Method of and apparatus for determining instants at which the instantaneous frequency of an electrical signal of changing frequency has predetermined values

1,066,478. Electric selective signalling. NATIONAL RESEARCH DEVELOPMENT CORPORATION. July 26, 1965 [Aug. 12. 1964], No. 32898/64. Heading G4H. The instants at which an electrical signal of changing frequency has frequencies which are integral multiples of a fixed frequency are determined. The speed of a magnetic rotor decelerating from a standard initial speed in a magnetic field due to magnetic losses is detected by two photo-cells sensing a toothed disc, the photo-cell outputs being used to amplitude modulate strobe pulses from an independent generator, the lower sideband frequencies in the two cases then being selected by respective filters after waveform broadening in ringing chokes. The sideband waveforms have a phase difference of Π90 degrees depending on whether the photocell frequency is greater than or less than the nearest integral multiple of the strobe frequency. The discontinuous phase change occurring at equality is detected using Schmitt triggers with a first threshold level receiving the filter output to produce outputs cross-coupled to two AND gates (some via monostables), pulses being produced by one of the gates until equality and then by the other. The two AND gates feed respective input sides of a bi-stable device, the two output sides of which are connected via monostables to respective further AND gates which also receive the outputs of Schmitt triggers having a common second threshold level and receiving the filter outputs. One of the further AND gates, depending on whether the speed is increasing or decreasing, produces an output pulse for each equality between the frequencies (see above) and these pulses are counted in a reversible counter. Those representing decrease are also fed to a further counter counting clock pulses to cause read out (to a punch) and reset. The counter reading is also punched out when the speed reaches zero as detected by a Schmitt trigger receiving a D.C. voltage obtained by an R-C circuit from the output of one of the photo-cells. Measurement of satellite (Doppler) and turbine speeds are also mentioned.