Why single ended trace require impedance of 50 Ohm?

Three factors which is highly influence PCB trace impedance calculation

1.crosstalk: dramatically various with nearest ref plane.Cutting height by half reduces the crosstalk by 4 times.

2.The height of the trace above the nearest ground plane: It should be minimum. Less height means less radiation.

3.Less height means less radiation..it 'll reduce the impedance: which is less susceptible to capacitive loading.

• As per this three consideration PCB need to be thin as possible as to reduce distance between reference plan and signal plan.So we can not compromise with height of trace.What stops you from pressing height of trace down to zero is the fact that most chips can not comfortably drive impedance less than about 50 Ohm.

  EXCEPTIONAL: The old National BTL family drives 17 Ohm 

                             Rambus which drives 27 Ohm  

• Now impedance is inversely proportional to cross sectional area of trace.We have another parameter width of trace to maintain impedance of trace which is also not possible because all high speed boards are highly dense board.

• It not wise to use always 50 Ohm trace because an application like NMOS 8080, which works on low operating frequency(ex. 100 kHz) doesnt have Electromagnetic Interference and that type of application can't drive 50 Ohm.For this kind of processor you should use thinest and heighest impedance line to minimize operating power. 

• As  per mechanical view increasing impedance require decreasing in width of trace.The tiny lithography that high impedance trace require becomes difficult to fabricate.Where as 50 Ohm trace gives much wider trace to be manufacture.

As per above conditions we should use 50 Ohm impedance of single ended trace. Other differential pair traces have impedance other than 50 Ohm like USB having impedance of 90 Ohm!!!. 

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