NBFM or Frequncy narrow band modulation is used in this schematic diagram of 27 MHz FM. This application circuit is a Motorola MC2833 - VHF transmitter with narrow-band FM modulation in a single chip.
The output power is 10 mW (+10 dBm) and the coverage is 10 - 20m.
P1 is used to adjust the gain of the microphone and adjust P2 for the deviation. Remember this NBFM 27MHz FM transmitter use with a maximum deviation of 5 kHz, this means you have to use a receiver NBFM a sufficient level for audio.
The transmitter is easy to adjust: cut C9, C18 and C9 for maximum output power with an artificial 50Ω load. Then listen to a radio receiver 27MHz and adjust P1 and P2 until you get the best modulation.
