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SIGNALS.  A MODERN VERSION USES A SINGLE CHIP SYNTHESIZER. THESE HAVE
ONBOARD FREQUENCY DIVIDERS THAT CONVERT TO A LOW FREQUENCY, WHICH CAN BE
COMPARED TO A REFERENCE CRYSTAL OSCILLATOR, TYPICALLY AT 10 MEGAHERTZ.  THEN
A PHASE LOCKED LOOP CONTROLS THE ONBOARD VOLTAGE CONTROLLED OSCILLATOR, VCO.
THE CHIPS TYPICALLY OPERATE IN THE 1 TO 4 GIGAHERTZ RANGE, SO ADDITIONAL
FREQUENCY MULTIPLIER STAGES MIGHT BE NEEDED FOR HIGHER BANDS. PHASE NOISE IS
CREATED WHEN ANY NOISE IN THE SYSTEM REACHES THE OSCILLATOR AND MODULATES
IT, CHANGING THE FREQUENCY SLIGHTLY.  A SLIGHT CHANGE AT A LOW FREQUENCY, 10
OR ABOUT 100 MEGAHERTZ, IS MAGNIFIED BY THE FREQUENCY MULTIPLICATION, SO IT
WORSENS BY 6 DB AT HIGH FREQUENCIES FOR EACH FREQUENCY DOUBLING OR ALMOST 40
DB WORSE FOR A 96 MULTIPLIER.  CRYSTALS HAVE VERY HIGH Q, SO THEY ARE LESS
SUSCEPTIBLE.  HOWEVER, THE VCO IN A SYNTHESIZER IS SENSITIVE TO VOLTAGE,
RESULTING IN LARGER FREQUENCY VARIATIONS FROM SMALL AMOUNTS OF NOISE.  FOR
EXAMPLE, IF THE VCO SENSITIVITY IS 100 MEGAHERTZ PER VOLT, 1 MICROVOLT OF
NOISE WILL RESULT IN 100 HERTZ WIDE PHASE NOISE. DIGITAL NOISE FROM THE
FREQUENCY DIVIDERS ON THE CHIP AND NOISE FROM POWER SUPPLIES REACHING THE
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