A Novel Frequency Estimator for Sampled Doppler Signals

In multigate pulsed Doppler systems with serial processing the Doppler signals are retained in their sampled form. The time-discrete nature of the Doppler signals affects the procedure to estimate the average frequency of the Doppler signal. Based on computer simulation studies it is demonstrated that a time-discrete frequency estimator based on either the density of zero-crossings or the time average of the instantaneous frequency exhibit a large relative error under poor signal-to-noise conditions or in the case of relatively wide band signals. Especially, the frequency estimator based on the instantaneous frequency functions poorly for average Doppler frequencies close to the Nyquist frequency. However, restricting the detected instantaneous frequency to a specific interval around its average improves the estimator performance considerably, while it allows tracking of center frequencies beyond the Nyquist frequency. A hardware realization of this modified estimator as incorporated in a high-resolution multigate pulsed Doppler system is described. In vitro and in vivo registrations as assessed with this system demonstrate the ability of the system to track frequencies close to and beyond the Nyquist frequency.