1. Field of the Invention
The present invention relates to an equalizing circuit configured to equalize a signal.
2. Description of the Related Art
In order to test whether or not a semiconductor device operates normally after the semiconductor device is manufactured, a semiconductor test apparatus (which will also be referred to simply as the “test apparatus” hereafter) is employed. The test apparatus receives a signal (signal under test) output from a DUT (device under test), and compares the signal under test with an expected value, so as to judge the quality (Pass or Fail) of the DUT, or so as to measure the amplitude margin or the timing margin of the signal under test.
U.S. Pat. No. 6,937,054 B2 Specification
U.S. Pat. No. 7,394,331 B2 Specification
In general, a receiver circuit included in the test apparatus and the DUT are electrically connected to each other via a transmission line and a connector. The characteristic impedance Zo (e.g., 50Ω) of the transmission line or the connector is designed so as to provide impedance matching with a circuit block to be connected. Ideally, such an arrangement causes no waveform distortion due to signal transmission via the transmission line or the connector. However, in reality, it is impossible to provide such impedance matching over the entire pass band. Accordingly, such a transmission line or the like functions as an undesirable filter which causes waveform distortion. That is to say, the receiver circuit of the test apparatus receives a distorted waveform even if the waveform output from the DUT is satisfactory. This prevents the performance of the DUT itself from being measured.
By providing an equalizer circuit configured to compensate for the distortion of the signal under test as a component upstream of the receiver circuit (e.g., comparator) of the test apparatus, such an arrangement is capable of improving the waveform distortion of the signal under test due to the transmission line or the like. For example, Patent document 1 discloses an equalizer circuit monolithically integrated together with a differential amplifier. Also, Patent document 2 discloses a passive equalizer employing an LRC.
The present invention has been made in view of such a situation. Accordingly, it is an exemplary purpose of an embodiment of the present invention to provide a variable equalizer circuit which is capable of adjusting the equalization level using a new approach that differs from conventional approaches.
An embodiment of the present invention relates to a variable equalizer circuit configured to equalize a signal received via a transmission line from a device which is a communication partner device. The variable equalizer circuit comprises: an input terminal connected to the transmission line; an output terminal; a first resistor arranged between the output terminal and a fixed voltage terminal, and configured to have a variable resistance; a first capacitor arranged between the output terminal and the fixed voltage terminal, arranged in parallel with the first resistor, and configured to have a variable capacitance; a second resistor arranged between the input terminal and the output terminal; a second capacitor arranged between the input terminal and the output terminal, and arranged in parallel with the second resistor; and a shunt resistor arranged on a path including the first capacitor and the second capacitor between the input terminal and the fixed voltage terminal.
Another embodiment of the present invention also relates to a variable equalizer circuit configured to equalize a signal received via a transmission line from a device which is a communication partner device. The variable equalizer circuit comprises: an input terminal connected to the transmission line; an output terminal; a first capacitor arranged between the output terminal and a fixed voltage terminal, and configured to have a variable capacitance; a second resistor arranged between the input terminal and the output terminal; a second capacitor arranged between the input terminal and the output terminal, and arranged in parallel with the second resistor; a shunt resistor arranged on a path comprising the first capacitor and the second capacitor between the input terminal and the fixed voltage terminal; and a level shifter configured to shift the voltage level of the output terminal, and to have a variable resistance between the output terminal and the fixed voltage terminal.
Such an equalizing circuit according to any one of the aforementioned embodiments functions as a high-frequency emphasis filter configured to emphasize the high-frequency component of the input signal, and has an advantage of being capable of adjusting the amount of boost and the time constant. Furthermore, such an equalizing circuit can be integrated on a semiconductor chip. Such an arrangement uses no inductor, thereby providing an advantage of a small circuit area, and an advantage of involving no unintended oscillation.
Yet another embodiment of the present invention relates to a test apparatus configured to receive a signal from a device under test via a transmission line, and to test the device under test. The test apparatus comprises: a variable equalizer circuit according to any one of the aforementioned embodiments, configured to equalize a signal received from the device under test; and a receiver circuit configured to receive an output signal from the variable equalizer circuit.
Such an embodiment is capable of testing a signal output from the device under test after it corrects signal distortion that occurs due to the transmission line or the like.
It should be noted that any combination of the aforementioned components may be made, and any component of the present invention or any manifestation thereof may be mutually substituted between a method, apparatus, and so forth, which are effective as an embodiment of the present invention.
Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which:
Description will be made below regarding preferred embodiments according to the present invention with reference to the drawings. The same or similar components, members, and processes are denoted by the same reference numerals, and redundant description thereof will be omitted as appropriate. The embodiments have been described for exemplary purposes only, and are by no means intended to restrict the present invention. Also, it is not necessarily essential for the present invention that all the features or a combination thereof be provided as described in the embodiments.
In the present specification, a state represented by the phrase “the member A is connected to the member B” includes a state in which the member A is indirectly connected to the member B via another member that does not affect the electric connection therebetween, in addition to a state in which the member A is physically and directly connected to the member B. Similarly, a state represented by the phrase “the member C is provided between the member A and the member B” includes a state in which the member A is indirectly connected to the member C, or the member B is indirectly connected to the member C via another member that does not affect the electric connection therebetween, in addition to a state in which the member A is directly connected to the member C, or the member B is directly connected to the member C.
The test apparatus 2 is connected to a DUT 1 via a transmission line 3. The test apparatus 2 judges the quality of the DUT 1 or identifies the defective portions based upon a signal output from the DUT 1. The DUT 1 includes a driver Dr and an output resistor Ru. The driver Dr1 applies a signal under test Vu to one terminal of the transmission line 3.
A terminator 6 includes a terminal driver Dr2 and a terminal resistor Rd. The terminal driver Dr2 applies a terminal voltage Vd to the other terminal of the transmission line 3 via the terminal resistor Rd. The terminator 6 may function as a transmitter circuit (driver) configured to output a signal to the DUT 1.
A receiver circuit 8 receives a signal under test Vu output from the DUT 1. For example, the receiver circuit is configured as a comparator or a buffer. The test apparatus 2 compares the signal under test thus received by the receiver circuit 8 with an expected value so as to judge the quality of the DUT 1. Alternatively, the test apparatus 2 measures the amplitude margin or the timing margin of the signal under test.
With such a test system, waveform distortion occurs in the signal under test output from the DUT 1 when it passes through the transmission line 3, an unshown connector, or the like (which will be referred to as the “transmission line” or the like hereafter). In order to compensate for such waveform distortion, the test apparatus 2 includes a variable equalizer circuit 100 arranged as an upstream component of the receiver circuit 8.
Description will be made regarding a specific configuration of the variable equalizer circuit 100.
The variable equalizer circuit 100 equalizes a signal Va input via an input terminal P1 thereof from the DUT 1 which is a communication partner device, at the same time attenuates this signal, and outputs the signal thus processed to the receiver circuit 8 via an output terminal P2.
The variable equalizer circuit 100 includes an equalizing unit 10 and a level shifter 20.
The equalizing unit 10 includes a first resistor R1, a second resistor R2, a first capacitor C1, a second capacitor C2, and at least one shunt resistor Rs.
The first resistor R1 is configured as a variable resistor, the resistance value of which is changeable. The first resistor R1 is arranged between the output terminal P2 and a fixed voltage terminal (ground terminal). The first capacitor C1 is configured as a variable capacitor, the capacitance of which is changeable. The first capacitor C1 is arranged between the output terminal P2 and the ground terminal, in parallel with the first resistor R1. The second resistor R2 is arranged between the input terminal P1 and the output terminal P2. The second capacitor C2 is arranged between the input terminal P1 and the output terminal P2, in parallel with the second resistor R2.
At least one shunt resistor Rs is arranged on a path including the first capacitor C1 and the second capacitor C2 between the input terminal P1 and the ground terminal.
The third resistor R3 is arranged between the input terminal P1 and a connection node (N1) that connects one terminal of the second resistor R2 and one terminal of the second capacitor C2. The third resistor R3 has resistance that is sufficiently greater than the characteristic impedance (50Ω) of the transmission line 3. For example, the third resistor R3 is preferably configured to have a resistance on the order of five to ten times the characteristic impedance of the transmission line 3. By setting the resistance of the third resistor R3 to be greater than the characteristic impedance of the transmission line 3, such an arrangement reduces the effects of the variable equalizer circuit 100 on the impedance matching between the terminator 6 and the DUT 1.
The fourth resistor Rc is arranged on a path in parallel with the first resistor R1, and in series with the first capacitor C1.
It should be noted that N-channel MOSFETs only or P-channel MOSFETs only may be employed, depending upon the relation between the electric potential at the first terminal P31 and the electric potential at the second terminal P32.
It should be noted that the configurations of the variable resistor and the variable capacitor are not restricted to such arrangements shown in
Returning to
Assuming that the level shifter shown in
R
SH
=R
SH1
//R
SH2
V
SH=(vdd·RSH2+vss·RSH1)/(RSH1+RSH2) (A1)
Here, “R1//R2” represents an operator that expresses the combined impedance of the resistors R1 and R2 arranged in parallel.
By solving Expression (A1) for RSH1 and RSH2, the following Expression (A2) is obtained.
R
SH1
=R
SH·(vdd−vss)/(VSH−vss)
R
SH2
=R
SH·(vdd−vss)/(Vdd−VSH)
The first variable resistor RSH1 and the second variable resistor RSH2 preferably have a configuration in which the multiple switches SW are arranged on one fixed voltage terminal Pvdd side and a configuration in which the multiple switches SW are arranged on the other fixed voltage terminal Pvss side, respectively. Each switch SW has a parasitic capacitance (not shown). However, by arranging the switches SW on the fixed voltage terminal side, such an arrangement reduces the parasitic capacitance that occurs at the output terminal P2. As a result, such an arrangement reduces the effects of such parasitic capacitance on a signal transmitted via a node to which the output terminal P2 is connected.
The above is the configuration of the variable equalizer circuit 100. Next, returning to
First, the DUT 1 outputs a signal under test to the test apparatus 2, and the signal under test thus output is input to the input terminal P1 of the variable equalizer circuit 100 shown in
A combination of the second resistor R2 and the second capacitor C2 functions as a peaking filter for the signal Va input to the input terminal P1. The capacitance C2 of the second capacitor C2 is determined so as to provide overcompensation.
Furthermore, the first resistor R1 is configured as a variable resistor and the first capacitor C1 is configured as a variable capacitor. By adjusting the first resistor R1 and the first capacitor C1, such an arrangement has a function of adjusting the overall characteristics of the variable equalizer circuit 100. Specifically, the first capacitor C1 having a capacitance C1 suppresses overcompensation provided by the second capacitor C2. Here, the capacitances of the first capacitor C1 and the second capacitor C2 are set such that the relation C2>C1 holds true. Furthermore, the amount of boost by the equalizer can be controlled using the resistance of the first resistor R1.
With such a test system shown in
The equalizing unit 10 equalizes the signal input to the input terminal P1, and at the same time attenuates this signal. The level shifter 20 shifts the level of the output signal of the equalizing unit 10, and outputs the resulting signal to the receiver circuit 8.
The above is the operation of the variable equalizer circuit 100. The advantage of the variable equalizer circuit 100 can be clearly understood in comparison with conventional techniques.
With such a variable equalizer circuit 300 shown in
That is to say, such parasitic capacitances counteract the desired functions of the equalizer circuit. This means that there is a reduction in the response speed of the circuit.
In contrast, with the variable equalizer circuit 100 shown in
In addition to such an advantage, the variable equalizer circuit 100 has the following advantages.
The variable equalizer circuit 100 is capable of changing the amount of boost and the time constant by adjusting the first capacitor C1 and the first resistor R1.
Furthermore, the variable equalizer circuit 100 includes resistors, capacitors, and transistors. This means that the variable equalizer circuit 100 has a configuration suitable for integration on a semiconductor chip. Furthermore, the variable equalizer circuit 100 includes no inductor. Thus, such an arrangement provides an advantage of a reduced circuit area and an advantage that unintended oscillation does not occur.
Furthermore, the variable equalizer circuit 100 attenuates the signal at the same time as the equalizing operation. Accordingly, such an arrangement reduces the voltage level to be input to the receiver circuit 8. Thus, such an arrangement allows the receiver circuit 8 to be configured using high-speed and low-voltage transistors. Thus, such an arrangement is capable of receiving a high-speed signal.
Furthermore, by providing the third resistor R3, such an arrangement reduces the effects of the variable equalizer circuit 100 on the impedance matching between the terminator 6 and the DUT 1. Furthermore, by providing the fourth resistor Rc, such an arrangement provides improved bandwidth characteristics.
Next, description will be made regarding a qualitative analysis of the variable equalizer circuit 100.
Here it is supposed that impedance matching is provided between the output resistor Ru of the DUT 1, the terminal resistor Rd of the terminator 6, and the characteristic impedance Zo of the transmission line 3. In this case, the impedance at the node N2 is represented by Zo/2.
Furthermore, the resistance of the third resistor R3 is sufficiently higher than the characteristic impedance Zo as described above. Accordingly, it can be assumed that the effects of the variable equalizer circuit 100 on the impedance matching between the terminator 6 and the DUT 1 are negligible.
R1 represents the resistance of the first resistor R1, R2 represents the resistance of the second resistor R2, R3 represents the resistance of the third resistor R3, Rc represents the resistance of the fourth resistor Rc, C1 represents the capacitance of the first capacitor C1, and C2 represents the capacitance of the second capacitor C2.
First, the following Expression (1) is obtained using Kirchhoff's current law.
i(t)=iR2(t)+iC2(t)=iSH(t)+iR1(t)+iC1(t) (1)
Each current is obtained as represented by Expressions (2) through (6). Here, G1=1/R1, G2=1/R2, G3=1/R3, and GSH=1/RSH. It should be noted that ici is separately calculated.
By generating the Laplace transform of the Expressions (1) through (6), the following Expressions (1)′ through (6)′ are obtained.
Next, directing attention to the relation between the iC1(t) and vc(t), the following Expression (7) is obtained. By generating the Laplace transform of Expression (7), Expression (7)′ is obtained. Furthermore, Expression (7)′ is solved with respect to IC1(s) by removing VP(s), thereby obtaining the following Expression (8).
By substituting Expressions (2)′ through (6)′ and (8) into Expression (1)′, Expression (9) is obtained.
The following Expression (10) is obtained from the left side and the middle of Expression (9).
Here, defining vA(t) to be a step function represented by Expression (11), the Laplace transform of vA(t) is represented by Expression (12). It should be noted that the value of VA1 does not appear in Expression (12). However, the information with respect to the initial state is included in Vc(0−) in expression (10), and accordingly, this poses no difficulty in the downstream calculation step. Furthermore, assuming that the circuit is static at the time point t<0, the following Expression (13) holds true.
Expression (10) and Expression (12) are substituted into the left side of Expression (9), and Expression (13) is substituted into the right side of Expression (9), thereby obtaining Expression (14). Furthermore, Expression (14) is transformed so as to provide the following Expression (15).
The coefficients A, T, U, P, and Q in Expression (15) are represented by the following Expressions (15-1) through (15-5).
Assuming that partial fraction decomposition of Expression (15) can be done as represented by Expression (16), α, β, γ, ω1, and ω2 are calculated. If α, β, γ, ω1, and ω2 are all real numbers, the inverse Laplace transform of Expression (16) can be calculated, thereby obtaining the response Vc(t) on the time domain. The reason why such a partial fraction decomposition can be done as represented by Expression (16) is that the variable equalizer circuit 100 shown in
Expression (15) must be identically equivalent to Expression (17). Thus, by making a comparison of each term between these Expressions, the following Expressions (18-1) through (18-5) are obtained.
A=γ+α+β (18-1)
T=γ·(ω1+ω2)+α·ω2+β ω1 (18-2)
U=γ·ω
1·ω2 (18-3)
P=ω1+ω2 (18-4)
Q=ω
1ω2 (18-5)
By solving the Expressions (18-1) through (18-5), the following Expressions (19-1) through (19-5) are obtained.
The inverse Laplace transform of Expression (16) is calculated, thereby obtaining the following Expression (20).
Expression (20) is defined only in the range 0<t. In the range t<0, assuming that the circuit is in the static state, vc(0−) is calculated.
The response waveform represented by Expression (20) is attached to the response waveform represented by Expression (21) at the time point t=0, thereby obtaining the response waveform vc(t) which represents the response that occurs when a step input represented by Expression (11) is applied.
Next, the attenuation rate is calculated.
Assuming that vA(t) is represented by a step function as represented by Expression (11), the circuit is in the static state at the time point t=∞. Thus, vc(∞) can be calculated as represented by the following Expression (22) based upon the equivalent circuit shown in
R2=1.75 kΩ
R3=250 Ω
Rc=2 kΩ
C1=60 fF
C2=300 fF
It can be understood that the amount of boost can be controlled mainly by changing the resistance value R1 of the first resistor R1.
The above-described embodiment has been described for exemplary purposes only, and is by no means intended to be interpreted restrictively. Rather, it can be readily conceived by those skilled in this art that various modifications may be made by making various combinations of the aforementioned components or processes, which are also encompassed in the technical scope of the present invention. Description will be made below regarding such modifications.
A third modification has a configuration obtained by eliminating the third resistor R3 from the configuration of the variable equalizer circuit 100 shown in
Some modifications described above may be combined with each other.
For example, the first modification may be combined with at least one from the second and third modifications.
For example, the second modification may be combined with at least one from the first, third, and fourth modifications.
For example, the third modification may be combined with at least one from the first, second, and fourth modifications.
For example, the fourth modification may be combined with at least one from the second and third modifications.
Various combinations and various modifications may be made without harming the advantages of the present invention, which are readily conceived by those skilled in this art.
Description has been made in the aforementioned embodiments regarding an arrangement in which the variable equalizer circuit 100 is employed in the test apparatus 2. However, an application of the variable equalizer circuit 100 is not restricted to such an arrangement. Rather, such a variable equalizer circuit can be applied to various kinds of devices configured to receive a signal from an external circuit.
Description has been made regarding the present invention with reference to the embodiments. However, the above-described embodiments show only the mechanisms and applications of the present invention for exemplary purposes only, and are by no means intended to be interpreted restrictively. Rather, various modifications and various changes in the layout can be made without departing from the spirit and scope of the present invention defined in appended claims.
This application is the U.S. National Stage of International Patent Application No. PCT/JP2010/002357 filed on Mar. 31, 2010, the disclosure of which is hereby incorporated by reference in its entirety.
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/JP2010/002357 | 3/31/2010 | WO | 00 | 10/26/2011 |