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CHOSE TWO FOR THE BEACON FREQUENCIES.  IF YOU ARE PROGRAMMING A
SYNTHESIZER, PICK ROUND NUMBERS AND CHECK CAREFULLY WITH A SPECTRUM
ANALYZER, EVEN IF YOU CANT MEASURE PHASE NOISE.  PRACTICAL LO
SUGGESTIONS.  IF YOU ARE CONSIDERING BUILDING A NEW MICROWAVE
TRANSVERTER, I RECOMMEND A SYNTHESIZER LOCKED TO A GPS REFERENCE FOR
ULTIMATE FREQUENCY ACCURACY.  ON THE OTHER HAND, YOU MIGHT ALREADY
HAVE A GOOD TRANSVERTER WHERE FREQUENCY ACCURACY OR STABILITY IS
MARGINAL.  MICROWAVE LENGTHS.  LOCAL OSCILLATORS FOR TRANSVERTERS.
THE MICROWAVE LENGTHS COLUMN IN THE JANUARY 2024 ISSUE OF QST
DISCUSSED TRANSVERTERS AND HOW THE LOCAL OSCILLATOR, LO, IS AN
ESSENTIAL PART.  THE LO PROVIDES A SIGNAL TO THE MIXER AT A
FREQUENCY THAT IS ADDED TO THE IF SIGNAL TO CREATE A MICROWAVE
SIGNAL FOR TRANSMISSION.  THE LO ALSO PROVIDES A SIGNAL TO THE MIXER
AT A FREQUENCY THAT IS SUBTRACTED FROM THE MICROWAVE FREQUENCY TO
RECEIVE IF SIGNALS ON A VHF OR UHF TRANSCEIVER. FINDING WEAK SIGNALS
OR OPERATING DIGITAL MODES REQUIRES AN LO THAT IS CLEAN, STABLE, AND
ON FREQUENCY, AND HAS LOW PHASE NOISE.  TRADITIONAL LOS STARTED WITH
A CRYSTAL OSCILLATOR, TYPICALLY AROUND 100 MEGAHERTZ.  THEN THE
CRYSTAL FREQUENCY WAS MULTIPLIED THROUGH A SERIES OF FREQUENCY
MULTIPLIERS, SUCH AS DOUBLERS, TRIPLERS, AND QUADRUPLERS, UP TO THE
ULTIMATE LO FREQUENCY.  FOR A 10 GIGAHERTZ LO, THE FREQUENCY MIGHT
BE MULTIPLIED 96 TIMES OR MORE.  A CRYSTAL OSCILLATOR USUALLY
PROVIDES A VERY CLEAN SIGNAL, BUT MANY MULTIPLIERS CAN CREATE OFF
FREQUENCY LOWER LEVEL SIGNALS THAT CAN REACH THE MIXER AND PRODUCE
UNWANTED SIGNALS. ONE RESULT CAN BE SPURIOUS SIGNALS IN THE
RECEIVER, WHICH CAN BE HARD TO DIFFERENTIATE FROM DESIRED WEAK
SIGNALS.  GOOD CRYSTALS NEEDED FOR THE LO FREQUENCIES HAVE BECOME
DIFFICULT TO FIND, SO THE TREND IS TO USE A FLEXIBLE FREQUENCY
SYNTHESIZER TO GENERATE LO 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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