Answer #51

The correct answer is B, 5KHz.

This question tests your conceptual knowledge about the limits of Pulsed wave Doppler. Since the machine sends out pulses and listens inbetween, it can misjudge events in time and space if listening time is too long. There is therefore a maximum upper limit at which PW Doppler can detect a Doppler shift, called the Nyquist limit.

It is equal to half of the pulse repetition frequency (PRF), which is the frequency of transmitting pulses (how often the machine sends out a pulse). PRF is the reciprocal of the time period between pulses, known as the pulse repetition period (PRP). Recall that period and frequency are reciprocals.

Since we are given a time between pulses of 0.1ms (PRP) the PRF is 1/0.1 or 10KHz. Since the Nyquist limit is PRF/2, the answer is 5KHz, the maximum Doppler shift we can detect. Anything above this will cause aliasing.