a-fo/2
b) fo*10
c) as high as possible
d) fo*4.
ANSWER: (b)(c)-both answers are correct.
According to Nyquist cryteria sampling frequency should be fmax*2, where fmax is the highest frequency component still present in signal, usually fo*2 if we deal with wide bandwidth transducer. So, frequency should be fo*4, and in most applications it is acceptable, especially if filtering of frequencies higher than Nyquist frequency is used. In that case signal could be fully restored using fo*4 sampled signal. But Nyquist cryteria in deed is valid for signal with known phase, no DC. Othervise sampling frequency should be fmax*4. Now we have fo*8 value. If wide bandwidth transducer is used, high frequencies are necessary for investigation too, so filtering of fo*2 frequencies is unwanted. In order to have measurement errors introduced by the 8bit quantisation and by sampling frequency equal, it should a bit higher, i.e. about 10*fo. In that case it is optimal. But if higher sampling rate is available and there is no reason to save space ocupied by samples, computation time etc, even >fo*10 sampling rate is acceptable.
Dr.Linas Svilainis, Kaunas University of Technology
| |Quiz UT-3| |UTonline| |