Sample-and-hold unit

As compared to the period of a demodulated Doppler signal, the opening time of the receiver gate is very short, and the sample-and-hold unit therefore receives only brief "glimpses" or samples of the Doppler signal. Multiple samples at slightly different phases of the Doppler signal provide the basis for "reconstruction" of the Doppler signal. Each time the Doppler signal passes the receiver gate, the sample-and-hold unit measures the instantaneous amplitude of the signal (Fig.1). The measuring or sampling period is in the order of 1 ms, which for practical purposes is "instantaneous" compared to the period of the Doppler signal. (The Doppler frequency shift may be in the order of 110 kHz, corresponding to periods of 1001 000 ms.) A voltage corresponding to the instantaneous amplitude is induced, and after the 1 ms sampling time, the voltage does not drop to zero, but is held at the same level until the next sampling is performed (Fig. 1). The result is a steplike signal consisting of two frequencies: the underlying low Doppler frequency, and a high frequency corresponding to each (nearly) vertical step. The high frequency component is filtered away by a low-pass filter, and the original Doppler signal is thus "reconstructed". Prior to spectral analysis by a Doppler shift analyser, the Doppler signal is also high- pass filtered to remove unwanted high-amplitude, low-frequency signals such as those from vessel walls.

The Doppler signal is sampled each time the receiver gate opens, which is once per pulse repetition period PRP . The sampling frequency is thus equal to the pulse repetition frequency PRF . According to Shannons sampling theorem, the maximum Doppler frequency that can be measured is half the sampling frequency, i.e. 1/2 PRF. The low-pass filter being used therefore has a cut-off frequency at this level, also called the Nyquist limit.

HJS