Abstract: A variable characteristic filter is described in which output signals are frequency analyzed by means of a frequency-to-voltage converter to derive a control signal. Variable impedance elements in the filter are controlled by the control signal to vary the filter characteristic automatically. The filter, including the frequency-to-voltage converter, may be such that the filter attenuation is reduced in the frequency region where the input or output signals mainly occur. The invention is particularly useful in Doppler blood-flow velocimeters to reduce noise.

Abstract: In an ultrasonic Doppler velocimeter for measuring blood flow it is necessary to change the frequency of insonation according to the site and size of a vessel to be investigated. Several different crystal pairs are employed in a single probe, each pair associated with a different frequency. All of the transmitting crystals are connected in parallel, and all of the receiving crystals are connected in parallel. As a result, the single probe is suitable for investigating blood flow in different blood vessel conditions. Optimum insonation frequency selection is achieved by use of a voltage controlled oscillator, and a frequency control circuit which compares powers received at different frequencies. Signals representative of blood velocities appear at the output of a scaler which ensures that the same velocity provides the same output independently of frequency of insonation.

Abstract: The invention is particularly related to the measurement of the area of a blood vessel and the volume flow rate of blood in such vessels. An ultrasonic Doppler velocimeter which derives a number of velocity signals dependent on the number of different velocities in a duct is coupled by way of a limiter to a frequency-to-voltage converter which therefore has an output with one component for each velocity signal. A further frequency-to-voltage converter provides a signal dependent on the number of velocity signals and since this number depends on the duct area, the output from the further converter is a measure of this area. By using another frequency-to-voltage converter and a mean detector, a mean velocity signal is obtained which when multiplied by the area signal generates volume flow rate.