| 摘要 |
PURPOSE:To enable frequency modulation at a comparatively lower frequency band, by converting a digital quantity once into an analog quanity and controlling the advanced time of a binary counter through the comparison with converted voltage, in a low frequency modulation circuit used for telephone lines. CONSTITUTION:A binary counter B starts advancing through the production of pulses, and an output B is in D/A conversion C and an analog quantity proportional to an intput digital quantity can be outputted H. This is compared D with an input terminal voltage and an output I is changed at the moment of crossing. This change triggers a monostable multivibrator E, resets the counter B surely and inverts an FF circuit F surely. When harmonics are removed from an output K through an LRFG, a sinusoidal wave L is obtained. Thus, when an input voltage decrease, since the output of comparator is changed while the output of D/A conversion is low, the output frequency is increased, and the output frequency is low through the increased input voltage, allowing frequency modulation at a very low frequency band. |
