Patent Publication Number: US-6987424-B1

Title: Narrow band clock multiplier unit

Description:
BACKGROUND 
   1. Field of the Invention 
   This invention relates to clocking in electronic systems and more particularly to managing jitter in high speed clocking environments. 
   2. Description of the Related Art 
   High speed clocks are used in transmission systems to synchronize the flow of data. Those high speed clocks may include jitter, which should be managed to prevent bit errors. Jitter is the variation in clock output frequency from a desired output frequency and can occur for a number of reasons. For example, jitter may be caused by noise introduced into the system from any of a variety of sources. A critical area of jitter management is in the transmit path of an optical/electrical interface where the outgoing light pulses typically have jitter within tight system requirements. 
   A typical transmit path of an electrical/optical interface is illustrated in  FIG. 1 . Low speed data that is supplied by the system on node  101  is written into a First In First Out (FIFO) memory  103  using a low speed clock supplied on node  105 . A clock multiplier unit (CMU)  107  generates a high speed clock on node  109 . The CMU  107  is configured as a phase-locked loop (PLL) and uses a lower speed reference clock supplied on node  111  as its input, which is multiplied to provide the high speed clock on node  109 . The high speed clock generated by the CMU  107  is used to read data out of FIFO  103  and send it to a laser driver and laser  113  for optical transmission. 
   A typical CMU utilizes a wideband PLL, which passes virtually all of the jitter present on its input (the reference clock) to the output. Thus, a very low jitter clock reference source, such as a high quality crystal oscillator, has been used in order to meet the transmit jitter generation requirements. In this context “wideband” is defined as a PLL with closed loop bandwidth above or at the high end of the jitter frequencies of interest for output jitter generation. For example, for an OC-48 Synchronous Optical Network (SONET) system, having a data rate of approximately 2.5 GHz, transmit jitter is specified for jitter frequencies from 12 KHz to 20 MHz. A wideband PLL typically used in such a system would have a closed loop bandwidth that encompasses a substantial portion of the frequency range of the jitter frequencies of interest, e.g., a closed loop bandwidth of 15 MHz. 
   The need for a high precision clock source adds expense to a system both in terms of the cost of a high precision crystal oscillator, particularly at frequencies above 100 MHz, as well as additional complexity in design and board layout. In addition, reference clocks are often distributed across backplanes, making low jitter difficult to obtain. It would be desirable to relax the tight jitter requirements of the reference clock in order to remove the need for a high precision clock source, which would simplify system design and implementation and reduce its cost. 
   SUMMARY 
   Accordingly, the invention provides in one embodiment a clock multiplier unit that has a narrowband PLL that multiplies an input reference clock to generate a higher speed clock that can be used, e.g., for supplying high speed serial data from a FIFO memory. The narrowband PLL attenuates jitter in the jitter frequencies of interest sufficiently to allow a relaxation of the jitter requirements for the input reference clock. A narrowband CMU can save costs in reference clock generation because of the lower jitter requirements or alternatively, in some embodiments, can entirely remove the need to separately generate a low jitter reference clock to drive the clock multiplier unit by instead using an existing low speed clock as the reference clock, such as the clock used to write data into the FIFO. 
   In one embodiment the invention provides a clock multiplier/multiplexer transmit circuit arrangement that includes an input terminal coupled to receive a reference clock signal. A clock multiplier circuit on the integrated circuit includes a phased-locked loop circuit coupled to receive the reference clock signal and to supply an output clock as a multiple of the input clock. The phase-locked loop has a narrowband transfer function to attenuate jitter that is present in the reference clock in a predetermined frequency range, thereby providing an output clock substantially free of jitter present in the reference clock. 
   In another embodiment the invention provides a method of operating an integrated circuit that includes receiving a reference clock at an input terminal of the integrated circuit and generating an output clock as a multiple of the reference clock utilizing a phase-locked loop having a narrow frequency band transfer function. The method further includes attenuating jitter present in the reference clock in a predetermined frequency range in the phase-locked loop circuit of the clock multiplier circuit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. 
       FIG. 1  shows a typical prior art architecture of a transmit path of an electrical/optical interface. 
       FIG. 2  shows an architecture of a transmit path of an electrical/optical interface according to an embodiment of the invention. 
       FIG. 3  shows an architecture of a transmit path of an electrical/optical interface according to an embodiment of the invention. 
       FIG. 4  shows a clock multiplier unit/multiplexer transmit circuit incorporating an embodiment of the present invention. 
       FIG. 5  shows a block diagram of a PLL suitable for use in an embodiment of the clock multiplier unit. 
       FIG. 6  shows an a portion of an exemplary PLL suitable for use in an embodiment of the clock multiplier unit. 
       FIG. 7  shows successive data bits of the error signal sent to the decimator circuit shown in  FIG. 6  and illustrates operation of the decimator circuit. 
       FIG. 8  illustrates a portion of an exemplary PLL suitable for use in an embodiment of the clock multiplier unit. 
       FIG. 9  illustrates how peaking may be selected in an embodiment of the invention. 
       FIG. 10  illustrates how loop bandwidth may be selected in an embodiment of the invention. 
       FIG. 11  illustrates a mask that the jitter transfer function from any receive port to any transmit port must meet in OC192 systems. 
   

   The use of the same reference symbols in different drawings indicates similar or identical items. 
   DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     FIG. 2  illustrates an architecture of an electrical/optical interface in a communications system in accordance with one embodiment of the present invention. The clock multiplier unit  207  utilizes a narrow band PLL that attenuates jitter on the reference clock  211  that is supplied to the PLL allowing the reference clock to be of lower precision as compared to previously described prior art systems. The narrowband CMU  207  does not pass a significant portion of the jitter present on its input clock to the output, thus easing the precision requirements for the input clock. 
   However, a narrow band PLL tends to internally generate more jitter than a wideband PLL. Therefore, in order for a narrowband PLL to meet the jitter requirements at the output, the generated jitter as well as the passed jitter should be considered. One aspect of controlling generated jitter is to use a low noise voltage controlled oscillator (VCO). Additional details on a suitable narrowband PLL and lower noise VCO are provided herein. A narrow band PLL as used herein means a PLL that has a closed loop bandwidth below or at the low end of the jitter frequencies of interest for output jitter generation. For example, for a clock multiplier unit utilized in an OC-48 system, a jitter bandwidth of, e.g., 40 KHz would be considered narrowband, where the jitter frequencies of interest are from 12 KHz to 20 MHz. In OC  192  systems, the jitter frequencies of interest range from approximately 50 KHz to 80 MHz. A jitter bandwidth of e.g., 175 KHz, would be considered narrowband. 
   Use of a low-noise VCO in a narrow band PLL helps the CMU to meet jitter specifications while relaxing the jitter requirements of the reference clock  211 , which drives the CMU. The relaxed jitter requirements allows a less expensive clock source than the high quality crystal oscillator typically used in previous systems. 
   Referring to  FIG. 3 , another embodiment is shown in which the low speed clock  105  is supplied to the narrowband CMU  207  as the reference clock. Low speed clock  105  is also used to clock data into FIFO  103 . Since that clock exists in the system already, the requirement for a separate reference clock can be eliminated altogether providing additional design simplification. 
   Referring to  FIG. 4 , an exemplary clock multiplier unit/multiplexer transmit circuit  401  that incorporates an embodiment of the present invention is illustrated. In one embodiment transmit circuit  401  is implemented as a single integrated circuit. The transmit circuit  401  receives parallel data on node  403  and supplies high speed serial data TXDOUT on node  405 . The serial data is supplied to a laser driver and laser circuit (not shown). Note that the parallel and serial data are shown as differential signals. Although those and others clock and data signals are shown in  FIG. 4  as differential, the clock and data signals may also be implemented as single-ended and the principles described herein are still applicable. 
   The transmit circuit  401  includes a narrow band CMU  407 , an embodiment of which is described further herein. The transmit circuit  401  incorporates a selector circuit (such as a multiplexer)  409 , which selects the source for the reference clock  411  supplied to the CMU  407  according to a reference clock select signal  413  (REFSEL), supplied on an input terminal of the integrated circuit  401 . In other embodiments, the reference clock select signal may be programmably set by, e.g., a serial communications port on the integrated circuit. The clocks that are selectable in the illustrated embodiment include a clock supplied on node  415  and the clock TXCLKIN supplied on node  419 , which is used to clock data into the FIFO. Use of the FIFO clock  419  as the CMU reference clock allows a simpler design of the module containing the clock multiplier unit/multiplexer transmit circuit  401  in that a separate reference clock no longer needs to be supplied. 
   The CMU  407  supplies a clock signal on node  417 , which is used to read data out of the FIFO  420 . In the illustrated embodiment, a 16:1 multiplexer is used to select the bit of the 16 bit word written into the FIFO for serial output on node  405 . 
   The CMU in the exemplary transmitter multiplies the frequency of the selected reference clock up to the serial transmit data rate. Examples of typical input reference data frequencies of interest are 78 MHz, 155 MHz and 622 MHZ, with typical output frequencies being 2.5 GHz and 10 GHz. Other embodiments may of course utilize different input and output clock frequencies. 
   The TXLOL output signal on node  423  provides an indication of the transmit CMU lock status. When the CMU has achieved lock with the selected reference, the TXLOL output is deasserted (driven high). The TXLOL signal will be asserted, indicating a transmit CMU loss-of-lock condition, when a valid clock signal is not detected on the selected reference clock input. The TXLOL signal will also be asserted during frequency calibration. Calibration is performed automatically when the exemplary transmit circuit is powered on, when a valid clock signal is detected on the selected reference clock input following a period when no valid clock was present, or when the frequency of the selected reference clock is outside of the CMU&#39;s PLL lock range. 
   In an embodiment of the invention, the closed loop bandwidth of the PLL is programmable. Thus, the PLL can be adjusted to better optimize output jitter given the reference clock input jitter specifications. If the reference clock has high jitter, the narrower bandwidths can be selected but if the reference clock has lower jitter, the loop bandwidth can be somewhat greater. In an embodiment, four bandwidth settings are provided for a variety of jitter transfer characteristics. The filter bandwidth may be selected via input terminals. In one embodiment, the filter bandwidth is selected via the BWSEL[1:0] control inputs on node  421 . In one embodiment used in OC192 applications, the selectable closed loop PLL bandwidths are 17.5 KHz, 90 KHz, 175 KHz, and 350 KHz. Other embodiments will have other appropriate cutoff frequencies based on the application. In a digital loop filter implementation, the loop filter bandwidth may be controlled by adjusting the digital filter without modifying external components, as would be the case in traditional analog PLL implementations. 
   Lower loop bandwidth settings (narrowband operation) make the CMU more tolerant to jitter on the reference clock source. As a result, circuitry used to generate and distribute the physical layer reference clocks can be simplified without compromising margin to the SONET/SDH jitter specifications. Higher loop bandwidth settings (wideband operation) are useful in applications where the reference clock is provided by a low jitter clock source. Wideband operation allows the PLL to more closely track the precision reference source. 
   In an embodiment, the serialization circuitry includes FIFO  420  and multiplexer  422 . The serialization circuitry may utilize a parallel to serial shift register. Low speed data on the parallel input bus, TXDIN[15:0] on node  403 , is latched into the FIFO on the rising edge of TXCLKIN. Data is clocked out of the FIFO and into the shift register by a clock that is produced by dividing down the high-speed transmit clock, TXCLKOUT, by a factor of 16. The high-speed serial data stream TXDOUT is clocked out of the shift register by TXCLKOUT. The TXCLK16OUT clock is provided as an output signal to support data word transfers between the transmitter and upstream devices using a counter clocking scheme. 
   The exemplary transmit circuit described above decouples the timing of the data transferred into the device via TXCLKIN from the data transferred out of the shift register. The FIFO is eight parallel words deep and accommodates any static phase delay that may be introduced between TXCLKOUT and TXCLKIN. Furthermore, the FIFO accommodates a phase drift, or wander, between TXCLKIN and TXCLKOUT of up to, e.g., plus or minus three parallel data words. The FIFO circuitry indicates an overflow or underflow condition by asserting the FIFOERR signal supplied on node  425 . This output can be used to re-center the FIFO read/write pointers by tying it directly to the FIFORST input on node  426 . The FIFORST signal causes re-centering of the FIFO read/write pointers. The exemplary transmit circuit automatically recenters the read/write pointers after the device is powered on, after an external reset via the RESET input, and each time the PLL transitions from an out-of-lock state to a locked state (when TXLOL transitions from low to high). 
   The exemplary transmit circuit provides the capability to select the order in which the data received on the parallel input bus TXDIN[15:0] is transmitted serially on the high-speed serial data output TXDOUT. Data on the parallel bus will be transmitted MSB first or LSB first depending on the setting of the TXMSBSEL input provided on node  427 . When TXMSBSEL is set low, TXDIN0 is transmitted first, followed in order by TXDIN1 through TXDIN15. When TXMSBSEL is set high, TXDIN15 is transmitted first, followed in order by TXDIN14 through TXDIN0. This feature can simplify printed circuit board (PCB) routing in applications where ICs are mounted on both sides of the PCB. 
   In one embodiment, to prevent the transmission of corrupted data into the network, the multiplier unit/multiplexer provides a control pin that can be used to force the high-speed serial data output TXDOUT to zero. When the TXSQLCH input on node  428  is set low, the TXDOUT signal is forced to a zero state. 
   In the illustrated embodiment, a clock disable pin, TXCLKDSBL, on node  429  can be used to disable the high-speed serial data clock output, TXCLKOUT. When the TXCLKDSBL pin is asserted, the positive and negative terminals of CLKOUT are tied internally to 1.5 V through 50 ohm on-chip resistors. That feature can be used to reduce power consumption in applications that do not use the high-speed transmit data clock. 
   As previously described, a narrowband PLL may generate undesirable jitter in the synthesis of the output clock. One embodiment of a narrow band PLL that generates sufficiently low jitter during output clock synthesis is now described. In one embodiment, the CMU  407  utilizes a digital signal processing (DSP) algorithm to replace the loop filter commonly found in analog PLL designs. This algorithm processes the phase detector error term and generates a digital control value to adjust the frequency of the voltage-controlled oscillator (VCO). The digital implementation requires no external loop filter components. That eliminates sensitive noise entry points, making the PLL implementation less susceptible to board-level noise sources, and making SONET/SDH jitter compliance easier to attain in the application. 
   Referring to  FIG. 5 , a high level block diagram of an embodiment of a phase-locked loop (PLL) suitable for use in the CMU is illustrated. The PLL includes a phase/frequency detector  501  which receives the reference clock supplied on node  503 . As illustrated in  FIG. 4 , reference clock  503  may be supplied by the multiplexer  409  ( FIG. 4 ). The phase and frequency detector detects the phase and frequency difference between the supplied reference clock a feedback signal supplied from the divide-by-N circuit  505 . The phase/frequency detector  501  supplies a signal to a digital loop filter  507  which supplies control signal generation circuit  508 , which in turn supplies control signals to the voltage controlled oscillator (VCO)  509 . The output of the VCO  509  supplied on node  511  is fed back through the divide-by-N circuit  505  to the phase/frequency detector  501  and provides the high speed output clock TXCLKOUT generated by the CMU. 
   Referring now to  FIG. 6 , a portion of a PLL is shown that includes a loop filter, and a phase detector  602 , which compares phase or phase/frequency between two signals A and B conveyed respectively on nodes  612  and  614  and generates an error signal on output node  616 . The A signal is the selected reference clock and the B signal is a feedback signal from the divide-by-N circuit coupled to the output of the VCO (not shown in  FIG. 6 ). The phase detector circuit  602  preferably includes a linear phase detection circuit followed by a delta-sigma modulator to generate a 1-bit digital bit-stream error signal. Other phase detectors, e.g., bang—bang phase detectors may be utilized in embodiments of the invention. In the illustrated embodiment in  FIG. 6 , the phase detector circuit  602  may be assumed to produce a 311 MHz 1-bit wide error signal  616 . Exemplary digital phase detector circuits are described in the U.S. patent application Ser. No. 09/902,542 by Perrott, filed Jul. 10, 2001, and also in the U.S. Provisional Application 60/360,461, filed Feb. 28, 2002, entitled “Method and Apparatus for Switching Between Input Clocks in a Phase-Locked Loop” by Huang, et. al., which are incorporated herein by reference. 
   The error signal  616  is received by a decimator circuit  604  that produces a lower frequency, wider bit-width error signal on its output node  618 , which is then conveyed to both a digital feed-forward block  608  and a digital integrating path filter  606 . In the example depicted, the decimated error signal is a 4-bit wide 78 MHz signal which is preferably calculated by adding each of several weighted sequential data bits in the input bit stream. For example, four sequential data bits may be each weighted by a factor of two and added to produce an unsigned output having a range from 0000 to 1000. In a preferred embodiment each 4-bit value of the decimated error signal may be calculated by adding: a first data bit weighted by a factor of one; the next three data bits each weighted by a factor of two; and a fifth data bit weighted by a factor of one. This technique simultaneously provides decimation, gain, and digital low-pass filtering. 
   Such a calculation is shown in  FIG. 7 , which depicts successive data bits of the error signal supplied to the decimator circuit  604 . A four-bit value SUM[3:0] is calculated by adding: BIT[2] weighted by a factor of one; BIT[3] weighted by a factor of two; BIT[4] weighted by a factor of two; BIT[5] weighted by a factor of two; and BIT[6] weighted by a factor of one. The next four-bit value SUM[i+1] is calculated by adding: BIT[6] and BIT[10], each weighted by a factor of one; and BIT[7], BIT[8], and BIT[9] after each is weighted by a factor of two. Consequently, the overall gain of this block is two (i.e., in aggregate each bit is weighted by a factor of two) and the overlapping summation technique results in a filtering effect. With an overall gain of two, and since a calculation is performed for every four sequential input data bits, the calculated output values may range from 0000 to 1000. This unsigned value is preferably converted to a signed value ranging from 1100 (−4) through 0000 (0) to 0100 (+4), although such conversion may be performed in the decimator circuit  604  or in the succeeding circuit stages. Alternatively, such succeeding stages may be designed to directly operate using an unsigned input value, without an explicit conversion to a signed value. In the descriptions herein, the decimator circuit  604  may be considered to be part of the PLL loop filter, but a similar decimation circuit or function may alternatively be considered to be part of a more complex phase detector circuit  602 . 
   Referring again to  FIG. 6 , the decimator circuit  604  produces a 4-bit wide error signal at a 78 MHz data rate on its output node  618 , which is then conveyed to both a feed-forward block  608  and an integrating path filter  606 . The feed-forward block  608  and integrating path filter  606  are preferably both digital circuits that generate output values such as 20-bit values, as shown, although it is understood that other circuits and other output widths may be employed as well. The respective outputs  620  and  622  of these blocks are added by digital adder  610  to preferably produce on output node  624  a 20-bit digital control signal DCVAL[19:0] for the VCO. The integrating path filter  606  preferably incorporates digital signal processing circuitry and/or digital signal processing techniques as described generally in the U.S. patent application Ser. No. 09/902,541 by Perrott, et al., filed Jul. 10, 2001. For some embodiments, the function depicted by the digital adder  610  may be incorporated into a subsequent circuit block which receives both output signals  620  and  622 . 
   Referring now to  FIG. 8 , an embodiment of a digital loop filter  630  is depicted. The 4-bit decimated error signal  618  is conveyed to a digital accumulator circuit  632  which sums the 4-bit decimated error signal  618  and accumulates up to a 46-bit result. The particular bit position of the digital accumulator circuit  632  into which the 4-bit decimated error signal  618  is added is configurable in accordance with a selected peaking setting for the PLL transfer function, as indicated in  FIG. 9 . For example, for a selected bandwidth of 12.5 Hz and peaking of 0.05 dB (i.e., the top line in the chart), the 4-bit decimated error signal DEC — ERR[3:0] conveyed on node  618  is added at bit positions LPF — ACCUM[3:0] of the low pass filter accumulator (indicated by the “I I I I” characters for integrating path), and a 46-bit result is accumulated on bits LPF — ACCUM[45:0]. Alternatively, for a selected bandwidth of 6400 Hz/0.05 dB, the 4-bit decimated error signal DEC — ERR[3:0] is added at bit positions LPF — ACCUM[21:18], and a 28-bit result is accumulated on bits LPF — ACCUM[45:18], with the lower 18 bit positions LPF — ACCUM[17:0] being ignored. 
   Irrespective of the bandwidth and peaking selected, the low-pass filter digital output signal conveyed on node  622  is preferably an upper portion of the accumulator, such as the upper 20-bits of the accumulator, namely LPF — ACCUM[45:26] for this example. As shown in  FIG. 8 , this output signal may be a 20-bit unsigned data word, although internally the upper 10 bits of the accumulator may preferably be signed bits. 
   The feed-forward filter  608  preferably has a gain of 4 and consequently has an output that is preferably a signed 6-bit data word. The feed forward path includes a low pass finite impulse response (FIR) filter. The feed forward filter generates a 6-bit output signal  620  (with sign extension), and the two signals  620  and  622  are added by digital adder circuit  610  to generate on node  624  (i.e., in this case, bus  624 ) a 20-bit VCO control signal DCVAL[19:0]. Referring to  FIG. 10 , the particular bit position of the 20-bit value into which the 6-bit feed-forward filter output signal is added in digital adder  610 , is configurable in accordance with a selected bandwidth for the PLL transfer function. Note that the 6 bits conveyed on node  620  may be supplied as part of a zero filled 20 bit value into which the 6 bits have been mapped according to a desired bandwidth. For example, the 6-bit feed-forward filter output signal may be mapped to be added into bit positions [5:0] indicated by the “F F F F F F” characters for a selected bandwidth of 12.5 Hz and peaking of 0.05 dB (i.e., the top line in the chart). It should be noted that the “F F F F F F” position determines the PLL&#39;s bandwidth while the “I I I I” position ( FIG. 9 ) and the “F F F F F F” position together determine the PLL&#39;s peaking. 
   An extremely long low pass filter time constant may be achieved in the digital accumulator circuit  632  by calculating up to an internal 46-bit result, but not all of these bits must necessarily be conveyed to subsequent circuits, such as the VCO control signal generation circuit, especially if such succeeding circuits cannot make use of the 46-bit resolution. In the exemplary embodiment shown, the output value of the low pass filter may be taken from only the upper 20-bits of the accumulator and represented herein also as LPF[19:0]. However, in certain embodiments, some additional precision may be maintained beyond just the upper 20-bit value by conveying a group of the next lower accumulator bits to a sigma-delta modulator circuit  634  that generates, for example, a 1-bit serial data bit stream representative of those bits, and adding the resulting 1-bit serial output bit stream to the upper 20-bits to generate another 20-bit result. Any given value of the DCVAL[19:0] signal will, of course, have only 20-bits of resolution, but a large group of sequential values of the DCVAL[19:0] signal will generate an average value which is representative of 32-bits of resolution, not just 20-bits, even though only 20-bits are conveyed to subsequent circuit blocks. 
   Additional details on generating the control word for the VCO and alternative PLL implementations are provided in provisional application 60/360,333, filed Feb. 28, 2002, entitled “DIGITAL EXPANDER APPARATUS AND METHOD FOR GENERATING MULTIPLE ANALOG CONTROL SIGNALS PARTICULARLY USEFUL FOR CONTROLLING A SUB-VARACTOR ARRAY OF A VOLTAGE CONTROLLED OSCILLATOR”, by Yunteng Huang and Bruno Garlepp, which application is incorporated herein by reference in its entirety, and in application Ser. No. 10/188,576, entitled “DIGITAL EXPANDER APPARATUS AND METHOD FOR GENERATING MULTIPLE ANALOG CONTROL SIGNALS PARTICULARLY USEFUL FOR CONTROLLING A SUB-VARACTOR ARRAY OF A VOLTAGE CONTROLLED OSCILLATOR”, naming Yunteng Huang and Bruno Garlepp as inventors, filed the same day as the present application, which is incorporated herein by reference. 
   Utilizing a low noise VCO is another aspect of achieving a narrow band PLL suitable for use in the CMU described herein. Referring again to  FIG. 5 , in one embodiment VCO  509  is implemented as an LC oscillator. Other types of controlled oscillators, such as a current controlled oscillator with a suitable control signal, are also contemplated. Because it is important to minimize jitter, one embodiment of a VCO suitable for use in the present invention uses an LC tank circuit having the inductor being implemented off-die, e.g., in the package, to achieve a higher Q value. In another embodiment an inductor having a small inductance L, and a high quality factor Q may be made to include a surface rotational portion instead of a coil to provide a low inductance inductor with sufficient quality factor. The surface rotational portion is a conducting structure which is substantially enclosed along the length of the structure and which has openings at each end of the structure and an opening along the length of the structure between the end openings. One example of such a structure is a cylinder having a lateral opening and openings at its ends or bases. Another example of such a structure is a duct-shape or rectangular structure. Any appropriately shaped surface may be used to rotate current, and varying thicknesses and lengths may be used as appropriate. Such an inductor is described in more detail in U.S. patent application Ser. No. 09/997,338, filed Nov. 30, 2001, by Ligang Zhang, which is incorporated by reference herein. LC oscillators are described in U.S. Pat. No. 6,137,372 to Welland, which is hereby incorporated by reference. Additional details of various aspects of a PLL and VCOs suitable for use in embodiments of the present invention can be found in the applications entitled “DIGITALLY-SYNTHESIZED LOOP FILTER CIRCUIT PARTICULARLY USEFUL FOR A PHASE LOCKED LOOP”, application Ser. No. 09/902,541, filed Jul. 10, 2001, and “FEEDBACK SYSTEM INCORPORATING SLOW DIGITAL SWITCHING FOR GLITCH-FREE STATE CHANGES”, application No. 60/300,699, filed Jun. 25, 2001, which are incorporated herein by reference. 
   An additional significant advantage of utilizing a narrowband CMU is that it can be used to meet the jitter transfer function system requirements, either in loopback or in normal operation. For example, in OC192 systems the jitter transfer function from any receive port to any transmit port must meet the mask illustrated in  FIG. 11 . 
   Given that the receive PLLs are always wideband in order to meet the input jitter tolerance specifications, the narrowband PLL needed to meet the transfer requirement has typically been implemented elsewhere. Such embodiments use discrete PLLs to generate clean reference clocks and to meet the jitter transfer specifications. This clean reference clock drives a wideband CMU to create the 10 Gb/s output. By using a narrowband CMU in accordance with the present invention, the CMU, reference clock jitter filtering, transmit clock generation, and jitter transfer function requirements can all be handled in one integrated circuit. 
   Note that OC48 systems do not have the narrowband jitter transfer requirements that OC  192  systems do. For OC48 systems, the receive PLL can meet both the input jitter tolerance and jitter transfer specification of f 3  db&lt;1.3 MHz. 
   While the invention has been largely described with respect to the embodiments set forth above, the invention is not necessarily limited to these embodiments. Variations and modifications of the embodiments disclosed herein may be made based on the description set forth herein, without departing from the scope and spirit of the invention as set forth in the following claims. Accordingly, other embodiments, variations, and improvements not described herein are not necessarily excluded from the scope of the invention, which is defined by the following appended claims.