| 摘要 |
The fabrication of a printed circuit board (26) usually includes the step of testing the board with a testing machine (24) to verify operability. The testing machine accomplishes such testing by transmitting test signals to the board via a transmission line (36) and then analyzing each response signals returned from the board in response to the test signals. To reduce the incidence of error, the testing machine (24) is compensated for the propagation delay of the line (12) which is measured by launching a first string of pulses into one end of the line whose opposite end is left open. A second string of pulses is sinultaneously launched into a programmable delay line (16) which delays each second pulse by an adjustable interval. After the generation of each first and second pulse, a check is made whether the first pulse has been reflected back to the first end of the transmission line at the same time the second pulse reaches the output of the delay line. If the pulses are not coincident, the delay setting of the delay line is increased and then a check is again made whether one of the first and second pulses are coincident. The delay of the delay line is successively increased until the pulse are coincident whereupon the delay of the delay line now equals twice the delay of the transmission line.
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