The synchronized module is each system’s heart, it for system’s in other each module present correct clock signal. Therefore needs and realizes to the synchronized module design gives specially the attention. This article to affected the system design the clock characteristic to carry on the inspection, and the reason which worsened to the signal has carried on the appraisal. This article has also analyzed the influence which the synchronization worsens, and to the Standardization organization to guarantee the transmission quality and each kind of transmission facility’s interoperability formulates the standard request has carried on the discussion.
Abstract:
The network synchronization and the clock produce are the high speed transmission system design important aspects. For through reduces the launch and the receive wrong raises the network efficiency, must make clock’s quality which system’s each stage must use maintains the specific rank. The network standard definition synchronous network’s architecture and on standard interface’s anticipated performance, guarantees the transmission quality and transmission facility’s seamless integration. Has the massive synchronized question, system design personnel when establishment system architecture must be very clear. This article elaborated each kind of origin which the clock worsens, like vibration and drifting. This article also discussed the reason which and the influence in the transmission system the clock worsens, and has analyzed the standard request, proposed each kind realizes the skill.
Basic concept: Vibration and drifting
The vibration general definition may be “an event the short deviation which appears to its ideal”. In the digital transmission system, vibrates is defined deviates its ideal position short change for the digital signal significant instant in the time. The significant instant may be one cycle is the T1 potential flow best sampling time. Although hoped that various units places appear in the T integral multiple position, but in fact will differ from. This kind of pulse position modulation was considered that is one kind of vibration. This is also called the digital signal the phase noise. Under chart, the actual signal border makes the periodic migration nearby the ideal signal border, has demonstrated the periodic vibration concept.
Figure 1. vibration hint
The vibration, is different with the phase noise, it as the unit expressed take unit interval ui (UI). A unit interval ui is equal to signal period (T), is equal to 360 degrees. The supposition event is E, nth time presents the expression is tE[n]. Then the instantaneous vibration may express is:
A group to the peak vibration value use is smallest including N vibration survey’s peak and the greatest instant vibrates the surveying computation to be as follows:
The drifting is the low frequency vibration. Between both’s model division spot is 10 Hz. The vibration and the drifting cause the influence can appear in the transmission system differently, but specific region.
Vibration type
According to having the reason, the vibration separable becomes two predominant types: Stochastic vibration and definite vibration. Stochastic vibration, if just, is uncertain, and so on causes by the stochastic noise effect like thermal noise. The stochastic vibration usually occurs in digital signal border transformation period, creates the stochastic midpoint crossing. Without a doubt, the stochastic vibration has Gauss probability density function (PDF), (Mu) and root-mean-square value (rms) (Sigma) decided by its average value. Because gaussian function’s tail infinite extends in the average value both sides, may be do not have the limiting value the instantaneous vibration and the peak to the peak vibration. Therefore the stochastic vibration usually uses its root-mean-square value to express and the survey.
Figure 2. by Gauss probability density function expression stochastic vibration
As for the vibration remainder, is more useful than the peak to the peak vibration the root-mean-square vibration, therefore needs to transform the stochastic vibration root-mean-square value Cheng Fengdao the peak value. To transform the root-mean-square vibration Cheng Fengdao the peak vibration, defined vibrated stochastically the gaussian function random limit (arbitrary limit). Error rate (BER) is in this kind of transformation useful parameter, once in its supposition gaussian function’s instantaneous vibration falls outside its compulsion limit namely presents the error code. Through the following two formulas, may obtain the root-mean-square to vibrate to the peak to the peak vibration conversion. 3
May obtain the next table from the formula, in the table the peak to the peak vibration correspondence different BER value.
The definite vibration has, therefore may predict that and has the determination scope limit. The consideration integrated circuit (IC) system, will have the massive crafts, the component and the system-level factor will affect the definite vibration. The dutyfactor will distort (DCD) and the pulse width distorts (PWD) to create the digital signal the distortion, will cause the zero crossing sector deviation ideal position, upward or downward migration. These distortions usually are along different create by the signal rise with the drop along between succession. If in the non-balanced system has between the electric potential drifting, the differential input to have the voltage excursion, the signal rise and the fall time presents the change and so on, also possibly creates this kind of distortion.
Figure 3, always vibrates double model expression
The data correlation vibrates between (DDJ) and the mark disturbs the (ISI) cause signal to have the different zero crossing sector level, causes each kind of only position to present the different signal conversion. This is also called pattern correlated jitter (PDJ). The signal way’s low frequency cut-off point and the high frequency band width will affect DDJ. When the signal way’s band width may carry on the comparison with the signal band width, the position will extend in the ortho-position time, will cause between the mark to disturb (ISI). The low frequency cut-off point will cause the low frequency component’s signal to present the distortion, but the systematic high frequency band width limit will cause the high frequency component performance to drop. 7
Sine vibration by sine pattern modulation signal border. This possibly is because the supplies overall system’s power source or even in system’s other vibrations create. The earth bounce and other power sources change also possibly create the sine vibration. The sine vibration widely uses in vibrating the environment test and the simulation. The non-correlated vibration possibly creates by the power source noise or the crosstalk and other electromagnetic interference.
When consideration vibration to digital signal influence, needs the entire definite vibration and the stochastic vibration consideration. The definite vibration and the stochastic vibration’s grand total result will have other one kind of probability distribution 4: The double model responds, its middle expresses the definite vibration, the rear part responds for Gauss, expression stochastic vibration component.
Vibration survey - TIE, MITE and TEDV
Time-gap error (TIE) is through obtains to the actual clock gap survey and to the ideal reference clock identical gap survey. In assigns time t, is called the observation gap by one time-gap to produce the time T(t) clock, it is opposite in clock Tref(t) TIE may through under formula expression. (x(t) is called the error function. )
TIE expressed that in the signal high frequency phase noise, has supplied the actual clock’s each cyclical deviation ideal situation direct information. TIE uses in calculating massive statistical derivation function like MTIE, TDEV and so on.
Greatest time-gap error (MTIE) defines as, in hour of observation in (t=nt0), assigns the clock signal to be opposite in an ideal clock signal biggest peak to the peak delay variation, this length all hour of observation in survey cycle (T). Uses the following formula to carry on estimated:
MTIE aims at the time slow changes or the drifting defines. When needs to analyze clock’s long-term characteristic, needs to carry on the survey to MTIE. The MTIE value is to a clock signal long-term stability one kind of weight.
Chart 4.TIE graph expression
TDEV is other statistical parameter, takes the integrated time the function to a signal anticipated time variation survey. DEV can also provide the related signal phase (time) the noise spectrum component information. In the TIE chart each spot standard deviation is to an observation gap computation, this observation gap has slid the entire measuring time. This value carries on the average in entire above measuring time by to obtain this specific gap TDEV value. Increases the observation gap, repeated measure process. TDEV is to the short-term stable one kind of weight, when appraisal clock oscillator performance useful. TDEV belongs to the Unit of time.
In high speed transmission system vibration and drifting reason
The most commonly used one kind of clock architecture is, in prepares on the board to move a low frequency clock, produces the synchronized high frequency clock in each transmission card. The low frequency clock or through establishes separately PLL in the integrated circuit to realize carries on the frequency multiplication to produce the high frequency clock. Through model PLL frequency multiplication, after the frequency multiplication, on clock’s phase noise increases for the original clock phase noise 20*log(N) power of exponent, N is the frequency multiplication coefficient. In addition, the PLL reference clock inputs the vibration will lengthen the locking time, when and the input jitter is oversized high speed PLL is even unable to realize locking. Will prepare on the board to use one kind of higher speed differential timing compared to use the low speed single end clock to have the better vibration performance.
Because VCO is more sensitive to the input voltage change, therefore the power source noise is increases the clock vibration a primary factor. The output clock vibration scope and the power source noise scope, the VCO gain are proportional, is in reverse proportion with the noise frequency. Because the wire resistance forms because the resistance drop and the wire inductance forms the inductance noise creates the power source or the earth bounce, will have the similar influence to the above output clock vibration. Carries on the full filtration on the system board to the power source, approaches the integrated circuit power source pin to provide the decoupling electric capacity, may guarantee that PLL obtains the higher vibration performance.
In the system board, the clock and the data are mutually independent, the launch and the receiving end in the start, maintain with the delay time aspect change are essential to the high speed. Because in the data and the clock way has the different active element to cause between the data and the clock way has the propagation delay difference, between the clock way wiring detention difference, between the data position wiring detention difference, between the data and the clock way the different load situation, block length difference and so on, possibly creates the above changes. When plan system vibration remainder, must the different signal way change consideration.
When carries on the transmission when section of distances, lights the existence vibration accumulation in the transmitter and the receiver. In the transmitter physics level realizes, DAC misalignment or laser misalignment and so on nonlinear responses will aggravate the signal distortion. In the transmission medium and the receiver, seems real the source besides the outside (mostly in copper wire), because causes the optical fiber because of the different frequency and the mudulation effect to distort, the receiver to realize (mainly the signal related phase deviation which with band width related) and the clock extraction electric circuit realizes causes, will aggravate the signal class the vibration.
Figure 5. comes from TIE the chart the MTIE deviation
Will make concrete (synchronized digit series) transmits to SDH, has the massive system-level event to cause the vibration. In (orbit digit series) the leg mapping is the SDH frame and (network module) carries on PDH through SDH NE the transmission in the model transmission system, in the PDH leg before SDH terminal multi-channel divider solution mapping, presents VC in each middle node place (hypothesized vessel) synchronization again. Has the gap clock to use in each leg mapping the STM-N frame and from the STM-N frame solution mapping, sends out with the expenses, the fixed packing and the adjustment phase should pulse, thus creates the mapping vibration. Will use the adjustment opportunity position to compensate the PDF leg medium frequency displacement the method to create the standby period vibration. Also has the indicator to adjust the mechanism, uses to comes from between the output STM-N frame phase fluctuation which initial NE input VC and local produces to carry on the compensation. According to frequency deviation, VC in the STM-N frame shuttle. This will cause VC to withdraw the spot to see in the potential flow the sudden change, causes to be called the indicator vibration the type vibration. All above system-level vibration will aggravate the total definite vibration.
Although all above factors can aggravate from the source to the destination between the signal dissemination vibration, the standard request still stipulated that must have in the transmission spot compared to the theory night-watch low vibration value. Thus, considered the clock frequency multiplication, the power source change, the electricity - light - electricity transforms the distorted signal influence which, the launch and the receive influence as well as the other cause actual signal worsen, will have a relatively very low vibration value in the source place driving signal’s clock.
Vibration to transceiver influence
In the ideal situation, the digital signal is carries on the sampling in two neighboring level switch point center point. The reason that the vibration will create the error code, will be because will be opposite in the ideal center point, it changed the signal border switch point. The error code is possible, because the signal class border change is too late ((unit interval ui is equal in the time compared to ideal center point late 0.5UI to signal one cycle)) or too early (in time compared to ideal center point 0.5UI is the result of early). When clock sampling border when signal class’s any one side misses 0.5UI, will present 50% error code probability, the supposition average transformation density will be 0.5. 7 if knew separately the definite vibration and the stochastic vibration, may through the above two digital and the peak to the peak vibration value and the root-mean-square vibration value relate in the together table, estimates the error rate. The calibration vibration, defines as the digital signal best sampling time with between the sampling clock’s short-term change which withdraws from it, may create the above error code. Regarding the commercial use, the source clock and the source launch connection vibration standard will be lower than 1UI by far.
The launch connection vibration standard usually matches with receiving end’s input jitter allowance. Regarding vibration measurement circuit cut-off frequency of filter, especially so. For example, in the SDH system, two kind of vibration survey band width, stipulated separately: Uses in the wide band survey filter (f1 to f4), uses in the high frequency band survey filter (f3 to f4). Value f1 refers to may the output clock signal narrowest clock cut-off frequency which uses in wiring system’s PLL. Will be lower than this band width the frequency vibration through the system, but high frequency vibration by factional absorptions. The value f3 expression input clock catches electric circuit’s band width. Will be higher than this frequency the vibration to cause the calibration vibration. The calibration vibration causes the luminous power to lose, needs the extra luminous power by against each kind of worsening. Therefore the limit transmitter end high frequency band frequency spectrum’s vibration is very important.
Drifting to transceiver influence
In the market sells the majority telecommunication receivers have used a buffer, adapts the chance fluctuation which in the line signaling exists. The following diagram 6 express this concept in detail. Restores the clock to send in the data the high resilience buffer, but the system clock sends out the data equipment’s core spot.
In orbit transmission system, transmitter and receiver work, in is mutually independent and in the extremely close frequency, fL and Fs express the transmitter and receiver’s frequency separately. When between both has the phase or the frequency difference, the elastic store will eliminate it, otherwise the buffer will appear owes carries or the overflow (is decided by difference scope and elastic buffer’s size), will create a time controllable frame glide (basic speed transmission) or a position adjustment (higher order asynchronous multi-channel multiplexer).
In the orbit application, has carried on the standardization according to the acceptable cushion glide to the frequency shift and the buffer depth. The initial network mainly uses in the speech transmission, will not create the pronunciation drop in quality under certain frequency threshold. The ITU-T standard stipulated that this change is /-50ppm. But starts along with the network to transmit the compression pronunciation, the facsimile form data, the video frequency as well as other type media application, as well as just started the synchronous network regarding the mistake and the re-transmit, the glide causes the efficiency to drop seriously.
In the synchronized transmission system, the system clock usual synchronization to uses in receiving the higher clock rank signal on the connection restoration clock. Will restore between the clock and the system clock the phase and the frequency instant and the accumulation difference by the elastic buffer absorption, will otherwise cause the elastic memory to overflow/owes carries (is decided by buffer size and change scope), will cause the indicator adjustment to retard either ahead of time in the frame transmission, the frame glide or the system somewhere presents the position adjustment.
In the synchronous system, all network module work in the identical average frequency, may eliminate the frame worsening through the indicator mechanism. These indicator mechanism ahead of time or detention payload in transmission frame position, thus adjustment receive and in the system clock exists frequency and phase change. In the SDH transceiver’s buffer must be smaller than the PDH transceiver, moreover irregularity and so on pen travel which possibly causes regarding the SDH system in have the limit. Therefore, compares with the PDH system, synchronous system’s request is stricter. Because between the network development’s history and the different network’s interoperability connection, in certain stages or other stages, these synchronous network will connect through the orbit network. therefore the PDH network’s clock architecture also needs to consider.
MTIE provided the clock to be opposite in the known ideal reference clock’s peak time change. Will use the MTIE value in the synchronized transmission and in commutation equipment’s elastic buffer’s design. In the elastic store, the buffer packing level and between the input digital signal and local system clock’s TIE is proportional. Guaranteed that the clock conforms to the related MTIE clock standard, the guarantee will not be able to surpass certain buffer threshold. Therefore, in the buffer design, its size is decided by the MTIE prescribed limit.
Figure 6, model transmission system’s receiver connection
System clock output phase perturbation to transceiver influence
A clock’s output phase change may through analyze its MTIE information to obtain. The drifting produces (in free oscillation pattern and synchronized pattern) mainly refers to the system to use the clock oscillator’s long-term stability, only receives oscillator’s stable influence in the free mode in the formula system’s stability. Produces besides the drifting, outputs the clock phase also to come under the massive system anomalous characteristic influence.
Specially speaking of a system synchromesh, bad or the worsening reference clock will transform the reference source from one to a normal reference clock will possibly cause the output phase perturbation. Transmission used with high speed PLL in traditional VCO which (pressure controls oscillator) when the change reference clock has used the cut condenser bank method. This kind of cut will transform to will output the clock to create the temporary phase deviation. Uses the ultra low vibration clock frequency multiplier electric circuit to be possible to solve this problem.
The high performance network clock when system’s all reference clock loses uses one kind is called “maintains” mechanism. This is produces the system last known good reference clock through the memory storage technology to realize. Will enter and the withdrawal maintains the pattern to will possibly output creates the phase perturbation. When is in maintains in the pattern, as a result of the accurate frequency’s regeneration is not very precise, will therefore continue to have the output phase error. The integrated circuit technology’s progress has caused maintains the precision has achieved 0.01ppb. The input reference clock worsening and (will not cause reference clock cut) to system’s maintenance test too few, also will create the output phase perturbation.
The system output perturbation is limited, is decided in the system in the low level acceptable input allowance. For example, conforms to the G.813 option 1 clock, in its phase perturbation permits the phase slope and the biggest phase error is limited for 1μS, the biggest phase slope is 7.5ppm, two 120ns phase error section, other part of phase slope is 0.05ppm. These digit correspond to the G.825 standing operating procedure input jitter allowance, this standard described in the SDH network to the vibration and the drifting control.
When the output phase is disturbed, phase error’s scope and the speed maintains, in the Standard structure suggested in the limit, may guarantee that carries on the end-to-end system to the signal worsening processes properly, thus avoids the data damage or the loss. For example, when the system synchromesh carries on the reference clock cuts, if the output phase error located at the code requirement within, the synchromesh may realize “not interrupted” the reference clock cut, instructed that the existence buffer overflows either owes carries, creates the pen travel, the position adjustment or the glide.
Conclusion
The network synchronization and the clock produce are in all high speed transmission network system the most important part. This article elaborated the different type which the clock worsens, is mainly vibrates and the drifting. The article also in detail elaborated has created the above worsened the reason, as well as how did they affect the transmission system. Carries on the systematic characteristic to the clock subsystem to design and to realize, will enhance overall system’s performance, cuts the error rate, easy to integrate, provides the higher transmission quality and the efficiency.
