1.  Field of the Invention
The present invention relates to a printed circuit board including a differential line for transmitting a high-speed digital signal. 
2.  Description of the Related Art
Up to now, a differential line is used to transmit a high-speed digital signal.  The differential line can reduce a radiation noise caused from the high-speed digital signal.  In order to suppress the reflection of signals on a printed circuit board, there are cases where a circuit termination of transmission side and a circuit termination of reception side are provided on a transmission line for connecting an output circuit with an input circuit.  The circuit termination of transmission side is located near the output circuit and the circuit termination of reception side is located near the input circuit.  However, complete impedance matching is not obtained between a connection point of the circuit termination of transmission side and a connection point of the circuit termination of reception side on the differential line, so that a slight number of reflected waves are generated.  In a high-speed circuit, the slight number of reflected waves are superimposed on a signal to generate noises, thereby reducing the quality of a signal waveform.  A reduction in quality of the signal waveform causes a bit error of the input circuit. 
Japanese Patent Application Laid-Open No.  2001-111408 discloses that a round trip transmission time of a signal transmitted between the connection point of the circuit termination of transmission side and the connection point of the circuit termination of reception side is set so as to become an integral multiple of a switching period of the signal.  Therefore, the amount of jitter caused by reflection at a mismatching point is reduced. 
In recent years, an increase in frequency of a digital signal has been accelerated, so that the influence of an electrical mismatching point of the differential line on the quality of a signal becomes significantly larger.  That is, each of the rise and fall times (Trf) of the signal becomes shorter with the increase in frequency of the digital signal.  In other words, a higher-order harmonic signal is used, so that a generated noise becomes larger.  Here, the rise and fall times (Trf) of the signal in the present invention is defined to be a time required for changing an amplitude of the signal from 20% to 80% and from 80% to 20%, respectively. 
A problem of the mismatching point disclosed by Japanese Patent Application Laid-Open No.  2001-111408 is caused by impedance at the connection point of the circuit termination of transmission side and an impedance at the connection point of the circuit termination of reception side.  However, when a frequency becomes equal to or larger than 1 GHz, the signal quality is influenced by changes in impedances not only at the connection point of the circuit termination of transmission side and the connection point of the circuit termination of transmission side but also at points which exist in all positions on the transmission line and which has been ignorable up to now, such as a connection pad for mounting ICs, a via hole (VIA), and a connector.  In particular, when there is a point whose impedance is varied by a value equal to or larger than ±10% of an impedance of a main line of the printed circuit board, the signal quality is influenced at the point, thereby increasing the risk of bit error of a circuit. 
In general, when waveform quality on high-speed transmitting is to be evaluated, an eye pattern is used.  The eye pattern to be displayed is obtained by the superimposition of a digital signal for each unit interval (UI) with the abscissa indicating time and the ordinate indicating voltage.  The waveform quality is determined by whether or not the eye pattern meets a voltage axis standard and a time axis standard of a hexagonal or rhombic standard value (i.e., mask pattern) of a transmitting system.  That is, when the signal is not overlapped with the mask pattern, it is determined that the signal has an excellent signal waveform.  On the other hand, when the signal is overlapped with the mask pattern, it is determined that the signal has a defective signal waveform.  Here, one unit interval (UI) means a signal period of a digital signal, that is, a minimum time interval necessary for switching between 0 and 1. 
An evaluation method using the eye pattern will be described in detail with reference to FIGS.  11A to 11D. FIG.  11A shows a waveform of a periodic differential transmission signal (1→0→1→0→1→0→).  FIG.  11B shows an eye pattern of this differential transmission signal.  Reference numeral 10 denotes the eye pattern and 11 denotes the mask pattern for determining the quality of the eye pattern 10.  An actual differential transmission signal is not necessarily the signal shown in FIG.  11A in which 0 and 1 are alternated.  That is, as shown in FIG.  11C, a signal in which values of 0 or 1 are successive (1→0→1→0→1→0→0→1→0→0) is used in many cases.  Even when such a signal is used, the superimposition of the waveform is performed for each UI similarly as in the case using the signal shown FIG.  11B to determine the quality of the signal. 
FIG.  11D shows an eye pattern in the case where a digital signal of approximately 2 Gbit/sec is transmitted on a differential line including connection pads and via holes (VIA) which become the mismatching points.  As is apparent from FIG.  11D, the signal waveform is overlapped with the mask pattern, so that it is likely to cause a bit error of a circuit.