CN1975688A

CN1975688A - Transmission delay and dithering measure

Info

Publication number: CN1975688A
Application number: CNA2006101631726A
Authority: CN
Inventors: 谈侃; J·C·卡尔芬; K·塞普
Original assignee: Tektronix Inc
Current assignee: Tektronix Inc
Priority date: 2005-11-29
Filing date: 2006-11-29
Publication date: 2007-06-06
Anticipated expiration: 2026-11-29
Also published as: TWI400460B; JP5025234B2; TW200739101A; CN1975688B; JP2007147619A

Abstract

A method using a real time oscillograph which uses crosscorrelation to mesure transmission delay and dithering measure, obtains waveform from two test point of the system in the measurement. Clock resumption is functioned in two waveforms to each celocity and offset of them. Time difference between two waveforms are calculated. Filtering wave is from the dithering of two test point, and apart mean crosscorrelation coefficient of two test point are calculated. Fraction delay is calculated using interpolation based on the LMS error, and each calculated component are sumed to calculate the transmission delay of two test point. The transmission delay can be used to alter the clock edge by comparing the clock edge of a waveform with the date conversion edge of another waveform, in order to measure the dithering.

Description

Transmission delay and jitter measurement

Technical field

The present invention relates to regularly measurement, and relate more particularly to transmission delay and jitter measurement in the clock drive system.

Background technology

In the clock drive system, reference clock is through different propagated, and the difference that the layout in these paths is introduced reference clock postpones.Along some circuit in these paths, introduced additional delay such as clock and data recovery (CDR) circuit.Be used for presenting many signal integrity challenges with the high-speed chip group of the many gigabits transmissions of per second data.Although high speed backplane looks just as P.e.c. (PC) plate, in fact can see communication system as.The reference clock that propagates into the base plate different piece has experienced different transmission delays.System design need be considered this transmission delay so that system performance is met the demands, such as the restriction to shake.

For the high-speed serial data standard, such as Peripheral component interface (PCI) Express and full buffer two-wire internal memory module (FB-DIMM) standard, Physical layer as described in Figure 1.Reference or system clock drive them by the phaselocked loop (PLL) that drives in transmitter (TX) and the receiver (RX).PLL has the transmission delay of seeing the shake propagation delay as.Usually, the PLL characteristic has the shake propagation delay that becomes with frequency.In operating frequency range, group delay is used to represent propagation delay.The transmission delay of PLL can be measured by spectrum analysis.The transmission line that comprises microstrip and microstrip line also has the transmission of the time domain of use (TDT) or the measured transmission delay of vector network analyzer (VNA).For system shown in Figure 1, if provide the transmission delay of each parts by parts producer, so by with each parts, suing for peace such as the transmission delay of PLL and transmission line obtains whole transmission delay.But even be not impossible, it also is very difficult using current available technology (spectrum analysis, TDT, VNA) directly to measure transmission delay.

What wanted is a kind of direct method that is used to measure transmission delay and jitter on the high-speed printed circuit board.

Summary of the invention

Thus, the invention provides and a kind ofly utilize simple crosscorrelation to measure the method for transmission delay and shake with real-time oscilloscope.Two test points in test from system are obtained waveform.The operation clock recovery is to obtain speed and side-play amount separately on two waveforms.According to the time deviation between two waveforms of side-play amount calculating separately.Filtering is from the shake of two test points, and removes average (mean-removed) cross-correlation coefficient according to the Jitter Calculation of filtering.Utilize interpolation to calculate fractional delay based on the LMS error, and calculating component separately is summed to calculate two transmission delays between the test point.Transmission delay can be then used in by a signal and another signal are carried out the edge " Utopian " reference clock relatively is provided, so that determine shake.

When the detailed description of reading in conjunction with claims and accompanying drawing subsequently, can therefrom know and learn purpose of the present invention, advantage and other novel feature.

Description of drawings

Fig. 1 is the block scheme of the typical physical layer of high speed transmission system, indicates the test point according to transmission delay of the present invention and jitter measurement.

Fig. 2 is the process flow diagram according to transmission delay measurement process of the present invention.

Fig. 3 is from the clock of the PCI-Express system corresponding with Fig. 1 and the curve map of data waveform.

Fig. 4 is the curve map according to the simple crosscorrelation between the shake of filtering of the present invention.

Fig. 5 is by the curve map of interpolation computing relay according to the present invention.

Embodiment

Refer again to Fig. 1, reference or system clock 12 are linked to transmitter 14 and receiver 16 by transmission line " passage T " 18 and " passage R " 20 separately.The PLL22 of transmitter 14 inboards multiplies each other reference clock and the data transfer rate that is used for the driving data bit sequence.Data from transmitter 14 propagate into receiver 16 by transmission line " data channel " 24.Path from " tp.1 " to " tp.4 " comprises two passages 18,24 and PLL22.These three parts all have low-frequency filter characteristics.Therefore, the low-frequency jitter of measuring at " tp.1 " with at " tp.4 " is interrelated.When two signals were interrelated, the delay between these two signals was the value that obtains cross-correlation coefficient between two signal peaks.When cross-correlation coefficient reached peak value, length of delay was optimum for lowest mean square (LMS) error.Between " tp.1 " and " tp.4 ", measure transmission delay Td_1.4 by calculating 2 cross-correlation coefficients of locating between the low-frequency jitter component.This cross-correlation method only provides separating at unit interval (UI).More accurate separating can obtain by interpolation, to realize the optimal value for the LMS error.

The process of measurement transmission delay shown in Figure 2 and shake.

● step 1: use suitable instrument, obtain waveform at two test point places such as real-time oscilloscope.Channel oscilloscope need be a deskew, and the mode with deskew comprises them when using probe.

● step 2: at a waveform, promptly " tp.1 " goes up operation constant clock recovery (CCR), to obtain clock frequency and side-play amount, T_CCR_Offset_tp.1.Then be multiplied by clock frequency to obtain data transfer rate.For example, the multiplication factor of PCI-Express standard can be 25.

● step 3: utilize data transfer rate at another waveform, promptly move CCR on the data waveform of " tp.4 ", to obtain side-play amount T_CCR_Offset_tp.4.This algorithm guarantees that the data transfer rate in the

step

2 and 3 is accurately identical---the bit rate of finding in step 2 (being converted to data transfer rate by multiply by mutually as required) is used for the bit rate of step 3 data.

● step 4: according to the time deviation between two waveforms calculating recovered clock

T_CCR_Offset＝T_CCR_Offset_tp.4-T_CCR_Offset_tp.1

Represent with UI.

● step 5: filtering is from the shake of two test points.Be multiplied by clock frequency on demand and need interpolative data shake by filtering.Filtering can be any form, depend on that between the signal at two some places what shake content has.In this example, low-frequency jitter has between two points, therefore uses low-pass filter.For the PCI-Express standard, cutoff frequency can be set to 2MHz.

● step 6: calculate from removing the average cross-correlation coefficient between 2 the filtering shake.Find and produce the peaked maximum-delay T_XCOR_Delay of cross-correlation coefficient (representing) with UI.When carrying out cross-correlation calculation, the scope that needs regulation to postpone, this is easy to determine.

● step 7: based on the LMS error approach, use interpolation to calculate fractional delay T_INTERP_Delay, to obtain accurately separating in a UI.

● step 8: sue for peace calculated transmission delay by component to above acquisition:

Td＝T_CCR_Offset+T_XCOR_Delay+T_INTERP_Delay

The result represents that with UI it can be converted into second, because the duration of UI is known.

It is poor that above-mentioned transmission delay method based on simple crosscorrelation can be applicable to measure in transmission delay or the system delay between any 2.For example, can directly measure the poor of transmission delay between " tp.4 " and " tp.5 ".At 2 signals of locating can be clock or data-signal, is from " tp.1 " from the clock to the clock to " tp.5 " promptly, and is from data to data from " tp.3 " to " tp.4 ".In order to obtain the measurement result of pinpoint accuracy, signal to noise ratio (snr) should be enough high, and this is genuine in this example, because the low-frequency jitter component signal in high speed data transmission system normally.For example, spread spectrum clock (SSC) is widely used for reference clock and has big low-frequency jitter.

In clock and data recovery (CDR) system as input with clock and data, such as exporting as among the CDR26 that imports from data and the reference clock of PLL28 in receiver 16, PLL is the clock multiplication device and has the characteristic of low pass from the shake of input reference clock.CDR26 calculates the optimal delay between its recovered clock and the data, thus the specified data buffer sizes.Be applicable as transmission delay and calculate and the cross-correlation procedure of description, to find the optimal delay between data and the recovered clock.

Obtain as shown in Figure 2 after the optimal delay, the binding that data cross edge and recovered clock are intersected between the edge can be determined (step 9) by utilize PLL to generate the recovered clock edge according to the reference clock waveform.Utilize optimal delay T _d(step 10) is adjusted the clock edge to produce " desirable " clock edge.(step 11) is intersected poor between the time of time and " ideal " clock edge intersection as the data edge equally, directly to calculate jitter measurement.

As an example, clock and data waveform are used for the PCI-Express system as shown in Figure 3.Centre and net result from the transmission delay measurement process are:

T_CCR_Offset＝-24.416(UI)

T_XCOR_Delay＝28(UI)

T_INTERP_Delay＝-0.0007(UI)

Td=3.5769 (UI) or 1.4308 (ns)

Data transfer rate is 25 times of the defeated rate of reference clock in the PCI-Express system, and the first intermediate result T_CCR_Offset is approximately-24UI, this means the first restore data edge about 24UI before the first recovered clock edge.This is consistent with the observation from Fig. 3.

From simple crosscorrelation curve shown in Figure 4, obtain the second intermediate result T_XCORR_Delay.The fractional value T_INTERP_Delay of the 3rd intermediate result provides more accurate and separates.Net result Td is a transmission delay.

In step 6, calculate the simple crosscorrelation between two signals.X (n) and y (n) under two signals of hypothesis, n=1,2 ..., N is interrelated, has optimum integer delay k, makes signal x (n) and inhibit signal y (n+k) have the poor of minimum.The good standard of difference is the difference of two squares or error.Because N is constant number, thus to square error to minimize with minimizing square error be the same.Therefore, the delay of LMS error to separate with separating of least mean-square error here be identical.

min _kJ(k)＝min _k{∑ _n(x(n)-y(n+k)) ²}＝∑ _nx(n) ²+∑ _ny(n) ²-2max _k{∑ _n(x(n) ^*y(n+k))}

＝‖x‖ ²+‖y‖ ²-2‖x‖‖y‖max _kr(k)

Wherein r (k) is a cross-correlation coefficient, and ‖ x ‖ and ‖ y ‖ are x (n) and y (n), n=1, and 2 ..., the 2-norm of N.When x (n) and y (n) were standardized, ‖ x ‖ and ‖ y ‖ equaled 1.Because ‖ x ‖ and ‖ y ‖ are constant, so that the specific k of cross-correlation coefficient maximum also makes the square error minimum at the same time.

Step 7 utilizes interpolation to calculate fractional delay, and as shown in Figure 5, wherein x (n) and z (n) are by interrelated.Have the optimum fractional delay that provides least error between signal x (n) and shift signal z (n+p), wherein p is the mark with UI unit, and z (n+p) is the interpolate value between z (n) and the z (n+1):

z(n+p)＝(1-p)*z(n)+p*z(n+1)

As N when being constant, good optimal standards is a square error, or is square error with being equal to.

min _pJ(p)＝min _p{∑ _n(x(n)-z(n+p)) ²}＝min _p{∑ _n(x(n)-z(n)-(z(n+1)-z(n))*p) ²}

Separating of this optimization problem by asking partial derivative to obtain to variable p:

p＝{∑ _n(x(n)-z(n))*(z(n+1)-z(n))}/{∑ _n(z(n+1)-z(n)) ²}

Therefore the invention provides a kind of transmission delay of real-time oscilloscope and method of jitter measurement effectively utilized, this method is based on simple crosscorrelation.

Claims

1. measuring method comprises step:

Two test points in system in test are obtained waveform;

The constant clock recovery of operation is to obtain offset value separately on two waveforms;

Calculate the time deviation between the offset value separately;

Filtering is from the shake of two test points;

Calculating is from removing the average cross-correlation coefficient between the institute filtering shake of two test points; With

By to time deviation with remove the average cross-correlation coefficient and sue for peace calculated transmission delay.

2. the method shown in claim 1 also comprises step: calculate fractional delay in case with the transmission delay calculation procedure in time deviation and remove the summation of average cross-correlation coefficient.

3. the method shown in claim 2, wherein the fractional delay calculation procedure comprises the step of using interpolation based on the least mean-square error method.

4. the method shown in claim 1, wherein operating procedure comprises step:

Acquisition is from the reference clock frequency of a waveform; And

This reference clock frequency be multiply by the acquisition data transfer rate mutually with a constant, and this data transfer rate is used for the operating procedure of another waveform.

5. the method shown in claim 4 also comprises step:

Determine reference clock edge from a waveform;

This reference clock edge is postponed described transmission delay, to obtain " ideal " clock edge;

Determine another waveform data conversion edge; And

Calculate shake according to described " ideal " clock edge and described date conversion edge.