Mechanical Engineering - Measurement

Multiple choice test

1 1. The Kurtosis coefficient:  none of the other ✕ o is negative when the signal has a sharper probability density function than a Gaussian distribution o is negative when the signal has a flatter probability density function than a Gaussian distribution o is negative when the signal has a non -symmetrical probability density function 2. Which window would you use to obtain amplitude of a random signal with unknown frequency components if you are not able to tune the acquisition parameters?  Hanning ✔ o Flat top o Exponential o Rectangular 3. In a Short Time Fourier Transform, the x and y axes show respectively: o none of the other o amplitude and frequency  time and frequency ✔ o time and amplitude 4. The autocorrelation of a signal at T=0 is 7 (units missing). The signal RMS is: o 3.18 o 1.13 o 7  2.64 ✔ 2 5. The CPDF curve of a gaussian distribution: o is nondimensional o is larger than the signal RMS o none of the other  is smaller than the signal RMS ✕ 6. Increasing the acquisition time T has positive effects on leakage because: o it allows to avoid aliasing o it allows to completely solve the problem of leakage o when T increases, the spectrum affected by leakage becomes as high as the correct spectrum  the change in resolution decreases the effect of both long -term and short -term leakage ✔ 7. The Fourier Transform of a real, odd signal is: o real, odd o real, even o imaginary, even  imaginary, odd ✔ 8. Superposition integral, the core of the linear system theory: o depends on the frequency resolution o none of the other  can be used to predict the frequency response of a linear system ✕ o is always imaginary 3 9. The average between the two signals Y1= 3*cos(2*pi*t + 2) and Y2= 3.1*cos(2*pi*t + 1.5), is a vector rotating at 2 Hz with arguments and frequencies: o |S| = 5.91, arg = -79.96 ° o |S| = 8.20, arg = -45.55 ° o |S| = 6.44, arg = 90 °  None of the other ✔ 10. The sampling property of the Dirac function: o is periodic o it is always linear also if the signal is nonlinear o none of the other o cannot accept multiple inputs 11. The LSB of a converter with Vmax = 10 V, Vmin= 0 V and B= 12 is: o 1.22 mV o 4.88 mV o None of the other  2440 uV (u = micro) ✔ 12. The Fourier Transform of a rectangular window: o is real o non of the others  is periodic ✕ o is complex 4 13. Describe the spectral leakage using the convolution theorem, in less than 10 lines. 14. Which window would you use to obtain the spectrum of the transient (impulse) response of a lightly damped 2DOF mechanical system? o Rectangular o Flat top o Exponential o Hanning 15. The convolution: o can be used to predict the time response of LTI systems o generates aliasing o is the inverse of Fourier Transform of the transfer function  can be used create periodic signals from sampled frequency responses ✕ 16. The Fourier Transform of Dirac function: o none of the other o is complex  it is always linear ✔ o the modulus depends on the delay tau 17. The Fourier Transform of a real, even signal is: o imaginary, odd  real, even ✔ o imaginary, even o real, odd 5 18. The LSB of a converter with Vmax= 2.5 V, Vmin= -2.5 V and B= 12 is: o 4.88 mV o None of the others  1.22 mV ✔ o 2440 uV (u = micro) 19. Which window would you use to analyse the spectrum of the impulse response of a lightly damped 3DOF mechanical system? o Hanning o Flat Top o Exponential o Rectangular 20. The 60th percentile of a Gaussian distribution with mean=0 and variance=1 is: o -0.034  none of the other o 1.475 ✔ o 2.134 21. The faster signal to identify the response of a linear time invariant is: o the gaussian noise  all the signals can be used, but we must consider the nonlinear effect of leakage ✕ o a single, short lasting, harmonic signal o a practical implementation of the Dirac function 19. The Cres F actor : o is useful to detect the maxima of a signal if the nois e is incorrelated with the signal itself o gives information about the flatness of a signal ’s PDF  none of the other ✔ o is useful to detect peaks only in periodic signals 20. If an operator is sta ble: o the response of the system can never go to infinite o is additive o it is always linear  its response is limited if the input is limited ✔ 21. The Fourier Tran sform of a real, even signal is:  real, even ✔ o real, odd o imaginary, even o imaginary, odd