How does a FM modulator work?
I have a doubt on FM modulator, I would like to know how the circuit for FM modulator works.
Basically, one generates a carrier frequency and how do they change the phase of the carrier frequency. Could anyone explain from the circuit point of view.
Thanks in advance.
Regards,
Prakash.
There are many types of FM modulators, a classical modulator consists of an oscillator controlled by the modulating signal.
In al LC oscillator, the capacitor is composed of some fixed value capacitors and a device whose capacitance depends on the voltage aplied (a varicap/varactor diode for example). The modulating voltage is applied on the varicap and determines variations of the capacitance and of the frequency of the oscillator, FM.
In an oscillator based on the charging/de-charging of a capacitor, the frequency of the output signal is proportional to the charging current of the capacitor. To produce FM of such an oscillator you need to change the charging current of the capacitor, so the modulating signal changes de charging current.
Because the phase is the integral of the frequency, every change in the frequecy determines a change in the phase of the signal.
Another type of FM modulator is based on the NCO (Numericaly Controlled Oscillator)/DDS(Direct Digital Synthesis) which produces an output signal (sinus) based on a memory/look up table. There is a fixed refference oscillator with higher frequency,a "frequency" register, a fixed size phase accumulator, a memory with the sin values, and a DAC. Every clock of the refference oscillator determines adding the "frequency" val to the phase accumulator. The content of the phase accumulator is the address of the memory, and the content of the selected address is converted to analog with the DAC. By changing the "frequency" value, the frequency of the output signal can be modified, so to frequency modulate the NCO you need to ADC the input signal and add it to the "frequency" register. This can produce only discrete FM.
in phase modulatiopn......
the arguement of Θ=ωt+Φ that is Φ is varied linearly as Kp*m(t) so that
u get the pm signal while in fm the instantaneous frequency is varied in the above fashion
usually if pm has message signal m(t) then the spectrum /sinusoid is also the fm of the integral of m(t)
chere
Try this,