| 摘要 |
945,268. Frequency discriminators. HAZELTINE CORPORATION. Dec. 19, 1961 [Jan. 24, 1961], No. 45539/61. Heading H3A. Relates to apparatus for measuring the repetition rate of a repetitive signal which comprises a means for amplitude limiting the repetitive signal, a differentiating means including an inductor for developing from the limited signal a series of pulses which in comparison to differentiated pulses developed in the absence of the inductor have a faster terminal decay than such pulses and means for rectifying the said pulses developed by the differentiating means so as to retain a series of same polarity pulses whose average value is representative of the repetition rate of the repetitive signal. The invention is particularly described with reference to detecting the modulation of a frequency modulated subcarrier of a frequency modulated multiplex signal, wherein the subcarrier is used as one of the modulating signals of a main carrier. Such a technique may be used to provide background music to normal programme material, or in stereo broadcasting. The main carrier is received in a unit 10, where the main modulating signal is detected and reproduced at 12 while the detected subcarrier is fed via a buffer amplifier 13 and a filter 14 to an amplifier 15 and a limiter 16, after which the subcarrier is fed to a detector 17. The detector comprises in addition to the conventional differentiating components 22, 23 an inductor 21, which speeds the recovery period of the differentiated wave (see curve B of Fig. 2a). A diode 24 rectifies the differentiated pulses, which are then passed to an integrator 26, 27, arranged to produce an output voltage depending upon the repetition rate of the input pulses. This signal is fed to an amplifier 18 and thence to an output reproduction stage 19. The arrangement is said to minimize the degradation in linearity which occurs in a conventional detector due to the failure of the differentiated pulses to return to zero before the following pulse occurs.
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