Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2000-05-25
2003-09-23
Cuneo, Kamand (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S754090, C324S758010
Reexamination Certificate
active
06624650
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to a device that automatically measures a characteristic impedance of a printed wiring circuit on a printed wiring board mainly using a coaxial probe.
BACKGROUND OF THE INVENTION
A CPU clock frequency and an operation clock for CPU interaction with external devices recently has been remarkably increased so that a processing speed of a computer or the like may be increased. With the speed increase of the operation clock, the performance requirement for the printed wiring board where a data signal runs also become demanding. By way of example, there is a requirement that the characteristic impedance between printed wiring circuits formed by signal conductors on the printed wiring board need to be matched and that the characteristic impedance and the impedance of the circuit load need to be matched. Without the match, a signal is partially reflected back to a signal source such that the signal transmitted to the load becomes so weak that the data may not be transmitted completely. This problem becomes more serious with an increase of the frequency.
Thus, to maintain the quality of the printed wiring board, it will be essential to measure the characteristic impedance of the printed wiring circuit. Conventionally, the characteristic impedance of the printed wiring circuit is measured manually using a coaxial probe such as illustrated in
FIG. 8. A
coaxial probe has a center probe (
201
) and a cylindrical plunger with a protrusion (
202
) with a sharp tip as a part of the edge, which is provided to surround the center probe (
201
). It is briefly explained how to use a coaxial probe below. A through-hole that is connected to ground is usually provided around the end of a signal conductor on the printed wiring board that handles high frequency. The impedance of the coaxial probe is set to match the designed impedance of the measured printed circuit. First, the center probe (
201
) is brought in contact with the end of the signal conductor, and the protrusion (
202
) is brought in contact with the through-hole. Then, a predetermined high frequency signal is sent from the center probe (
201
), and the reflected wave signal from the circuit is received by the center probe (
201
). Now, the protrusion (
202
) is connected to ground. As the impedance of the coaxial probe and the impedance of the printed wiring circuit are designed to be equal, there should be no reflected wave signal if the circuit was manufactured according to the design. However, if there is deviation in the impedance of the printed wiring circuit from the designed one, the reflected wave is reflected at the part with the deviated impedance. Thus, if the period of time from when the high frequency signal is sent to when the reflected wave returns and the strength or amplitude of the reflected wave are measured, it is determined what level of the impedance deviation is present and at which part.
OBJECT OF THE INVENTION
The operation clocks of equipment connected to computers are expected to increase in speed further in the future. Along with this trend, the need to measure the characteristic impedance to assure the quality of the printed wiring board will increase significantly. Then, the conventional method of manually measuring the impedance of the printed wiring circuit using a coaxial probe may not be good enough. It will be necessary to automate this process. Meanwhile, as a testing device for the printed wiring board, a device with a contact stylus automatically and two-dimensionally moved by a program to be in contact with the testing point of the printed wiring board to conduct a test for a line cut, etc. (hereafter called “an automatic printed wiring board testing device”) has been used conventionally. Thus, it is efficient and desirable to move a coaxial probe by means of this automatic printed wiring board testing device to test the testing point on the printed wiring board.
However, if a coaxial probe is fixed to the operating part of automatic printed wiring board testing device, the protrusion (
202
) may sometimes not be in contact with the through-hole when the center probe (
201
) is placed in contact with the edge of the signal conductor since the positional relationship between center probe (
201
) and the protrusion (
202
) will not be changed and since the position of a through-hole provided on the printed wiring board around the signal conductor is not fixed.
Besides, several different distances between the edge of the signal conductor and the through-hole may be used on the printed wiring board while the distance between the center probe (
201
) and the protrusion (
202
) of the coaxial probe is fixed. Thus, they may not match all the time. In some cases, when the center probe (
201
) is brought into contact with the edge of signal conductor, the protrusion (
202
) may not be in contact with the through-hole. Therefore, a good measurement may not be performed simply by applying the conventional coaxial probe to the automatic printed wiring board testing device.
The object of the present invention is to provide an impedance measuring device for the automatic measurement of the characteristic impedance of the printed wiring circuit on the printed wiring board principally by loading a coaxial probe in the automatic printed wiring board testing device.
SUMMARY OF THE INVENTION
The present invention seeks to solve the problem so as to provide an impedance measuring device for the printed wiring board that has a test probe unit such as a coaxial probe that includes a first and a second contact styli whose top tips point to the same direction wherein the second contact stylus is placed with a certain distance from the first contact stylus and a test probe driving means for moving the test probe unit to any place over the printed wiring board to be measured in order to make the first and the second contact styli contact certain measuring points on the printed wiring board, wherein the impedance measuring device further comprises a second contact stylus turning means for turning said second contact probe around said first contact stylus.
With this constitution, even when the direction is varied in which the through-hole is provided relative to the edge of signal conductor on the board to be tested, the second contact stylus turning means can turn the second contact stylus to match the direction of the second contact stylus in relation to the first contact stylus toward the direction of the through-hole in relation to the edge of the signal conductor. In this manner the first contact stylus may be brought into contact with the edge of the signal conductor, and at the same time the second contact stylus may be brought into contact with the through-hole so that the impedance of the signal conductor may be measured. Now, the impedance measuring device may be employed to measure the impedance with two styli contacted to two measuring points, although the impedance measuring of the signal conductor is shown as an example that is, the area of use of the device is not limited to the impedance measurement of signal conductors. It can be widely applied to any use wherein contact styli are placed in contact with two points to be measured. In this manner, with the impedance measuring device in accordance with the invention, the measurement can be made even when the direction of a point from the other is varied where the two points are provided for measurement. Therefore, the device can contribute to the realization of an automatic impedance measurement that uses a test probe unit such as a coaxial probe.
The second contact stylus turning means preferably turns the second contact stylus by turning the whole test probe unit around the first contact stylus as the central axis of turning. That is, if only the second contact stylus is designed to turn, a mechanism that enables the second contact stylus to turn independently form the first contact stylus movement needs to be provided in the vicinity of the top end of the test probe unit so that the mechanism must be small. On
Hollington Jermele
Kilpatrick & Stockton LLP
Microcraft KK
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