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There are several types of frequency synthesizers.
They are mostly "digital" in the sense that frequency control /switching is made digitally, by a computer or a switch matrix.
Such synthesizers can have a "parallel" control input, where switch setting determine output frequency. Or, they can have "serial" control, by a computer-generated serial code.
"Analog" synthesizers are simply mechanically or electrically tuned oscillators which are, at predetermined frequencies, stopped-locked to a frequency usually generated as an integer multiple of a reference frequency, from a stable lower-frequency reference oscillator.
"Direct-Digital Synthesizers" are computer-controlled devices constructing the resulting signal (sinusoidal or other ) by a special procedure.
There is another class of synthesizer which relies on different crystal oscillators being individually switched with their outputs being mixed and filtered to get the required products. I.E. xtal oscillator 1 = 30 MHz to 31 MHz in .1 MHz steps (10 crystals) Oscillator 20 MHz to 19.9 MHz in .01 MHz steps(again 10 crystal), OSC= 30, OSC 2 = 20, mixer and filter output = 10MHz, OSC 1 set to 30.1, mixer output now 10.1 MHz, if OSC 2 set to 19.99, output from mixer is now 10.11 MHz. Loads of crystal to be used and matched. Not much to recommend this method now.
Frank
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