With the rapid development of electronic technology, the application of embedded system is more and more extensive. In many applications, people no longer consider the function and performance, but reliability and compatibility. Print circuit board (PCB) is the basic support of circuit components and devices in electronic products. The design quality often directly affects the reliability and compatibility of embedded system. In the past, in some low-speed circuit boards, the clock frequency is only about 10 MHz, the main challenge of circuit board or packaging design is how to spread all the signal lines on the double-layer board and how to assemble without breaking the package. Since the interconnect does not affect system performance, the electrical characteristics of the interconnect are not important. In this sense the signal in the low speed circuit board interconnection is clear and transparent. However, with the development of the embedded system, the circuits used are basically high-frequency circuits. As the clock frequency increases, the rising edge of the signal also becomes shorter. The capacitive and inductive reactance generated by the printed circuit to the passing signal will be far greater than the resistance of the printed circuit itself, which seriously affects the integrity of the signal. For embedded systems, signal integrity effects become important when the clock frequency exceeds 100 MHz or the rise edge is less than 1 ns.
PCB manufacturer: improve signal integrity of embedded system PCB
In PCB, signal line is the main carrier of signal transmission. The routing of signal line will directly determine the superiority of signal transmission, thus directly affecting the performance of the whole system. Improper wiring will seriously cause a variety of signal integrity problems, circuit timing, noise and electromagnetic interference (EMI), will seriously affect the performance of the system. In this paper, starting from the actual electrical characteristics of signal lines in high-speed digital circuits, the electrical characteristics model is established to find out the main reasons affecting the signal integrity and the methods to solve the problems.
1 signal integrity
Signal integrity refers to the quality of the signal on the signal line, that is, the ability of the signal to respond with correct timing and voltage level in the circuit; signal with good signal integrity refers to the required voltage level value when needed. Poor signal integrity is not caused by a single factor, but by a combination of factors in the plate-level design. Signal integrity problems are reflected in many aspects, including delay, reflection, crosstalk, overshoot, oscillation, ground projectile, etc.
Delay: Delay means that the signal is transmitted at a limited speed on the transmission line of the PCB board, and the signal is sent from the sending end to the receiving end, where there is a transmission Delay. Signal delay will affect the timing of the system. Transmission delay mainly depends on the length of the wire and the dielectric constant of the surrounding medium. In high-speed digital system, the length of signal transmission line is the most direct factor affecting the phase difference of clock pulse. The phase difference of the clock pulse reduces the predictability of the arrival of the signal.
Reflection: Reflection is the Reflection of a signal on a signal line. When the signal delay time is much longer than the signal jump time, the signal line must be regarded as the transmission line. When the characteristic impedance of the transmission line does not match the load impedance, a portion of the signal power (voltage or current) is transmitted to the line and to the load, but a portion is reflected. If the load impedance is less than the original impedance, the reflection is negative; Conversely, reflection is positive. Such reflections can be caused by changes in the geometry of the wiring, incorrect wiring, transmission through the connector, and discontinuity of the power supply plane.
Crosstalk: Crosstalk is the coupling between two signal lines, and the mutual inductance and capacitance between the two signal lines cause the noise on the signal line. Capacitive coupling leads to coupling current, while inductive coupling leads to coupling voltage. Crosstalk noise results from the electromagnetic coupling between the signal line network, between the signal system and the power supply distribution system, and through the hole. String winding may cause false clock, intermittent data error and so on, and affect the transmission quality of adjacent signals. In reality, crosstalk cannot be eliminated completely, but it can be controlled within the tolerance of the system. The parameters of PCB layer, the spacing between signal lines, the electrical characteristics of the drive end and the receiver end, and the connection mode of the baseline end all have some influence on the crosstalk.
Overshoot and Undershoot: Overshoot is when the first peak or valley exceeds the set voltage. For the rising edge, it is the highest voltage. For the down edge, this is the lowest voltage. Overshoot is when the next valley or peak exceeds the set voltage. Excessive overshoot can cause the protection diode to work, resulting in its premature failure. Too much down can cause false clocks or data errors.
(Ringing) and surrounding oscillations (Rounding) : oscillation phenomenon is a recurring overshoot and undershoot. The oscillation of the signal is caused by the inductance and capacitance on the line, which belongs to the underdamped state, while the circumferential oscillation belongs to the overdamped state. Oscillations and wrap-around oscillations, like reflections, are caused by a variety of factors and can be reduced by proper termination, but cannot be completely eliminated.
Ground level rebound and reflux noise: when the circuit has bigger surge current can cause noise level rebound, such as a large amount of chip output at the same time open, there will be a larger transient current flow in the chip from the power source of the plate plane, chip packages and resistance and inductance of the power plane will cause the power supply noise, it will not produce in the real ground plane voltage fluctuation and change, the noise will affect the action of other components. The increase of load capacitance, the decrease of load resistance, the increase of ground inductance, and the increase of the number of switching devices will lead to the increase of ground projectile.
Analysis of electrical characteristics of transmission channel
In multilayer PCB, most transmission lines are not only arranged on a single layer, but also interlaced on multiple layers, which are connected through holes. Therefore, in a multilayer PCB, a typical transmission channel mainly includes three parts: transmission line, routing corner and through hole. At low frequencies, printed and wired through holes can be viewed as common electrical connections to different device pins, with little effect on signal quality. However, in the case of high frequency, printed line, corner and through hole can not only consider its connectivity, but also should consider the impact of its high frequency electrical characteristics and parasitic parameters.
2.1 electrical characteristics analysis of transmission lines in high-speed PCB
In high-speed PCB design, inevitably, to use a large amount of signal cables, and the length is differ, signal through wire delay time compared with the change of time signal itself can no longer be ignored, signal transmission, at the speed of electromagnetic waves on the cable wire at this time is a complex network with a resistor, capacitor, inductor, need to use distributed parameter system model to describe the transmission line model. A transmission line is used to transmit a signal from one end to the other, consisting of two wires of a certain length, one called a signal path and one called a return path. In low frequency circuits, the characteristics of transmission lines are expressed as pure resistance electrical characteristics. In high-speed PCB, with the increase of transmission signal frequency, the capacitive impedance between conductors decreases, and the inductive impedance on the conductors increases, the signal line will no longer only show pure resistance, that is, the signal will not only be transmitted on the conductors, but also in the medium between conductors. If the signal frequency increases further, when j L>>R, 1/(j C)<