A microprocessor is connected to one end of a transmission line. The transmission line has a characteristic impedance of 100 ?. The transmission line is driven by the microprocessor at the near end which has an impedance of 50 ?. The line is terminated by 300 ? at the far end.

Question

a. If a step voltage of 5V is applied by the microprocessor, what is the magnitude of the pulse that initially
propagates along the bus?
b. What is the coefficient of reflection at each end of the bus?
c. What is the voltage level at the far end of the bus after time tpd, where tpd is the end?to?end propagation
delay of the bus? That is, calculate the voltage at the far end of the bus as the initial pulse reaches the end.

Answer 1

a. The initial step is divided between a potential divider formed from the line’s impedance and the source
resistance. That is, the pulse is
5 × 100/(100 + 50) = 5 × 2/3 = 3.3333V

b. The reflection coefficient is given by (Rl ? Z0)/(Rl + Z0). The source reflection coefficient is
(50 ? 100)/(50 + 100) = ?50/150 = ?0.3333.
The end reflection coefficient is (300 ? 100)/(300 + 100) = 200/400 = 0.5

c. When the 3.333V pulse reaches the far end, part of it is reflected. Since the reflection coefficient is 0.5, the
reflected component is 3.3333 × 0.5 and the total voltage is 3.3333 + 3.3333 × 0.5 = 3.3333 + 1.6666 = 4.9999.