Abstract:
A shaped pulse generation circuit for applications such as automatic test equipment pin drivers employs an active feedback circuit that adds a pre-emphasis to the output pulses, thereby compensating for the effect of the transmission system between the pin driver and the device under test. Current pulses are applied to the pin driver output transistors in conjunction with the production of rising and falling output pulse edges to produce edge overshoots that are mitigated during pulse transit to the device under test. The driver circuit together with the pre-emphasis active feedback circuit can be integrated onto a single chip, with an additional programming circuit employed to control the amount of pre-emphasis.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the generation and transmission of electrical pulses with desired waveforms, particularly in the context of automatic test equipment (ATE) pin drivers. 
     2. Description of the Related Art 
     Pin drivers are commonly used in automated test equipment systems to generate test signals in the form of digital or analog voltages for delivery to the input pins of a device under test (DUT). They may be used, for example, to test memory chips having many input pins. Proper waveshape and timing accuracy of the signal at the DUT-are primary objectives of the overall system design, and are figures of merit which distinguish between different available ATE systems. 
     Due to increasing system speeds, waveform fidelity has become harder to maintain. While a pin driver itself may be able to produce a pure, undegraded signal, various elements of the overall system have a detrimental effect on waveform fidelity. These include the board on which the pin driver is attached (commonly called the channel card), the relays used to direct the signal, and the connectors which transmit the signal to other boards. The large number of pins on test devices such as memory chips also leads to long signal paths. The net effect is to lower the bandwidth of the test pulse signals that are delivered to the DUT. Although the driver itself may produce crisp, clean pulses with sharp corners, the net effect of the signal path between the pin driver and the DUT is to round off the corners of the test pulses, causing the transitions between the pulse edges and the full scale pulse value to be slower and more gradual than is desired. This is illustrated in FIG. 1, in which the clean pulse  2  generated by the pin driver is indicated by a solid line, with the rounded corners  4  induced by the transmission system indicated by dashed lines. 
     Attempts to remediate the pulse degradation have typically been directed towards reducing the effects of the transmission system. For example, inductive tuning networks have been provided along the signal path to produce a ringing which compensates for the path&#39;s effect on the pulse shape. Another approach involves adding copper stubs to the circuit board with a characteristic impedance that matches that of the transmission elements. Efforts have also been made to develop improved relays whose effect on the final pulse delivered to the DUT is reduced. All of these approaches are directed at modifying the transmission system so as to reduce or compensate for its effects on the clean pulse, and require a significant modification to the channel card. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide an improved way to deliver a sharp pulse signal to a DUT, without having to modify the transmission system or significantly impacting the channel card. It relies upon modifying the original pulse signal itself, rather than the transmission system, to generate the pulse with a pre-emphasis that compensates for the effects of the transmission system, so that the final pulse delivered to the DUT has a desired clean waveform. 
     The pre-emphasis is implemented with an active feedback circuit in the pulse forming circuitry that adds a pre-emphasis to the edges of the pulse, preferably causing leading and trailing edges of the pulse to overshoot by an amount that compensates for the rounding effect of the transmission system. The feedback circuit is preferably integrated together with the basic pulse forming circuit in a common integrated circuit (IC) chip, except for an off-chip circuit that is used to program the amount of pre-emphasis to be added. 
     In one embodiment the feedback circuit modifies the current drive to an output transistor from which the pulse is produced. It includes an amplifier that receives a feedback from the output pulse, and an RC circuit which causes the amplifier to produce a pre-emphasis current for the output transistor as an edge overshoot. A pair of amplifiers are preferably employed to provide positive and negative overshoots for the leading and trailing pulse edges. 
     These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram, described above, illustrating an ideal voltage pulse and the distortion imparted by its transmission system; 
     FIG. 2 is a simplified schematic diagram of a pin driver system which incorporates the invention; 
     FIG. 3 is a diagram illustrating the nature of the pre-emphasis provided by the invention; 
     FIG. 4 is a schematic diagram of a feedback pre-emphasis circuit in accordance with the invention, together with a reverse buffer amplifier for the pin driver; 
     FIGS. 5A,  5 B and  5 C are graphs respectively illustrating an output voltage pulse waveform at various locations in the system, current waveforms generated internally within the pre-emphasis circuit, and current waveforms which induce the pre-emphasis output; 
     FIG. 6 is a 3-dimensional graph illustrating the pre-emphasis signal as a function of resistance and capacitance values in an RC programming circuit employed by the pre-emphasis circuitry; and 
     FIG. 7 is a schematic diagram of an alternate circuit for generating a pre-emphasis signal. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 illustrates a pin driver system which incorporates the invention. It includes a pin driver with a control circuit  6  and an output stage. The pin driver can be an available device such as the Analog Devices, Inc. model AD 5 3004. This device uses a class A-B (“push-pull”) type output stage. An npn bipolar output transistor Q 1  and an output resistor R 1  are connected in series between a positive voltage reference Vcc and an output node  8  on the “push” or positive side of the output stage, while another output resistor R 2  and pnp bipolar output transistor Q 2  are connected in series between node  8  and a negative voltage reference Vee on the “pull” or negative side of the output stage. A current source I 1  controlled by one output of the pin driver control circuit  6  drives current into the base of Q 1 , while another current source I 2  controlled by another output of the pin driver control circuit draws current out of the base of Q 2 . Vcc and Vee are typically +10 volts and −5.2 volts, respectively. The current sources are typically 15 mA, and the output resistors 4 ohms. The output node  8  provides test pulses to a DUT  10  through a transmission system  12  which typically includes relays, cabling and connectors, the collective impedance of which is represented by Zt. 
     A voltage which establishes a high reference level (level  26  in FIG. 3, discussed below) during the “push” phase of operation is provided in the base of Q 1  over output line  13   a  from the pin driver control circuit. Similarly, a voltage which establishes a low reference level (ground voltage in FIG. 3) during the “pull” phase of operation is provided to the base of Q 2  over output line  13   b  from the pin driver control circuit. 
     To initiate a positive pulse, the pin driver control circuit  6  causes I 1  to transmit current into the base of Q 1 , which in turn causes the base voltage to rise. Functioning as an emitter follower, the emitter voltage of Q 1  also rises, bringing up the voltage at the output node  8 . To reduce the output voltage, the pin driver control circuit  6  causes I 2  to draw current out of the base of Q 2 , in turn causing Q 2 &#39;s base voltage and thus its emitter voltage to drop, along with the voltage at output node  8 . 
     The output node is connected via a known unity gain reverse buffer amplifier  14  to pre-emphasis feedback circuitry which is the subject of the invention. A suitable reverse buffer amplifier is discussed as element  106  in U.S. Pat. No. 5,434,446 to Hilton et al., “Parasitic Capacitance Cancellation Circuit”. 
     The reverse buffer amplifier  14  is part of a feedback circuit  15  which includes a pair of voltage-to-current converting amplifiers  16  and  18  connected to dual outputs of the reverse buffer amplifier. Amplifier  16  responds to a rising voltage at the output node  8  (which produces an equal rising voltage at the output of unity gain reverse buffer amplifier  14 ) by producing an output current that adds to the current drive from I 1  into the base of output transistor Q 1 . This causes the voltage rise at the base and emitter of Q 1 , and also at the output node  8 , to transiently overshoot the value that would be reached from the current drive supplied from I 1  by itself. Thus, as illustrated in FIG. 3, the leading edge  20  of an output voltage pulse  22  at output voltage node  8  will spike to a peak overshoot level  24  that is greater than the maximum pulse level  26  due to I 1 . Similarly, amplifier  18  responds to a falling voltage at output node  8  to produce a transient boost in the current drawn out of the base of output transistor Q 2 , causing an overshoot  28  in the trailing edge  30  of output pulse  22  to a level less than that due solely to I 2 . 
     The overshoots  24  and  28  constitute a pre-emphasis of the ideal pulse waveform  2  in FIG. 1 that compensates for the effects of the pulse transmission system, yielding a much closer approximation of the ideal waveform for the pulse that is actually applied to the DUT after transmission through the pulse transmission system  12 . The amount of overshoot is programmed into the system by means of a programming element, which in the illustrated embodiment is shown as an RC circuit consisting of resistor Rp and capacitor Cp in series between the output of the feedback circuit and ground. The magnitude of over-shoot can be adjusted, to the first order, simply by changing the resistance value, while its duration is established by the RC time constant. Alternate programming circuits, such as an RLC circuit or an active cell that produces a desired waveshape, could be used instead of an RC circuit. 
     One of the distinct advantages of the invention is that the pre-emphasis feedback circuit can be integrated in common with the pin driver on a common substrate, illustrated as IC chip  32  in FIG.  2 . The feedback amplifiers  16 ,  18  employ active transistor elements that can be integrated with the remainder of the pin driver circuitry in a single IC chip. The only elements that would normally be off-chip would be the components of the programming circuit. This is advantageous, since different programming component values might be desired for the same type of pin driver chip in different situations; these values can be optimized for each individual application. However, the programming circuit can be on-chip if desired. 
     FIG. 4 is a schematic diagram showing one implementation of a pre-emphasis feedback circuit  15 , including a simplified reverse buffer amplifier  14  for which typical ohmic resistance values are given. The reverse buffer amplifier is a push-pull emitter follower unity gain buffer. It receives a voltage input from the driver circuit&#39;s output node  8 , and produces corresponding output voltages on lines  34  and  36 . 
     The pre-emphasis circuit  15 , much like the push-pull portion of the driver output stage, includes a positive current drive section consisting of a resistor R 3 , a diode-connected transistor Q 4 , a drive transistor Q 5  and another resistor R 4  connected in that order between Vcc and the RpCp circuit. Similarly, a negative current drive circuit is included consisting of a resistor R 5 , a drive transistor Q 6 , a diode-connected transistor Q 7  and another resistor R 6  connected in that order between the RpCp circuit and Vee. A high value resistor R 7  is connected between the common base-collector terminals for Q 4  and Q 7  to maintain a standing current of about 1 mA, which prevents Q 5  and Q 6  from turning completely off during a transition. The Q 5  and Q 6  drive transistors are respectively npn and pnp bipolar devices whose bases are driven respectively by voltage outputs  34  and  36  from the reverse buffer amplifier  14 . In response to a positive going signal on drive line  34 , Q 5  develops a dynamic current spike in excess of its quiescent current (typically about 4-5 mA), with a magnitude and shape that are determined by the selected values for Rp and Cp; during this time the voltage on drive line  36  changes in tandem with that on drive line  34 , reducing the current through negative drive Q 6 . Conversely, a negative going pin driver output voltage on output node  8  produces a negative going drive voltage on line  36  that causes Q 6  to conduct a dynamic negative current spike, the magnitude and duration of which are again determined by the selected values for Rp and Cp. 
     The currents through drive transistors Q 5  and Q 6  pass through diode-connected transistors Q 4  and Q 7 , re-spectively, from which they are mirrored to inject the pre-emphasis base current spikes into the pin driver out-put transistors Q 1  and Q 2 . The first mirror leg consists of series connected pnp and npn transistors Q 8  and Q 9 , whose bases are connected in common with the bases of current drive transistors Q 4  and Q 7 , respectively, and whose emitters are respectively connected to Vcc and Vee through resistors R 8  and R 9 . The second mirror leg is identical to the first, including transistors Q 10  and Q 11  and resistors R 10  and R 11 , which correspond respectively to Q 8 , Q 9 , R 8  and R 9 . An input to Q 1  is taken from the collectors of Q 8  and Q 9  in the first mirror leg, and an input to the base of Q 2  from the collectors of Q 10  and Q 11  in the second mirror leg. In this particular embodiment the resistance values of R 8  and R 10  in the upper mirror legs are greater than that of R 3  in the mirror drive circuit, and similarly the resistance values of R 9  and R 11  in the lower mirror legs are greater than that of R 6  in the mirror drive circuit, to yield lower reflected currents in the mirror legs than in the mirror drive. Depending upon the particular application, other resistor ratios may be employed. 
     In response to a positive going leading pulse edge on input lines  34  and  36  to the pre-emphasis circuit, npn current drive transistor Q 5  conducts a current spike governed by the RpCp programming circuit. Since Cp will initially appear as a short circuit, a sharp rise in current will occur, followed by a rapid reduction back to zero as Cp charges. The Q 5  current spike also flows through Q 4 , from where it is mirrored into Q 8  and Q 10  in a proportion determined by the transistor sizes and resistor ratios. The current through Q 6  and Q 7  decreases, and is mirrored into Q 9  and Q 11 . Accordingly, a positive pre-emphasis current spike is sent to the high impedance bases of the pin driver output transistors Q 1  and Q 2  from the first and second mirror legs. Since npn transistor Q 1  is conducting, the current spike increases its base and emitter voltages to produce a corresponding positive pre-emphasis voltage (corresponding to pre-emphasis  24  in FIG. 3) in the output voltage pulse that is delivered through the pulse transmission system  12  to the DUT  10 . 
     Conversely, a negative going trailing edge voltage on input lines  34  and  36  to the pre-emphasis circuit reduces the current through Q 5  and increases the current through Q 6 . This activates the lower portion of the current mirror, causing mirror leg transistors Q 9  and Q 11  to draw pre-emphasis current pulses out of the bases of Q 1  and Q 2 . The result is a transient reduction in the voltage at pin driver output node  8 , corresponding to negative pre-emphasis overshoot  28  at the end of the trailing pulse edge in FIG.  3 . Again, the magnitude and duration of this negative overshoot is governed by the values of Rp and Cp. 
     With the circuit shown in FIG. 4, the necessary current spike into the high impedance output transistor base to effect a 5% overshoot is about 3 mA. To cancel out the quiescent current through Q 5  and Q 6 , the collectors of Q 8  and Q 9  and of Q 10  and Q 11  are connected together. The resulting current out to the output transistors is only the difference during transients, plus a small error term. 
     The current through the external RpCp circuit needs to be balanced with the pre-emphasis current required for the driver output stage, while maintaining a minimum preemphasis pulse width and keeping all transistors in the forward active region. In one implementation Rp was set at 250 ohms and Cp at 3 pF. The value of Rp establishes the amplitude of the pre-emphasis current, while the RpCp time constant sets its duration. Overshoots of 6.8%-7.9% were simulated for various circuit fabrication processes for a 3 volt rising edge, and 4.8%-6.9% for the trailing falling edge. 
     The durations of the pin driver rising and falling edges are relatively long; a slew rate of 2.4V/1.7 nsec is typical. By contrast, the reaction time of the preemphasis circuit is quite rapid, typically a few hundred psec. This allows for an effective feedback circuit that adds timely overshoots to pin driver pulse edges. 
     Illustrations of the output voltage pulse that would be produced with the circuit described above, at the output node  8  (trace  38 ) and at the DUT  10  (trace  40 ), are shown in FIG. 5A for a rising pulse edge; the circuit&#39;s response to a falling pulse edge would be a mirror image. A distinct pre-emphasis spike  42  can be observed on top of the leading rising pulse edge at the pin driver output node  8 . This spike is substantially eliminated, through the effect of the transmission network, in the final pulse delivered to the DUT. The pulse at the DUT has a sharper corner than the pulse that would result in the absence of the invention&#39;s pre-emphasis. A similar improvement is produced in the trailing pulse edge. 
     FIG. 5B illustrates the positive current spike  44  through resistor R 4 , and the reduction in current flow  46  through resistor R 5  in response to a rising voltage edge at the input to the pre-emphasis circuit. The resulting current spike into the RpCp programming circuit is indicated by trace  48 . 
     FIG. 5C indicates the reflected currents in the first mirror leg that flow through upper transistor Q 8  (trace  50 ) and the lower transistor Q 9  (trace  52 ) in response to the currents  44  and  46  through the mirror drive leg, respectively. The difference between these currents is supplied to the pin driver output transistor Q 1  to add a positive voltage overshoot to a rising output pulse edge. The current magnitudes in the mirror legs, illustrated in FIG. 5C, are reduced from the current magnitudes in the drive leg, illustrated in FIG. 5B, due to the resistor ratioing of the mirror. The circuit&#39;s response to a falling pulse edge is complementary to its response, described above, to a rising edge. 
     A 3-dimensional plot of the overshoot percentage as a function of both Rp and Cp is presented in FIG.  6 . It can be seen that, for a constant capacitance value, the overshoot percentage increases with falling resistance. Since the programming capacitor Cp initially appears as a short circuit to an output pulse transient, the magnitude of the output pulse overshoot will be scaled inversely to the value of Rp. 
     While a particular embodiment of the invention has been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. For example, alternate ways to generate a pre-emphasis could be used, such as the phase controlled differential transistor pair shown in FIG.  7 . In this circuit, instead of the feedback circuit described above, a pair of differentially connected transistors Q 12  and Q 13  with respective load resistors R 12  and R 13  receive base inputs from the pin driver&#39;s output node  8 , with a current source I 3  pulling current from the transistors&#39; emitters. A programming circuit, consisting of resistor Rp′ connected between the bases of Q 12  and Q 13  and capacitor Cp′ connected between the base of Q 13  and ground, establishes a time constant for the voltage rise at the base of Q 13 . A differential current amplifier  54  amplifies the current differential between the collectors of Q 12  and Q 13 , and provides a current output to the base of Q 1 . The base voltage of Q 1  rises rapidly in response to a rapidly rising pulse edge at the pin driver output node  8 , while the base voltage of Q 13  rises more gradually because of the Rp′ Cp′ circuit. This produces a transient differential between the emitter currents for Q 12  and Q 13  that is amplified by amplifier  54  to yield a pre-emphasis current pulse for Q 1 . This current can be mirrored to provide a pre-emphasis for Q 2  also. Accordingly, it is intended that the invention be limited only in terms of the appended claims.