Transmission line impedance design
With more and more high-speed electronic systems, high-speed electronic signal pcb of the problems caused by the alignment of more and more: for example, be a big signal on and overshoot, resulting in unwanted oscillations, ringing and interference. These will seriously affect the reliability of electronic equipment, and even make electronic device does not work. Therefore, the transmission line impedance control PCB design as a key and difficult.
Transmission line is a reference to one side or both sides of a special plane with distributed parameter wire. Ideal transmission line can be simplified as shown below lumped circuit:
Where L is a unit length inductance, C is capacitance per unit length. The actual transmission lines are lossy, the equivalent lumped parameter circuit is as follows:
Where R is the resistance per unit length, and parallel resistance due to relatively large resistance, the general said that the conductance G, is the reciprocal of resistance. Resistors and inductors in series with PCB trace resistance can be calculated
Transmission line is one of the most important parameters of characteristic impedance, which is defined as
Another parameter is the transmission delay
By transmission line structure can be divided into microstrip, stripline and asymmetric stripline, etc., you can see the link structure in the introduction.
Here are some typical impedance values: (Note: 1oz = 0.035 mm, where W, H, T refer to the meaning of the definition of microstrip line)
So in a product should be what kind of transmission line with a better arrangement, microstrip or stripline (or asymmetrical stripline). Should be based on these types of transmission line characteristics to determine, should pay attention to following aspects:
First, whether the process needs to meet the PCB? For example, the minimum line width and so on PCB's thickness, the ribbon cable in the same line width of the minimum thickness of the case, this time is likely to exceed the minimum width requirements PCB mill.
Second, anti-jamming capability requirements. Transmission lines may interfere with other circuits or other circuits may be interference. Such as PA to the antenna in the phone may interfere with the transmission line to the audio circuit to produce a 217Hz the TDMA noise, or the transmission line will be the local clock circuit of high harmonics from the fallout of cell phone receiver sensitivity in a lower number of channels. As the ribbon cable buried in the ground plane between the two, so the best EMI effect, that we should choose the ribbon cable selection (or asymmetric stripline).
3, transmission loss. Under the same conditions as the characteristic impedance stripline traces narrow, and thus may be more wear and tear. (Microstrip line due to take the course, the equivalent parallel resistor width smaller, generally the equivalent parallel resistance of the impact on the transmission loss will be less than the equivalent series resistance implications, but it depends on the width and the PCB materials )