timing margin may also be determined. Such voltage and timing margin measurements allow characteristics of the communication channel to be determined. In some embodiments, the channel may also be characterized based on a bit error rate and/or the pulse response in one or more sub-channels in a respective data transmission/receiving link circuit, such as data transmission/receiving link circuit  64   a  ( FIG. 1 ). In some embodiments, the channel may also be characterized based on a bit error rate and/or the pulse response in one or more of data transmission/receiving link circuits  64  or data transmission/receiving link circuits  68 . 
     In other embodiments, the escope  700  may be used to characterize a channel, including one or more notches, such as the first notch  316  ( FIG. 4 ), in the frequency domain. Such a frequency-domain measurement may be performed using a dedicated measurement channel having a single mixer and no low-pass or band-pass filters. Alternatively, the corner frequencies of one or more low-pass and/or bandpass filters in a sub-channel circuit, such as passband sub-channel  116   c  ( FIG. 2   b ), may be appropriately adjusted and any additional mixers may be disabled. While other sub-channels are disabled, a DC-signal may be transmitted over the dedicated measurement channel or the respective sub-channel. By varying the carrier frequency of the sinusoidal or harmonic signal generated by an oscillator, such as oscillator  128  ( FIG. 2   b ), a frequency range of interest may be swept. At each carrier frequency, the escope  700  may be used to measure a maximum received signal magnitude, which is inversely proportional to the channel loss. 
     After determining one or more channel characteristics, a respective low-pass filter corner frequency, a respective clock signal, a respective bandpass filter bandwidth and/or a respective carrier or fundamental frequency may be adjusted by control logic  78  ( FIG. 1 ) for one or more sub-channels in one or more data transmission/receiving circuits, such as data transmission/receiving circuit  64   a  ( FIG. 1 ). In some embodiments, determination of one or more channel characteristics and adjustment of one or more sub-channel circuit values, such as those listed above, in one or more data transmission/receiving circuits, such as data transmission/receiving circuit  64   a  ( FIG. 1 ), may be repeated iteratively. 
     In some embodiments, in the respective data transmission/receiving circuit, such as data transmission/receiving circuit  64   a  ( FIG. 1 ), a band of frequencies corresponding to a respective sub-channel, such as the first band of frequencies  318  ( FIG. 4 ), may be fixed and another band of frequencies, such as the second band of frequencies  320  ( FIG. 4 ), may be an integer multiple of the first band of frequencies  318  ( FIG. 4 ). This allows the use of a single clock in determining one or more eye patterns for one or more sub-channels using the escope  700 . 
     Some embodiments of the data transmission/receiving circuits, such as data transmission/receiving circuit  64   a  ( FIG. 1 ), may also include preceding, transmission equalization and/or receiving equalization. 
     The adjustable link apparatus and method are well-suited for use in communication between two or more semiconductor chips or dies, for example, in electronic interconnects and data buses. In particular, the apparatus and method are well-suited for use in improving the utilization of available bandwidth in communication channels between semiconductor chips on the same printed circuit board (PCB) or between semiconductor chips on different printed circuit boards that are connected through a backplane, signal lines or a coaxial cable at data rates exceeding multiple Gbps (gigabits per second), for example rates of at least 2, 5 or 10 Gbps, depending on the embodiment. 
     The adjustable link apparatus and method are also well-suited for use in improving communication between modules in an integrated circuit. The adjustable link may be used in communication between a memory controller chip and a dynamic random access memory (DRAM) chip. The DRAM chip may be either on the same printed circuit board as the controller or embedded in a memory module. In addition, the adjustable link apparatus and method are also well-suited for use in improving communication at data rates exceeding multiple Gbps, such as 2, 5 or 10 Gbps, depending on the embodiment, between a buffer chip and a DRAM chip, both of which are on the same memory module. The apparatus and methods described herein may also be applied to other memory technologies, such as static random access memory (SRAM) and electrically erasable programmable read-only memory (EEPROM). 
     Devices and circuits described herein can be implemented using computer aided design tools available in the art, and embodied by computer readable files containing software descriptions of such circuits, at behavioral, register transfer, logic component, transistor and layout geometry level descriptions stored on storage media or communicated by carrier waves. Data formats in which such descriptions can be implemented include, but are not limited to, formats supporting behavioral languages like C, formats supporting register transfer level RTL languages like Verilog and VHDL, and formats supporting geometry description languages like GDSII, GDSIII, GDSIV, CIF, MEBES and other suitable formats and languages. Data transfers of such files on machine readable media including carrier waves can be done electronically over the diverse media on the Internet or through email, for example. Physical files can be implemented on machine readable media such as 4 mm magnetic tape, 8 mm magnetic tape, 3½ inch floppy media, CDs, DVDs and so on. 
     The foregoing descriptions of specific embodiments of the present embodiments are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Rather, it should be appreciated that many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.