Systems and methods for injection of test jitter in data bit-streams

[0001] The present invention is generally related to digital circuits and specifically related to systems and methods for injection of test jitter in data bit-streams.

[0002] As data rates for new generation integrated circuits (ICs) continue to increase, the amount of a type of noise known as jitter in a data stream is becoming an important factor in the quality of data output. As used herein, jitter may be defined as uncertainties in the relative position of data transitions relative to a fixed reference edge. The number of applications for high speed interfaces have increased. Resultantly, a signal is typically sent over several different networks before reaching its destination. As a signal goes through each of these networks, jitter is introduced into the data stream. When this data reaches the intended receiver, even though the data stream began as a clean signal, it ends up as a dirty signal. Even though data streams with very low jitter may be generated, the amount of jitter in the received data stream may be quite large. This results when data output from a transmitter transits through different devices, each of these devices will, in turn, introduce one or another type of jitter into the data stream. Thus, data received by the target receiver may have different amounts of jitter, depending on the path the data took, the type of data pattern and/or other factors. So, when jitter levels in a data stream are high and the performance of the receiver is relatively marginal to begin with, the receiver may not function properly.

[0003] There are several types of existing jitter injection modules (JIMs). JIMs are intended to introduce jitter in the output data of a transmitter of a high speed data stream, simulating a data communication environment. An existing JIM may be a stand-alone module or may be a module embedded in an IC design.

[0004] In order to introduce jitter in a transmitted data stream, an existing external JIM may be inserted between a transmitter and a receiver. However, existing external JIMs have several disadvantages. When there are multiple high speed data channels in an IC, each of these channels requires its own separate JIM. Also, existing JIMs do not have a provision to introduce jitter in a controllable manner, in such a manner as to control the amount of jitter or to provide a variety of different types of jitter. For external JIMs, with, for example, a co-located transmitter and receiver, an external loop back may be used to introduce jitter. For an internal loop back, there is no existing manner to create jitter to measure quality of the receiver.

[0005] An existing embedded JIM is typically inserted in the path of a high speed clock or the data path. This method is an invasive manner for accomplishing jitter injection into a data stream. This technique also has disadvantages. Any module connected to a high throughput, or high bit rate, data stream has a potential to degrade the performance of a transmitter connected thereto. Hence, an IC embedded JIM is not a preferred method to inject jitter in the transmitted data stream. In an IC there are typically multiple high speed data channels. Therefore, an embedded JIM module must be incorporated in each of the transmitter channels, which will be a significant waste of physical IC space.

[0006] The present invention is directed to systems and methods that provide a non-invasive design for test approach by which jitter may be injected into a data stream in a controlled manner to study the effects of jitter on the performance of a receiver of a data stream.

[0007] In order to test receiver ICs the present invention provides systems and methods by which jitter may be introduced in a high speed data stream, in a controlled and non-invasive manner at a very low cost. Artificially introducing jitter enables measurement of the quality of a receiver, either receiving the signal or incorporated into the IC in accordance with embodiments of the present invention. Specifically, the present invention solves the aforedescribed problems in that it is incorporated into the transmitter IC itself, enabling low cost introduction of jitter to measure quality of a receiver, whether used in an internal loop back or an external loop back configuration. Examples of broadband applications which may advantageously employ the present invention include Fiber Channel, Gigabit Ethernet and new generation interfaces for third generation input/output (3GI/O) wireless hardware.

[0008] Advantageously, the present invention does not require any external or internal modules to inject jitter into a high speed data stream. Jitter is introduced in the data stream by modulating reference voltages of source and/or sink current generators typically present in ICs that are susceptible to the effects of jitter, such as high speed data transmitter ICs, and the like. Also, the present invention enables characteristics of the injected jitter to be controlled. For example, a desired amount of jitter to be introduced and a desired amplitude for this jitter to be selected. Furthermore, in accordance with the present invention, a desired type of jitter can be introduced. Such types of jitter may include in-band or out-of-band jitter, sinusoidal wave form jitter, pattern dependent jitter, phase dependent jitter, interference dependent jitter, or other types of jitter known to those skilled in the art. As a further advantage, jitter may be selectively introduced in any, or all, of the multiple high speed data channels in an IC, in accordance with the present invention.

[0009] An embodiment of a method for injecting test jitter in a data bit stream comprises modulating first and second voltage generators to control a rise and fall times of an output signal, respectively. A pair of input voltages are received by a differential pair. At least one current sink device is operated using a first control voltage provided by at least one of the voltage generators to provide an output voltage in response to the input voltages received by the differential pair. A plurality of current sources are operated to provide the output signal using a reference voltage provided by one of the voltage generators in response to the input voltages received by the differential pair, wherein simultaneous variation of the rise and fall times together with the input voltages define the output signal and jitter output by the current sink device and the current sources.

[0010] An embodiment of a system for injecting test jitter in one or more independent data bit streams, the system comprises at least one modulator and a plurality of data bit stream generators each of the generators modulated by at least one of the modulators. A differential pair has a first output of the differential pair controlled by a first input voltage. A second output of the differential pair is controlled by a second input voltage. A current sink device for each data bit stream is operated by a first control voltage provided by a first of the voltage generators. Each current sink device provides an output voltage in response to the input voltages provided to the differential pair. A plurality of current source devices are controlled by a second control voltage provided by a second of the voltage generators. Each of the current sources provide an output voltage in response to the input voltages provided to the differential pair. Each current source and each current sink device provide an output voltage in response to the input voltages provided to the differential pair. Simultaneous variation of the first and second control voltages in combination with the input voltages defines an output signal and jitter.

[0011] An embodiment of a circuit adapted to inject test jitter in one or more independent data bit streams comprises at least one modulator and a plurality of data bit stream generators each of the generators modulated by at least one of the modulators. An output buffer of the circuit comprises a differential pair with a first output of the differential pair controlled by a first input voltage and a second output of the differential pair controlled by a second input voltage. A current sink device in the output buffer for each data bit stream is operated by a first control voltage provided by a first of the voltage generators. Each current sink device provides an output voltage in response to the input voltages provided to the differential pair. A plurality of load capacitors in the output buffer each receive, store and discharge one of the output voltages. A plurality of output buffer current source devices are controlled by a second control voltage provided by a second of the voltage generators. Each of the current sources provides an output voltage in response to the input voltages provided to the differential pair. Each current source and each current sink device provide an output voltage in response to the input voltages provided to the differential pair. Simultaneous variation of the first and second control voltages in combination with the input voltages defines an output signal and jitter.

[0012] The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.

[0013] For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:

[0014] FIG. 1 is a partial schematic of a common mode logic (CML) circuit differential output employing an embodiment of the present jitter injection system and method;

[0015] FIG. 2 is a diagrammatic, partial schematic, illustration of a transceiver employing an embodiment of the present jitter injection system and method;

[0016] FIG. 3 is a graphical representation of jitter injection in accordance with the present invention; and

[0017] FIG. 4 is a flow chart of a method of jitter injection in a data bit stream in accordance with an embodiment of the present invention.

[0018] Turning to FIG. 1, a partial schematic of a common mode logic (CML) circuit differential output 100 employing an embodiment of the present jitter injection system is illustrated. In high speed transmitters, data is typically moved from one point to another in differential output format. These differential outputs are generated using differential transistor pairs made up of metal oxide semiconductor (MOS), common mode logic (CML) or other types of differential output buffers. An example of such a differential output is shown by box 101 in FIG. 1.

[0019] Reference signal voltage V_CS in FIG. 1 determines the amount of current that will be provided for differential transistor pair 102, Q1 (103) and Q2 (104). This may be represented by the formula: $\begin{array}{cc}{I}_{\mathrm{cs}}=\frac{}{2}*{\left(V_{\mathrm{cs}}-V_t\right)}^2\mathrm{for}0<V_{\mathrm{cs}}-V_t<V_{\mathrm{ds}}& \left(\mathrm{Eq}.1\right)\end{array}$

[0020] where I_CS is the current that flows from the drain to the source of current sink transistor Q3 (105). Current sink Q3 (105) synchronizes the current provided by Q1 (103) and Q2 (104). is the MOS transistor gain factor; V_CS is the gate voltage for transistor Q3 (105); V_t is the threshold voltage; and V_ds is the drain source voltage of current sink transistor Q3 (105). Given that I_cs is a function of the gate voltage V_CS as given by Eq. 1 and depicted in FIG. 3, the value of I_cs can be controlled by changing the value of gate voltage V_CS.

[0021] Transistor differential pair 102, Q1 (103) and Q2 (104), has two inputs, V_in and V_inb. Due to the nature of the configuration of the differential inputs, when V_in is logically high, then V_inb is logically expected to be at a logically low state. Conversely, when V_inb is high, V_in is expected to be low.

[0022] In a typical IC application, the two differential outputs, V_out and V_outb, are connected to respective differential inputs of a receiver. The input loads of the receiver including the loads from each of the outputs, V_out and V_outb, may be represented by capacitive loads, C_L and C_Lb as shown at capacitors 110 and 111 in FIG. 1. The voltage across these load capacitances, C_L and C_Lb can be represented by:

[0023] where V_out and V_outb are the voltage across the capacitances, C_L and C_Lb.

[0024] If capacitors C_L and C_Lb are allowed to charge for t seconds, then the charge accumulated by the capacitors C_L and C_Lb could be represented by:

[0025] where I and I_b are the currents flowing into the capacitors C_L and C_Lb. Hence, equations, Eq. 2 and Eq. 2a may be rewritten as:

[0026] where I and I_b are the currents flowing into the capacitors C_L and C_Lb and t is the time taken to charge capacitors C_L and C_Lb to respective voltages V_out.and V_outb. Given that C_L and C_Lb are constants and if the value of V_out and V_outb are fixed, then:

[0027] where I and I_b are the current that is charging or discharging capacitors C_L and C_Lb, respectively. Equations 4 and 4a show that the time to charge the capacitors to a predetermined level, rising time, or the time to discharge the capacitor to a predetermined level, fall time, is inversely proportional to the current I or I_b.

[0028] In FIG. 1, the source current I_sc that charges the capacitor C_L (110) is provided by active load transistors Q5 (116) and the source current I_scb that charges the capacitor C_Lb (111) is provided by active load transistor Q4 (115). On the other hand, the sink current I_sk that discharges capacitors C_L (110) and C_Lb (111) is provided by transistor Q3 (105). In a normal mode of operation of an existing transmitter, the value of the reference voltages V_CS and V_CS2 controls the source and sink currents and these voltages are well characterized and kept constant.

[0029] When a voltage V_CS2 is applied to current sources Q4 (115) and Q5 (116) such that it will become operational in the saturation region it will provide current to Q1 (103) and Q2 (104) in addition to load capacitors C_L (110) and C_Lb (111), as indicated by equation-1. So if V_in is high relative to V_inb, then transistor Q1 (104) will also be operating in the saturation region and hence current flows from Q4 (115) through Q1 (103) to Q3 (105). In addition, since transistor Q1 is operating in the saturation region, it will also discharge the load capacitor C_Lb (111). In this mode, since V_inb is low, transistor Q2 will be in an inactive state and hence there will not be any flow of current through Q2 (114) to Q3 (105). That being the case, the current from Q5 (116) will be used exclusively to charge the load capacitor C_L (110).

[0030] The amount of time taken to charge and discharge the capacitors as mentioned above is governed by Eq. 4 and Eq. 4a. In accordance with the present invention, V_out is fixed and that the size of capacitor C_L (110) and C_Lb, (111) are fixed. Thus the time to charge capacitor C_L or C_Lb is inversely proportional to the current. Thus, this relationship may be used to provide jitter in accordance with the present invention. By varying the amount of current, the time a signal takes to go from V_in, to V_outb, or V_inb to V_out, can be controlled. The larger the current, the less time it will take, and the smaller the current, the more time it will take. So if the output voltage of the voltage source V_CS is increased then that will increase the amount of current I_sk that will flow through the current sink transistor Q3 (105). This relationship, the current sink I_sk and the voltage V_CS, is governed by Equation 1 above. So when V_CS is increased the value of I_sk will also increase. Equations 3 and 3a indicate that the time to discharge capacitor C_L or C_Lb is inversely proportional to the current, in this case I_sk. So if I_sk is of a larger value then the amount of time to discharge capacitor C_L or C_Lb will be smaller. In such a case, a high to low transition at the output will appear to have taken place at an earlier time. On the contrary when the value of the voltage V_CS is decreased the value of I_sk will become smaller. Consequently, the time to discharge load capacitor C_L or C_Lb will be longer. In such a case, a high to low transition at the output will appear to have taken place at a later time. This is illustrated in FIG. 3.

[0031] FIG. 3 graphically shows how the arrival time of the rising edge of the output signal, V_out can be varied by changing the values of the reference voltages, V_cs2. Similarly, the arrival time of the falling edge of the output signal V_out can be changed by varying the value of V_cs. In FIG. 3 the x-axis depicts arrival time of a signal, and the y-axis represents the output current from the output port V_out.

[0032] As the voltage source V_CS controls the fall time of the output signal as described above, the voltage source V_CS2 controls the rise time of the output signal. The voltage source V_CS2 provides the source current I_sc to charge capacitor C_L and I_scb to charge capacitor C_Lb. The relationship of I_sc and I_scb to V_CS2 may be governed by Equation 1. Equation 1 shows that the source currents I_sc and I_scb will increase when voltage V_CS2 is increased. Equation 3 shows that when I_sc and I_scb are large, the time to charge capacitor C_L or C_Lb will be smaller. In such a case the output signal will transition from low to high at a faster rate. It will appear as if a low to high output signal became available at the output at an earlier time. If on the other hand when voltage V_CS2 is decreased, equation-1 shows that the source currents I_sc and I_scb will decrease. Equation-3 shows that when I_sc and I_scb are small, it will take a longer time to charge capacitor C_L or C_Lb. Consequently, it will appear as if a low to high output signal became available at the output at an later time.

[0033] Thus, according to an embodiment of the present invention, in a test mode, the rise and fall time of output voltages (V_out and V_outb) may be changed by modulating the value of the sink and source reference voltages V_CS and V_CS2. The effect of this variation on the rise and fall time of the output voltage will be injection of jitter into the data stream transmitted by differential output 101. This modulation may be provided by employing modulators 120 and 121 to modulate the output of voltage generators 130 and 131, respectively.

[0034] V_CS generator 130 and V_CS2 generator 131 may be comprised of a set of transistors that provide the appropriate voltage V_CS, to transistor Q3 (105) and voltage V_CS2 to Q4 (115) and Q5 (116). By controlling or modulating generator 130, the amount of voltage V_CS provided to Q3 (105) can be controlled or modulated. In the same way, V_CS2 generator 131 can be modulated to change the V_CS, voltage at Q4 (115) and Q5 (116). This modulation at the generator may be used to introduce jitter. The modulator may take the form of a voltage wave source generating waveforms, such as sine waves, triangular waves, pseudo random binary sequences (PRBS), or other random signals.

[0035] In a differential output buffer employing an embodiment of the present invention, the active load transistors Q4 (115) and Q5 (116) may alternatively be replaced with passive loads such as resistors. In such a case, the rise time of the output signal may not be externally controllable. However, the fall time of the output signal may be controlled by modulating V_CS to introduce jitter.

[0036] An embodiment of a jitter injection and measurement circuit 200 employed in conjunction with a high speed transceiver 201 and for use with a high speed data stream is shown in FIG. 2. New generation network processors, Internet applications, fiber channel applications and the like, employ many transmitters and receivers within a chip. In this embodiment automatic test equipment (ATE) 202 provides a modulating signal, for a desired type of jitter, to V_CS and V_CS2 generators 130 and 131. Alternatively, a built-in self test (BIST)-circuit may be substituted for ATE 202 module or a combination of a BIST and an ATE may be used. The amount of jitter may be measured by an on-chip timing interval analyzer (TIA) module 203 or other types of jitter measurement techniques, modules or devices. The jitter measurement is preferably fed back to ATE 202 via line 204.

[0037] More specifically, FIG. 2 diagrammatically illustrates use of V_CS and V_CS2 modulating signals to modulate V_CS and V_CS2 generators 130 and 131 which are connected to multiple CML output buffers 205(1n). Generators 130 and 131 provide jitter to the buffer of each transmitter 210(1)-210(n), simultaneously. Therefore, by controlling one set of generators, several transmitters may be modulated. Thusly, CML buffers 205 (1N) may be modulated to introduce jitter in an output data stream Tx-V_out or Tx-V_outb. The output data stream is directed into receiver 202. Preferably, a time interval analyzer (TIA) 203 at the front of receiver 206 measures the amount of jitter. TIA 203 returns the value of the jitter at receiver 206, which ATE 202 may use to control modulation. ATE 201 provides a modulated signal to V_CS and V_CS2 generators 130 and 131. Each transmitter may adjust the modulation simultaneously. However, an embodiment of the present invention enables modulating one or fewer transmitters in accordance with the present invention.

[0038] FIG. 2 shows jitter injected at the final output stage of transmitters 210(1)-210(n). However, it is not necessary that the jitter be injected only at the final output stage. Depending on the application, jitter may be selectively injected anywhere in the data and/or clock path by modulating the rise and fall times of one or more appropriate intermediate differential buffers, in accordance with the present invention.

[0039] A minimum amount of hardware is required on a chip to provide a jitter testing mechanism in accordance with the present invention. The present invention may be incorporated at the transmitter output(s), and all the modulating voltage is provided by ATE 202. Advantageously, minimal overhead costs are involved for a chip incorporating the present systems and methods for injection of test jitter in a data bit stream. A CML buffer is typically already part of the transmitter output buffers for most chips employing high speed transmitters or transceivers. A control to an outside ATE enables the ATE to provide the modulating signal in accordance with the present invention.

[0040] Turning to FIG. 4, method 400 for jitter injection in a data bit stream in accordance with an embodiment of the present invention is flow charted. At 401 a first voltage generator modulates a first control voltage to control the rise time of an output signal. A second voltage generator modulates a second control voltage to control the fall time of the output signal at box 402. A differential pair receives a pair of input voltages. See box 403. A current sink device is operated using a control voltage provided by one of the generators to provide an output voltage in response to the input voltages received by the differential pair at box 404. At 405 current sources provide the output signal using a reference voltage provided by one of the generators in response to the input voltages received by the differential pair. Thusly, jitter is provided in the output signal as detailed in box 406. Specifically, simultaneous variation of the rise and fall times of the output signal and the input voltages define the output signal and jitter output by the current sinks and the current sources.

[0041] Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.