Answers

1. C
2. B
3. B
4. A
5. D
6. C
7. A
8. C
9. B
10. B
11. D
12. A
13. C
14. D
15. B
16. C
17. C
18. C
19. A
20. B
21. B
22. C
23. C
24. C

Explanations

Q1. The general rule for the calculating the Bandwidth of signal is with rise time Tr is

Band Width = 0.35/Tr

Q2. If an oscilloscope has a rise time ( = 0.35/ band_width) of Tr1 and the signal has a actual rise time of Tr2, the observed bandwidth is given by

Tr = v(Tr12+Tr22)
Or,
Tr2 = v(Tr2-Tr12)
Here Tr = 60 ps

Tr1 = 0.35 / (10 GHz) = 35 ps

Substituting these values, we get

Tr2 = 48.73 ps

Q3. The impedance of the capacitor will given by the absolute value of

Z = 1/(2pfC) - 2pfL

For the first capacitor,

f = 10 MHz
C=10uF
L=2nH


Substituting these values,

Z = 0.124 Ohms

For the second capacitor

f = 10 MHz
C=4.7uF
L=0.5nH
Substituting these values,

Z = 0.028 Ohms or 28 milli Ohms

Obviously the second capacitor is a better choice for 10 MHz frequency .

Q4. The test pad acts as an additional capacitance. This decreases the trace impedance.

Q5. Even the capacitors with low ESR loose their effectiveness at frequency as high as 1 GHz because of the finite inductance of its leads and because of the finite inductance of the trace connecting the capacitor to power and ground plane. The parallel plate capacitance formed by a power and ground plane works at highest frequencies.

Q6. If a series termination as well as the end parallel termination is used the chances of reflections are greatly reduced. First the signal will get terminated at the receiver. If because of some mismatch, there is a signal reflection, it will travel back and will get absorbed at the matched series source termination. The scheme will however, reduce the signal amplitude at the receiver.

Q7. The sum of the driver source resistance and the external source resistance should match the characteristic impedance of the transmission line.

Q8. The speed of the electrical signal flowing is copper is reduced by the dielectric media around it. A stripline is slowest. The solder mask on one side of the microstrip acts as a partial dielectric and reduces the signal propagation speed. Microstrip without the soldermask is fastest as it has least effective dielectric around it.

Q9. Because of the low inductance the power planes are effective for filtering very high frequency power supply noise. Unfortunately, low values of the capacitance does not make them effective at low frequencies .

Q10. Even though BGA assembly has matured, they can not be directly inspected. XRays of the BGA parts can only reveal shorts, they can not find open.

Q17. The magnitude of the reflected signal will depend upon the difference in the impedance of the transmission line and the capacitor. The capacitor provides a low impedance. A higher transmission line impedance will lead to higher reflection

Q11. DDR2 has in fact, higher clock latency than DDR. This somewhat reduces gain achieved by way of increased frequency. A DDR2 at 400 MT/s will be slower that the corresponding DDR at 400 MT/s.

Q19. As a result of the coupling the differential signals travel faster than the single ended signal. Because of this, whenever, it is required to maintain a timing relationship between a differential signal and a data signal, the differential signal is routed for a length greater than the length of the data signal.

Q21. A two layer PCB is sometimes used by RF designers to get a low cost prototype. Some RF ICs have very small number of pins unlike the digital designs with large number of address and data pins. A 0.063” thick PCB will require a larger trace width. To achieve a smaller trace width use a 0.033” thick PCB and high Er.

Q22. Although, the reduced voltage signaling does reduce the power consumption and the EMI, it becomes more susceptible to the external noise. If we do want to use a low voltage signaling, especially, single ended low voltage signaling, we should be careful about the crosstalk and keep the signal away from nearby high speed signals.

Q23. Higher impedance design takes less power. It simplifies the connector and cable system design because we can spread the signal and return current at reasonable distance. However, higher impedance means that the signal trace and return ground plane separation is increased, leading to more cross talk.


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