common circuits fm receiver fm transmitter synthesizer

•   The microphone converts sound pressure wave to electrical signals.
•   These audio voltages are amplified by the audio amplifier. The amplified audio is used to control the deviation of the frequency controlled oscillator.

•   The oscillator frequency is at the carrier frequency, in the FM band.
•   The low power of the frequency modulated carrier is boosted by the radio frequency amplifier.
•   The aerial is driven by the amplifier and produces an electromagnetic wave.
•   Under normal conditions the transmitted signal will travel as far as the horizon.
•   The Phase Locked Loop (PLL) synchronizes a local oscillator with a remote one. This ensures that the local oscillator is at the same frequency and in phase with the remote one
•   The local oscillator is voltage controlled (it is a VCO). This means that its frequency is controlled by varying a DC voltage input
•   The output signal of the VCO is fed back to a phase detector via a buffer. The buffer isolates the VCO from the loading caused by the detector and external circuits. It avoids "pulling" of the oscillator frequency. If there is no reference input signal then the VCO will oscillate at its natural "free running" frequency.
•   The other input to the phase detector is the reference signal, which we wish to lock the frequency of the VCO to. 
•   If there is a difference in frequency or phase between the two inputs then an error signal is produced at the output of the phase detector
•   This error signal is fed to the VCO via a filter and a DC amplifier to produce an error correcting voltage .The filter is a low pass type which determines the range of frequencies over which the VCO can vary. The DC amplifier amplifies the DC voltage level to a value suitable to control the VCO
•   The error correcting voltage may be either negative or positive depending on whether the VCO frequency is higher or lower than the reference frequency.
•   The effect of the error correcting voltage is to pull the VCO back to the same frequency as the reference frequency, and in phase with it

Modulation techniques
C4FM is a 4-carrier modulation format where the carrier is shifted in frequency at a particular rate (time) to a particular location around a center frequency. This allows for each of the 4"states" to represent a binary number. Each state is a "Symbol" which contains two bits of information.As you can see, each of the carriers depicts a particular symbol. This is a Simplified representation of C4FM since, in reality, the carriers would not be seen as individual, stand alone carriers shifted off center from the fundamental frequency. Although each carrier has a fixed offset (-1800 Hz, -600 Hz, +600 Hz, +1800 Hz), the carrier never returns to center frequency. Each transition of the carrier is encoded to start from where it was last positioned.
As can be seen, there are 4 decision points (indicated by the red dots) that represent the frequency states of the C4FM signal at a particular time. Each are valid decision points that tell us what the value of the symbol will be (11, 10, 00 or01). Each of the decision points has to fall within a valid range to be recognized as a symbol. If it falls outside the "window" (shown as the blue circle),
it becomes an invalid symbol and the demodulator in the radio will decode it as an erroneous symbol and will therefore see a higher "BER" or Bit Error Rate.