A circuit for detecting and holding the peak voltage of an analog signal is provided. A constant current source controlled by a digital voltage comparator charges a capacitor. The capacitor is charged to a voltage value equivalent to the peak voltage of the analog signal. By selecting the value of the capacitor and the amplitude of the constant current from the current source, slew rate limiting can be achieved, giving the circuit high noise immunity.

A method and an apparatus for indicating in real time the occurrence of and measuring the frequency or period of the basic oscillation of a generally periodic unknown signal with statistically distributed spectral components is disclosed. Using an autocorrelation technique, the time interval from the initial peak value to the next successive peak value of the autocorrelation curve is measured and the unknown frequency or period is determined therefrom. A periodically occurring maximum of the next successive peak value is determined, and, on its appearance, a signal is produced, said signal approximating the occurrence in real time of the generally periodic signal.

An input signal of unknown frequency is applied to a sampler which is driven by a sampling signal of frequency f.sub.O to produce an intermediate signal of frequency f.sub.IF. The sampling frequency f.sub.O varies in response to a randomly, or pseudo-randomly varying signal which in turn causes the signal of frequency f.sub.IF to also vary. f.sub.IF will vary in the same way as f.sub.O, but with a scale factor of .+-. N, where N is the harmonic number. Correlation techniques are used to determine the harmonic number and the sideband to enable calculation of the input frequency. Errors in the determination of the harmonic number and sideband due to frequency modulation of the input signal are reduced, due to the variation imposed on the sampling frequency f.sub.O.

A method and apparatus makes rapid frequency measurements by measuring time intervals for a series of blocks of event counts with the number of events in each block held constant. This makes the numerator of the events/time relationship constant so it does not have to be measured, processed or stored. The frequency of the signal is determined by measuring the time interval, then taking the inverse of the measured value and multiplying by the appropriate constant. A fast inverse circuit uses a Taylor series expansion technique implemented in digital circuit, with the slope resolution adjusted for regions of small slope to improve accuracy.

Determining the fetal heart rate comprises irradiating an ultrasound beam in the direction of the fetal heart. The moving parts of the fetal heart cause a Doppler shift which is used to determine the fetal heart rate via an autocorrelation function. Moving parts of the fetal heart with different velocities and Doppler shifts originating from other physiological sources generate secondary maxima in the autocorrelation function, so that an accurate heart rate determination becomes nearly impossible. This problem is overcome by parallel processing of the demodulated ultrasound signal in various pass-bands, for which purpose the signal is fed to filters (28, 30) of different frequency characteristics. The output of the pass band with the better result is used for fetal heart rate circulation.