Jitter generation

Abstract

The present invention relates to a method for generating jitter in a digital data signal, the digital data signal having a predetermined data pattern being stored in a memory, the method comprising the steps of reading out the digital data signal from the memory using a clock signal provided by a clock source and modulating the clock signal provided by the clock source according to clock-control data, wherein the clock-control data represents the jitter to be generated in the digital data signal read out from the memory.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improvement of jitter generation, in particular for generating jitter in a stimulus signal for a Device Under Test (DUT).

2. Description of the Related Art

Integrated Circuits (IC) generally need to be tested to assure proper operation. This—in particular—is required during IC development and manufacturing. In the latter case, the ICs are usually tested before final application. During test, the IC, as Device Under Test (DUT), is exposed to various types of stimulus signals, and its responses are measured, processed and usually compared to an expected response of a good device. Automated Test Equipments (ATE) usually perform these tasks according to a device-specific test program. Examples for ATE are the Agilent 83000 and 93000 families of Semiconductor Test Systems of Agilent Technologies as disclosed e.g. under http://www.ate.agilent.com/ste/products/intelligent_test/SOC_test/SOC_Tech_Oview.shtml. Details of those families are also disclosed e.g. in EP-A-859318, EP-A-864977, EP-A-886214, EP-A-882991, EP-A-1092983, U.S. Pat. No. 5,499,248, U.S. Pat. No. 5,453,995.

EP 1 162 739 A discloses a filter that adds jitter and level noise to a digital data signal. The generated jitter depends on the digital data signal according to the characteristic of the filter.

SUMMARY OF THE INVENTION

It is an object of the invention to provide improved jitter generation, in particular to provide improved jitter generation in a digital data signal being a stimulus signal for a Device Under Test (DUT).

In accordance with a first aspect, the present invention provides a method for generating jitter in a digital data signal, the digital data signal having a predetermined data pattern being stored in a memory, the method having the steps of: reading out the digital data signal from the memory using a clock signal provided by a clock source, modulating the clock signal provided by the clock source according to clock-control data, wherein the clock-control data represents the jitter to be generated in the digital data signal read out from the memory.

In accordance with a second aspect, the present invention provides a software program or product, preferably stored on a data carrier, for executing, when running on a data processing system such as a computer, a method for generating jitter in a digital data signal, the digital data signal having a predetermined data pattern being stored in a memory, the method having the steps of: reading out the digital data signal from the memory using a clock signal provided by a clock source, modulating the clock signal provided by the clock source according to clock-control data, wherein the clock-control data represents the jitter to be generated in the digital data signal read out from the memory.

In accordance with a third aspect, the present invention provides a system for generating jitter in a digital data signal, the digital data signal having a predetermined data pattern being stored in a memory, the system having: a clock source providing a clock signal used for reading out the digital data signal from the memory, a modulator for modulating the clock signal provided by the clock source according to clock-control data, wherein the clock-control data represents the jitter to be generated in the digital data signal read out from the memory.

According to the present invention a phase or frequency modulated clock signal is used for generating the desired jitter in the digital data signal, e.g. to generate the desired jitter in a stimulus signal for a DUT, by using the modulated clock signal for clocking the data pattern. Using the present invention, high-speed digital inputs of a DUT can be tested. In particular the present invention provides a testability of clock data recovery circuit to track slow jitter, so called in-band jitter, which a phase-locked-loop of a clock data recovery circuit shall track, as well as high speed jitter, so called out-of-band jitter, which has to be tolerated by the phase-locked-loop of the clock data recovery circuit.

The jitter generated according to the present invention can be adjusted independently from the digital data signal, i.e. random jitter can be generated as well as deterministic jitter. The jitter amplitude and the jitter spectrum is fully programmable, therefore can be easily adjusted to the particular application. The electronic circuits for jitter generation can be integrated in CMOS semiconductor technology. The jitter can be generated individually for each pin of the DUT to be tested.

The present invention also relates to a software program or product for executing the method for synchronizing digital clock signals when running on a data processing system such as a computer. Preferably, the program or product is stored on a data carrier.

Furthermore, the present invention relates to a system for generating jitter in a digital data signal according to the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and many of the attendant advantages of the present invention will be readily appreciated and become better understood by reference to the following detailed description when considering in connection with the accompanied drawings. Features that are substantially or functionally equal or similar will be referred to with the same reference signs.

These and other objects and features of the present invention will become clear from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a block diagram of a first embodiment,

FIG. 2 shows a block diagram of a second embodiment,

FIG. 3 shows a block diagram of a third embodiment,

FIG. 4 shows a block diagram of a first possibility to realize the clock source,

FIG. 5 shows two possible embodiments for providing a sequence N(k),

FIG. 6 shows a block diagram of a second possibility to realize the clock source, and

FIG. 7 shows a block diagram of a third possibility to realize the clock source.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a block diagram of a first embodiment of the present invention. The system shown in FIG. 1 can be located in an Automated Test Equipment (ATE) providing on an output line 2 a digital data signal to be used as a stimulus signal for a Device Under Test (DUT, not shown). The digital data signal having a predetermined data pattern which is provided by a data pattern source 4 which is located within the ATE in the first embodiment. The data pattern is particularly adapted for testing the function of the DUT, wherein the response of the DUT is evaluated by the ATE.

The data pattern is input via data pattern line 6 in memory 8 according to a first clock 10 which is distributed via first clock line 12 to the data pattern source 4 for reading-out the data pattern as well as to memory 8 for reading-in the data pattern. The memory 8 has a first-in-first-out (FIFO) architecture. The data stored in memory 8 are output to output line 2 via a driver unit 16 according to a clock signal provided by a clock source 14 on a clock line 18 to said memory 8.

The clock signal provided by the clock source 14 is modulated according to clock-control data stored in a clock-control data source 20 which can comprise a corresponding clock-control pattern memory, a look-up table, a counter etc. The clock-control data are distributed to said clock source 14 via clock-control data line 22. The clock-control data representing said jitter in said digital data signal read-out from said memory 8 and provided to the DUT via output line 2. The clock-control data provides full phase and/or frequency control of the clock on clock line 18. Accordingly, the clock by which the data pattern read out from the memory 8 is phase or frequency modulated according to the clock-control data of data source 10.

The clock-control data can be fully programmed so that the jitter generation can easily be adapted to a particular application or a particular test of the DUT. The amount of jitter to be generated is only limited by the storage capacity of memory 8. Any jitter resolution can be generated only dependent on the clock-control data.

FIG. 2 shows a block diagram of a second embodiment of the present invention. In the second embodiment the data pattern defining the stimulus for the DUT is provided by the DUT itself, representing a so called loop-back test. The signal provided from the DUT may comprise DUT dependent or line dependent jitter and thus is input via DUT pattern line 128 via driver unit 124 to a clock data recovery unit 126 comprising a phase-locked-loop. The clock data recovery unit 126 recovers the data pattern on data pattern line 106 and recovers a first clock on first clock line 112 according to which the data pattern is input in memory 108.

Reading out said digital data signal from said memory 108 via driver unit 116 to output line 102 is identical as described for the first embodiment, i.e. a clock signal of a clock source 114 is used that s modulated according to clock-control data of a clock-control data source 120.

FIG. 3 shows a block diagram of a third embodiment of the present invention. Similar to the second embodiment also in the third embodiment the data pattern is provided by the DUT via DUT pattern line 228. As a difference to the second embodiment, in the third embodiment the data pattern is input in a data recovery unit 230 comprising no phase-locked loop, but receiving from external the first clock via first clock line 212. Accordingly the data recovery unit 230 has only to recover the data pattern within the signal received from the DUT and to input said data pattern via data pattern line 206 to the memory 208.

The remaining portion of the third embodiment is identical to the corresponding portions of the first and second embodiments, i.e. a digital data are read-out from memory 208 via driver unit 216 to output line 202 using a clock signal provided by clock source 214, said clock signal being modulated according to clock-control data of clock-control data source 220.

In the second and third embodiment the jitter generated in the digital data signal on output line 102, 202 is independent from jitter comprised by the signal received from the DUT because time is restored by clock data recovery unit 126 in the second embodiment, or by the external clock signal provided on first clock line 212 in the third embodiment which can be provided by the ATE. Thus, the generated jitter is independent of the amount of jitter at the DUT output.

FIG. 4 shows a block diagram of a first possibility to realize the clock source 14, 114, 214 comprising a fractional-N phase-locked-loop (PLL) or a delta-sigma-modulated (DSM) fractional-N PLL. The input frequency fin which represents a reference frequency 72 is divided by M using a divider 52 resulting in fM=fin/M which is input in a phase detector 54. The output signal of the phase detector 54 is input in a low pass filter 56, the output signal of which controls a voltage-controlled-oscillator (VCO) 58 having an output frequency fout.

The output signal of the VCO is fed back to the phase detector 54 via a divider 60 with adjustable divisor Navg being defined as Navg=average of a sequence N(k) of values N. Thus, divider 60 having an output signal with frequency fN=fout/Navg which is input in the phase detector 54. Accordingly, the quotient fout/fin is adjustable as Navg/M, wherein this quotient can be adjusted very fine and particularly fractional of N depending on the values of the sequence N(k). Furthermore, the phase of output frequency fout can be adjusted in accordance to the values of the sequence N(k) and thus the output signal of the VCO on clock line 18 can be phase modulated according to the sequence N(k). Due to the modulation of fout representing the clock signal for reading out memory 8, the output bit stream of memory 8 on output line 2 comprises a jitter according to the modulation of the output frequency fout and thus according to the sequence N(k) which represents the clock-control data and which is provided via clock-control data line 22 from clock-control data source 20.

FIG. 5 shows two possible embodiments for providing a sequence N(k) of values N for the divider 60. The sequence N1(k) can be provided directly by a data sequencer 62 receiving data from a pattern memory 64 and being controlled by control line 66. Alternatively, the sequence N2(k) can be provided by dedicated hardware, e.g. a delta-sigma modulator (DSM) 68, which can be realized in hardware or software.

FIG. 6 shows a block diagram of a second possibility to realize the clock source 14, 114, 214 comprising a Direct Digital Synthesis (DDS) unit 70. A reference frequency 72 or reference clock is provided to a phase accumulator 74, a look-up table 76 and a Digital-to-Analogue Converter (DAC) 78. The DDS-unit 70 is fed by clock-control data source 20 via clock-control data line 22. The phase of clock-control data is accumulated and input to look-up table 76 in which the digital sampling values of the ideal sine waveforms are stored. The DAC 78 converts the digital sampling values to the corresponding analogue signal which is subsequently filtered by a low pass filter 80, which may also be located external to the DDS-unit 70. The output of the low pass filter 80 is finally shaped by a comparator 82 being located external to the DDS-unit 70 and the resulting signal is output on clock line 18 as a clock being modulated according to the clock-control data. Unlimited phase variations are possible and the modulations speed is limited only by the DAC 78 and the low pass filter 80.

FIG. 7 shows a block diagram of a third possibility to realize the clock source 14, 114, 214 comprising a ring oscillator 90. A multiplexer 92 is controlled by the clock-control data source 20 via clock-control data line 22. The phase of the modulated clock on clock line 18 depends on the tap 94, 96 selected by the multiplexer 92 for switching to clock line 18 according to clock-control data. The modulation speed is very high. Interpolation between adjacent taps 94, 96 for fine resolution is possible. Multiple periods of phase shifting can be realized when the multiplexer 92 rotates multiple times through loop.

While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.

Claims

1. A method for generating jitter in a digital data signal, said digital data signal having a predetermined data pattern being stored in a memory, said method comprising the steps of: reading out said digital data signal from said memory using a clock signal provided by a clock source, modulating said clock signal provided by said clock source according to clock-control data, wherein said clock-control data represents said jitter to be generated in said digital data signal read out from said memory.

2. The method of claim 1, wherein said step of modulating of said clock signal comprises a phase modulation.

3. The method of claim 1, wherein said clock-control data are programmable.

4. The method of claim 1, wherein said digital data signal having said predetermined data pattern are input in said memory using a first clock, said first clock being different from said clock signal used for read out said digital data signal from said memory.

5. The method of claim 4, wherein said first clock is recovered from a bit stream also comprising said predetermined data pattern of said digital data signal.

6. The method of claim 1, wherein said memory has a first-in-first-out (FIFO) architecture.

7. The method of claim 1, wherein said step of modulating said clock signal comprising a N/M phase-locked-loop modulation of a reference clock, and/or comprising a fractional-N phase-locked-loop modulation of a reference clock, wherein N and M being integer.

8. The method of claim 1, wherein said step of modulating said clock signal comprising a direct-digital-synthesis (DSS) modulation.

9. The method of claim 1, wherein said step of modulating said clock signal comprising ring-oscillator modulation.

10. The method of claim 1, wherein said digital data signal is a stimulus signal for a Device Under Test (DUT), and in that a response signal of said Device Under Test (DUT) on said stimulus signal is returned to said Automated Test Equipment (ATE) for evaluation.

11. The method of claim 10, wherein said data pattern is provided by said Automated Test Equipment (ATE).

12. The method of claim 10, wherein said data pattern is provided by said Device Under Test (DUT).

13. A software program or product, preferably stored on a data carrier, for executing, when running on a data processing system such as a computer, a method for generating jitter in a digital data signal, said digital data signal having a predetermined data pattern being stored in a memory, said method comprising the steps of: reading out said digital data signal from said memory using a clock signal provided by a clock source, modulating said clock signal provided by said clock source according to clock-control data, wherein said clock-control data represents said jitter to be generated in said digital data signal read out from said memory.

14. A system for generating jitter in a digital data signal, said digital data signal having a predetermined data pattern being stored in a memory, said system comprising: a clock source providing a clock signal used for reading out said digital data signal from said memory, a modulator for modulating said clock signal provided by said clock source according to clock-control data, wherein said clock-control data represents said jitter to be generated in said digital data signal read out from said memory.