JP3109626B2 - Transmission line length measurement method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transmission line length measuring apparatus for measuring a transmission line length from a reflected waveform obtained by transmitting a pulse to a transmission line having an open end, and more particularly to an integrated circuit having a plurality of test pins. Measurement of transmission line length suitable for high-accuracy measurement of the transmission line length from the test pattern output / test result judgment circuit installed corresponding to these test pins to the input / output pins of the circuit under test in a test device, etc. About the method.

[0002]

2. Description of the Related Art In order to maintain test timing accuracy of an integrated circuit test apparatus, timing correction is required. In the timing correction, the transmission line length from the test pattern output / test result judgment circuit installed corresponding to the test pin of the integrated circuit test equipment to the input / output pin of the circuit under test varies between test pins. Correcting the timing error caused by this is one of the main processing items. In other words, it is necessary to measure the propagation delay time when propagating through the transmission line in one direction with high accuracy.

Conventionally, in such a measurement of the transmission line length, as shown in FIG. 5, for example, as shown in FIG. Of a staircase wave obtained by superimposing a reflected wave on a traveling wave when opening is opened, 1) a waveform of an edge a due to a traveling wave, 2) a waveform of an edge b due to a reflected wave, and 3) reflection from an edge due to a traveling wave The three elements with the time difference T to the edge due to the wave are measured, and the transfer function of the transmission line is determined from the three elements in the second block 3 and the third block 3. The fifth block 5 calculates the propagation delay time of the transmission line with high accuracy by analyzing the time delay of the output signal response. The law has been proposed.

According to this method, an increase in delay time due to waveform degradation determined by a frequency band limitation of a transmission line and a slew rate of an input signal can be measured separately for a traveling wave component and a reflected wave component. It is possible to accurately calculate the propagation delay time of only the forward path from the propagation delay time.

[0005]

However, the far end of the transmission line is constituted by an IC socket, a package, or the like, and a frequency band limiting factor such as a capacitance component due to such a structure is localized and reaches the far end. When the influence is almost the same as the frequency band limitation on the line, the measurement error increases for the following reason. In other words, the frequency band limitation that occurs between the incident end and the far end causes waveform deterioration in both the traveling wave and the reflected wave, in other words, the rising /
This causes an increase in the fall time, giving a greater delay time to the reflected wave. Accordingly, the propagation delay time of only the actual outward path is slightly smaller than the value obtained by simply halving the observed propagation delay time of the round trip path. On the other hand, the frequency band limitation occurring at the far end causes waveform deterioration only for the traveling wave, and has no effect on the reflection path. Therefore, the increase in the delay time due to the frequency band limitation at the far end contributes only to the propagation delay time in the forward path, so that the observed propagation delay time in the return path is simply reduced by half.
Conversely, the propagation delay time of only the actual forward path slightly increases with respect to the set value. As a specific example, the rise time is 200 ps in a lossless transmission line having a characteristic impedance of 50 ohms.
When a 5 pF capacitance is added as a frequency band limiting factor at the midpoint between the incident end and the far end when the pulse of (1) is applied, the propagation delay time of the reciprocating path obtained by the reflected wave measurement is simply halved. Propagation delay time of only the actual outward path is about 20p
s small. However, if a capacitance of 5 pF is added to the far end, the actual propagation delay time of only the forward path increases by about 10 ps. Therefore, unless the distribution of the frequency band limiting factor is grasped, it is difficult to accurately estimate the propagation delay time of only the outward path from the observed propagation delay time of the round trip path. However, conventionally, it has not been possible to separate the frequency band limiting factor occurring between the incident end and the far end from the frequency band limiting factor located at the far end.

An object of the present invention is to solve the conventional problems and to provide a method for measuring a transmission line length with high accuracy.

[0007]

In order to achieve such an object, a first aspect of the present invention is to transmit a pulse to a transmission line having a far open end and to measure a reflected waveform of the transmission line based on a reflected waveform observed. In a method of determining a transfer function and calculating a propagation delay time of the transmission line by analyzing a time delay of an output signal response to an individual input signal to measure a transmission line length, a structure that causes a frequency band limitation is used. For the transmission line associated with the far end, the first step of observing the reflected waveform on the transmission line excluding the structure, and the structure from the reflected waveform obtained in the first step A second step of determining the transfer function H1 of the excluded transmission line, a third step of observing the reflected waveform on the transmission line with the structure, and a reflected waveform obtained in the third step And the second Transfer functions H1 obtained in step
A fourth step of determining a transfer function H2 of a transmission line associated with the structure from the above, and calculating a time delay of an output signal response to an individual input signal using the transfer function H2 obtained in the fourth step A fifth step of measuring the propagation delay time of the transmission line associated with the far end by the structure causing the frequency band limitation.

[0008] A second embodiment of the present invention relates to the transfer function H1.
In determining H2 and H2, each of the structure causing the frequency band limitation and the transmission line excluding the structure is
It is characterized by approximation by cascade connection of a fixed delay time and a time constant component given by a product of a characteristic impedance of the transmission line and a capacitance component.

[0009]

According to the present invention, the structure causing the frequency band limitation is located at the far end with respect to the transmission line excluding the structure and the transmission line excluding the structure with respect to the transmission line having the structure. Observing the reflected waveform in the open state, the transfer function that can separately express the frequency band limiting factor generated between the incident end and the far end and the frequency band limiting factor localized at the far end using both observation data Then, the transmission line propagation delay time is obtained by using this transfer function.

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a flowchart showing one embodiment of a method for measuring a transmission line length according to the present invention.

One end point of the transmission line (hereinafter referred to as far end)
Is opened, and a pulse signal is applied to this transmission line from the other end point (hereinafter, referred to as the near end). When the output impedance at the near end matches the characteristic impedance of the transmission line, the state shown in FIG. Thus, at the near end, a step-like waveform in which the reflected wave is superimposed on the traveling wave is observed. Here, the incident wave v _INC , the transmitted wave v _TRANS , and the reflected wave v at the far end
_The following relationship holds for _REFL .

[0013]

## EQU1 ## v _REFL = v _TRANS −v _INC (1) When the structure A as a frequency band limiting factor such as a capacitance component is not attached to the far end, the transmitted wave v _TRANS is the total reflection of the incident wave v _INC. Because of this, the amplitude simply doubles.
Therefore, as shown in equation (2), the reflected wave v _REFL matches the incident wave v _INC .

[0014]

[Number 2] _{v REFL = 2v INC -v INC =} v INC ... (2) Therefore the incident end, i.e. the reflected wave has reached the observation end is equal to the response waveform incident wave has propagated through the transmission line twice. In other words, the response waveform is equal to a response waveform transmitted through a line in which the transmission lines are connected in two stages in series. This means that the transfer function of the forward path and the transfer function of the return path coincide with each other. For example, as described in the embodiment of Japanese Patent Application No. Hei. Measure the three components of the reflected waveform observed at the incident end with the end open, that is, the waveform and the incident waveform propagated on the round trip path, and the delay time from the incident wave to the reflected wave (corresponding to the first step 11 in FIG. 1). ), It is possible to obtain a transfer function only for the outward path (corresponding to the second step 12 in FIG. 1).

On the other hand, the relationship between the incident wave v _INC , the transmitted wave v _TRANS , and the reflected wave v _REFL is shown in FIG. 3 in a case where the capacitance component of the structure A as a frequency band limiting factor is loaded at the far end.
Shown in As shown in FIG. 3, the incident wave becomes blunt due to the capacitance component and becomes a transmitted wave. The reflected wave at the far end is affected by the waveform dullness. However, when the reflected wave propagates on the return path to the incident end, the capacitance component is naturally not affected at all. Therefore, since the transfer function of the forward path and the transfer function of the return path do not match, it is impossible to uniquely determine the transfer function of only the forward path from the reflected waveform observed at the incident end, that is, the waveform propagated on the return path. .

Since the reflected wave at the far end can be represented by the difference between the incident wave and the transmitted wave at the far end as in the equation (1), the reflected wave that has propagated back to the incident end further passes through the return path. The difference between the transmitted response waveform and the response waveform of the incident wave at the far end further propagated on the return path can be expressed. In the frequency space (s space), the transfer function of the single path of the transmission line excluding the far-end frequency band limiting factor is H ₁ (s), and the transfer function of the far-end frequency band limiting factor is H
₀ (s), the reflected wave response V _RFL at the incident end
Note that (s) can be expressed by the following equation, noting that the transmitted wave has twice the amplitude of the incident wave V _IN (s).

[0017]

Equation 3] _{V RFL (s) = (2H} 0 (s) -1) H 1 2 (s) V IN (s) ... (3) As mentioned earlier, excluding a frequency band limiting factor for the far-end Since the transfer function H ₁ (s) of a single line of the transmission line can be obtained from the transmission line excluding the frequency band limiting factor at the far end, a structure A as a frequency band limiting factor at the far end is further added. The reflection response of the transmission line in the open state at the far end is similarly measured (corresponding to the third step 13 in FIG. 1),
The transfer function H ₀ (s) of the frequency band limiting factor at the far end can be obtained by numerical analysis so that the measurement result satisfies the expression (3). Thus, a single-path transfer function H ₂ (s) of the transmission line accompanied by the structure A as a frequency band limiting factor at the far end is obtained, and the transmitted wave V at the far end is obtained.
_TRANS (s) is obtained as follows (corresponding to the fourth step 14 in FIG. 1).

[0018]

V _TRANS (s) = 2H ₁ (s) H ₀ (s) V _IN (s) = H ₂ (s) V _IN (s) (4) Equation (4) is transformed by, for example, Laplace inversion. If converted to time space, the time response v _TRANS (t) of the transmitted wave at the far end
Is calculated, and the propagation delay time for the incident wave, which is the final object, to propagate on the outward path can be calculated (corresponding to the fifth step 15 in FIG. 1).

For example, assuming a current connection state between an integrated circuit test apparatus and a device under test, a high-frequency component is attenuated by band limitation of a microstrip line, a fine coaxial cable, a connector, and the like that constitute a transmission line. Further, when an input terminal, a package, a socket, and the like of the device under test are present at the far end of the transmission line, the high-frequency component is attenuated by the time constant component determined by the capacitance component of the structure A and the characteristic impedance of the line. . This effect is a main cause of deterioration of the signal waveform obtained from the output end of the transmission line with respect to the input signal.

Therefore, for example, as in the embodiment of the approximate model of the transmission line, which is a component of the present invention shown in FIG.
The transmission line B excluding the structure A causing the frequency band limitation at the far end is replaced with a fixed delay time t, the characteristic impedance Z ₀ of the transmission line B and the transmission line B excluding the structure A.
Similarly, the structure A is similarly modeled by a cascade connection of a time constant component τ ₁ given by the product of the capacitance component C ₁ and the fixed delay time dt, the characteristic impedance Z ₀ of the transmission line B and the structure A Time constant component τ given by the product of the capacitance component C ₀ and
Modeling is performed by cascade connection with _0, and the transfer function of the outward path can be approximated as cascade connection of these models. Since all of the above models can be configured by cascade connection of the delay component and the time constant component, the voltage response in the time domain can be easily obtained using the Laplace transform / Laplace inverse transform. Therefore, by using the present model, it is possible to relatively easily realize a highly accurate measurement of the propagation delay time of the transmission line in consideration of the delay increase caused by the frequency band limiting factor associated with the structure A at the far end.

Here, assuming a measuring circuit such as an oscilloscope for actually observing a waveform, the measuring circuit has a small input terminal capacitance. In consideration of the influence, it is also possible to measure the input terminal capacitance in advance and add it to the above transmission line model. Specifically, for example, it may be described as a known time constant component given by the product of the characteristic impedance Z _{0 of the} line and the input terminal capacitance.

In practicing the present invention, various modifications other than the above-described approximate model of the transmission line are conceivable. For example, a transfer function is represented by a frequency polynomial by a fast Fourier transform from an observed reflected waveform. You may. In short, by observing the reflected waveforms on both the transmission line excluding the far-end frequency band limiting factor and the transmission line with this factor, the frequency that occurs in a distributed manner between the incident end and the far end The transfer function may be obtained in a form in which the band limiting factor and the frequency band limiting factor located at the far end are separated.

[0023]

As is apparent from the above description, according to the present invention, the increase in delay time due to waveform deterioration determined by the frequency characteristics of the transmission line and the frequency characteristics of the input signal is reduced between the incident end and the far end. Since the frequency band limiting factor occurring in a distributed manner and the frequency band limiting factor localized at the far end can be separated, the transmission line length can be measured with high accuracy even when the frequency band limitation at the far end is large.

[Brief description of the drawings]

FIG. 1 is a flowchart showing one embodiment of a transmission line length measuring method according to the present invention.

FIG. 2 is a schematic diagram showing a reflection waveform observed at the near end when a pulse is transmitted from the near end with the far end of the transmission line open.

FIG. 3 is a schematic diagram showing an incident wave component, a transmitted wave component, and a reflected wave component at the far end when a pulse is transmitted from the near end with the far end of the transmission line open.

FIG. 4 is an example of an approximate model of a forward path of a transmission line with a frequency band limiting factor attached to a far end, which is used when calculating a transmission line transfer function in the present invention.

FIG. 5 is a flowchart showing an example of a conventional transmission line length measuring method.

[Explanation of symbols]

 11 first step 12 second step 13 third step 14 fourth step 15 fifth step

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-269675 (JP, A) JP-A-2-16462 (JP, A) JP-A-2-98674 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G01R 31/28 G01R 27/04 G01R 27/28

Claims (2)

(57) [Claims] 1. A transfer function of a transmission line is determined from a reflection waveform observed by transmitting a pulse to a transmission line having an open end, and a time delay of an output signal response to an individual input signal is analyzed. In the method of calculating the propagation delay time of a transmission line and measuring the transmission line length, a structure causing a frequency band limitation is applied to a transmission line attached to a far end and to a transmission line excluding the structure. A first step of observing the reflection waveform; a second step of determining a transfer function H1 of the transmission line excluding the structure from the reflection waveform obtained in the first step; A third step of observing the reflection waveform with respect to the transmission line; and a transmission line associated with the structure from the reflection waveform obtained in the third step and the transfer function H1 obtained in the second step. A fourth step of determining the transfer function H2 of the following, and calculating the time delay of the output signal response to the individual input signal using the transfer function H2 obtained in the fourth step, thereby causing the frequency band limitation. A fifth step of measuring a propagation delay time of the transmission line associated with the far end portion of the structure. 2. When determining the transfer functions H1 and H2, each of the structure that causes the frequency band limitation and the transmission line excluding the structure is subjected to a fixed delay time, a characteristic impedance of the transmission line, 2. The method of measuring a transmission line length according to claim 1, wherein the transmission line length is approximated by cascade connection with a time constant component given by a product of the capacitance component.