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EMC Question of the Week: September 17, 2018
What value of shunt capacitor should be used to make a low-pass filter with an insertion loss of 20 dB at 65 MHz in a circuit where both the source and load impedances are 50 ohms?
- 940 pF
- 979 pF
- 985 pF
- 0.001 μF
Answer
The best answer is "d". The key point of this problem being the number of significant figures in the answer. In the laboratory, a 50-ohm trace or cable is typically 50 ohms plus or minus about 10% or more. Even a precision cable will have a tolerance of a few percent. Since two of the numbers in the problem statement were specified to 1 significant figure, any answer with more than 1 significant figure implies an accuracy that isn't there.
In a real printed circuit board design, a 0.001 μF capacitor is a relatively inexpensive component. On the other hand precision capacitors with any of the other nominal values in this problem would cost much more.
Finally, for anybody that worked this problem as if each of the values had infinite precision, the correct result to 4 significant figures would be 974.5 pF.
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