Mean frequency via zero crossings (medical ultrasound)

It is shown that the mean frequency of a Doppler shifted signal can be estimated, in principle, in the time domain, using the zero crossings of the signal. This is of note as zero crossing detectors have been criticised for the inaccuracy in measuring the mean frequency. By using the complex analytic signal, it is demonstrated that the use of zero crossing methods to determine mean frequency is much more accurate than is generally appreciated, provided that envelope information is incorporated into the algorithm. It is shown that interference effects cause large spike-like excursions in the instantaneous frequency of received RF echoes and hence will also affect their zero crossing rate. This applies also to the demodulated Doppler shifted signals and thus the estimated mean frequency. However, it is shown that the relation between the envelope and instantaneous frequency of such data segments allows the algorithm to minimize the size of the effect.<>