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# DSP and maintainability true/false questions

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TQ5
Indicate if the statement is true or false. If a statement is false, indicate the correct answer.
1. In vibration analysis, we often assume that an accelerometer signal is stationary. A
stationary signal has the property that the statistical properties of the signal do not change with time.
2. The error due to under-sampling a continuous signal can be avoided by simply
amplifying the signal.
3. The discrete Fourier transform (DFT) of a signal is periodic with period being a
function of signal sampling frequency.
4. Proximity or displacement sensors are commonly used to measure the movement of the shaft in a ball bearing.

5. Magnetic mounting is the best approach for mounting accelerometers on to bearing

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In vibration analysis, we often assume that an accelerometer signal is stationary. A stationary signal has the property that the statistical properties of the signal do not change with time.

True stationary signals are statistically constant with time so, as an example, if you look at such a signal now and in an hour's time it would look statistically the same.

The error due to under-sampling a continuous signal can be avoided by simply amplifying the signal.

False, under-sampling produces aliases which when reconstructed fall within the Nyquist bandwidth. Amplification only acts to increase both wanted signal and the distortion, proper bandwidth filtering is the only answer

The discrete Fourier transform (DFT) of a signal is periodic with
period being a function of signal sampling frequency.

True the DFT (for a number N discrete samples) is given by the following summation, each component a function of the sampling frequency Ï‰=2Ï€/N

F(jÏ‰)=âˆ‘_(k=0)^(N-1)â–’ã€–f(k)e^(-jkÏ‰T) ã€—

Proximity or displacement sensors are commonly used to measure
the movement of the shaft in a ball bearing.

True inductive displacement sensors are used to measure shaft vibration (movement) in ball bearing assemblies, an example here

Magnetic mounting is the best approach for mounting accelerometers on to bearing

False, Stud mounting is the best type of mounting for accelerometers as it effectively fuses the device to the test surface thus transferring maximum response between test device and the accelerometer. Magnetic mounting should only be used with caution when equipment is moved from place to place. The results from Magnetic mounting become increasing inaccurate above frequencies of 1kHz

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