What is the frequency deviation for a 12.21 MHz reactance modulated oscillator in a 5 kHz deviation, 146.52 MHz FM phone transmitter?
Why In a multiplier-chain FM transmitter, the deviation is multiplied by exactly the same factor as the carrier frequency. Here 146.52 MHz / 12.21 MHz = 12, so the oscillator only needs 5000 Hz / 12 = 416.7 Hz of deviation to produce 5 kHz at the output.
Watch out The choice of 5 kHz assumes deviation passes through the multipliers unchanged, but multiplying frequency by 12 also multiplies the instantaneous frequency swing by 12; 60 kHz is the result of multiplying instead of dividing.
Find the multiplication factor (146.52/12.21 = 12), then divide the final deviation by it: 5 kHz / 12 = 417 Hz.