In this method, voltage-controlled oscillators (VCO) are used to vary the
frequency of the carrier signal in accordance with the baseband signal amplitude
variations. These oscillators use devices with reactance that can be varied by the
application of a voltage, where the reactance causes the instantaneous frequency
of the VCO to change proportionally. The most commonly used variable reactanS
device is the voltage-variable capacitor called a varactor. The voltage-variable
capacitor may be obtained, for example, by using a reverse biased p-n
junction diode. The larger the reverse voltage applied to such a diode, the
smaller the transition capacitance will be of the diode. By incorporating such a
device into a standard Hartley or Colpitts oscillator, FM signals can be generated.
Figure 5.6 shows a simple reactance modulator. While VCOs offer a simple
way to generate narrowband FM signals, the stability of the center frequency
(carrier) of the VCO becomes a major issue when it is used for widehand FM generation.
The stability of the VCO can be improved by incorporating a phase
locked loop (PLL) which locks the center frequency to a stable crystal reference
frequency.
frequency of the carrier signal in accordance with the baseband signal amplitude
variations. These oscillators use devices with reactance that can be varied by the
application of a voltage, where the reactance causes the instantaneous frequency
of the VCO to change proportionally. The most commonly used variable reactanS
device is the voltage-variable capacitor called a varactor. The voltage-variable
capacitor may be obtained, for example, by using a reverse biased p-n
junction diode. The larger the reverse voltage applied to such a diode, the
smaller the transition capacitance will be of the diode. By incorporating such a
device into a standard Hartley or Colpitts oscillator, FM signals can be generated.
Figure 5.6 shows a simple reactance modulator. While VCOs offer a simple
way to generate narrowband FM signals, the stability of the center frequency
(carrier) of the VCO becomes a major issue when it is used for widehand FM generation.
The stability of the VCO can be improved by incorporating a phase
locked loop (PLL) which locks the center frequency to a stable crystal reference
frequency.
