Information on Frequency Synthesiser - Direct Method
In the direct frequency synthesiser method the required carrier is
obtained through a process of frequency multiplication, mixing and filtering.
Addition and subtraction are achieved by mixer circuits from which
the sum or difference frequency is selected. Multiplication is achieved
using frequency multipliers or harmonic generators and division relies on
digital techniques using re-generative dividers or bi-stable filters. While the
process has the advantage that the stability and accuracy of
the output frequency is set by the reference crystal oscillator
source, the method has major disadvantages in that the mixing
process generates spurious frequencies and requires a number of costly
filters.