| 摘要 |
PURPOSE:To make the modulation frequency characteristic at a low frequency flat up to a DC signal component by providing a multi-band switching function of VHF and UHF bands and a multi-channel access function and specifying a gain set externally to a variable gain amplifier in an FM modulation circuit of an FM radio equipment applying voice transmission in a voice band (0.3-3kHz) and signal transmission at low band (nearly 1-300Hz). CONSTITUTION:The FM modulation function is realized at a high frequency of equivalent band (0.3-3kHz) by the 1st PLL(phase locked loop) comprising a voltage controlled oscillator VCO1, a frequency divider 1, a phase comparator 3, a loop filter 4 and a level adder 5. Further, the FM modulation function at a low band (1-300Hz) or equivalent is provided by using the 2nd PLL formed by adding a voltage controlled oscillator VCO 2, a frequency divider 2 and a variable gain amplifier 6 to the said PLL. Moreover, the gain of the variable gain amplifier 6 is set to a ratio of the VCO sensitivity of the voltage controlled oscillator VCO 1 divided by a frequency division N1 of the frequency divider 1 to the VCO sensitivity of the VCO 2 divided by the frequency division N2 of the frequency divider 2. Thus, the characteristic of the modulation gain in the FM modulation is made flat. |
