Abstract: A synchronous frequency synthesis method for a radio transmitter-receiver, having the steps of applying the reference frequency of a fixed-frequency oscillator to a fixed frequency divider and to a second mixer for converting the reference frequency into a second intermediate frequency of the receiving stage, dividing the controlled frequency of a voltage-controlled oscillator by a variable frequency divider, multiplying the controlled frequency to produce a transmitting carrier frequency, applying the controlled frequency to the first mixer of the receiving stage, applying the divided frequency of the fixed frequency divider to a phase detector so as to produce comparison frequencies, inputting either one of the two frequency division step numbers, obtained from the controlled frequencies for transmission and reception and the comparison frequencies, to a shift-applying code converter adapted to convert the difference between the two frequency division step numbers into the sum of or difference between itsel