‰  NOW 40 WPM  ‰  TEXT IS FROM JANUARY 2014 QST  PAGE 46‰
SETTING THE VFO.  WITH THE TRANSMITTER OUTPUT CONNECTED TO A DUMMY LOAD,
SET THE VFO TO THE CORRECT FREQUENCY FOR THE BAND OF INTEREST.  TRY THE
TRANSMITTER ON SEVERAL BANDS, SINCE THE HIGHER BANDS USUALLY NEED MORE
DRIVE.  IF THE TRANSMITTER NEEDS MORE DRIVE USUALLY INDICATED BY
INSUFFICIENT GRID CURRENT IN THE FINAL AMPLIFIER, INSTABILITY, OR HAVING TO
TURN THE TRANSMITTER DRIVE CONTROL UP AN EXCESSIVE AMOUNT, THEN TRY ADDING
THE 1 2 VOLTAGE TRANSFORMER.  THE NEED F

GREEN BOARD IS CONNECTED BY WIRES TO THE FRONT PANEL LCD DISPLAY BOARD
RIGHT SIDE, AND THE BUFFER AMPLIFIER RED BOARD IS ON THE UPPER LEFT.  YOU
CAN USE ANY LOW CURRENT 12 V DC SUPPLY.  THE ENTIRE VFO DRAWS ABOUT 90 MA.
MY POWER SUPPLY IS SEEN ALONG THE BOTTOM OF THE PHOTO SEE THE QST IN DEPTH
WEB PAGE FOR MORE DETAILS.  BUILDING THE DIRECT DIGITAL SYNTHESIZER FOR THE
DDS CIRCUIT I CHOSE THE N3ZI DDS 2 DELUXE KIT, WHICH FEATURES THE STANDARD
LCD DISPLAY AND HAS THE SURFACE MOUNT CHIP COMPONENTS ALREADY INSTALLED.
YOU MUST FIRST PURCHASE AND BUILD THE SYNTHESIZER AND GET IT WORKING
PROPERLY BEFORE PROCEEDING.  WHEN BUILDING THE SYNTHESIZER CONNECT THE
ROTARY ENCODER AND UP/DOWN SWITCHES TO THE DDS CIRCUIT BOARD WITH WIRES SO
THAT THEY CAN BE MOUNTED ON THE FRONT PANEL SEE FIGURE 3.  WIRED
CONNECTIONS GIVE YOU MORE FREEDOM WHEN MOUNTING THE CIRCUIT BOARD AND
RELATED COMPONENTS.  SYNTHESIZER OUTPUT LEVEL CRYSTAL CONTROLLED
TRANSMITTERS FROM THE 50S AND 60S REQUIRE SEVERAL VOLTS PEAK TO PEAK P P OF
RF TO PROPERLY DRIVE THEM FROM THEIR VFO INPUTS.  MY EICO 720 MANUAL
RECOMMENDS 10 20 V AT THE VFO INPUT.  THE SYNTHESIZER OUTPUT, HOWEVER,

A MODERN KEYED VFO FOR VINTAGE TRANSMITTERS YOUR CLASSIC TRANSMITTER CAN
RIVAL THE SIGNAL QUALITY OF A MODERN TRANSCEIVER WITH THIS VFO.  FOR
STRAIGHT KEY NIGHT, I WANTED TO PUT MY 50 YEAR OLD EICO 720 TRANSMITTER ON
THE AIR, WITH THE CONVENIENCE OF A VFO RATHER THAN CRYSTAL CONTROL.  I
WANTED THE FEATURES OF A MODERN VFO DIGITAL FREQUENCY READOUT, MEMORIES,
AND FREEDOM FROM DRIFT AND CHIRP.  I DESIGNED A VFO BASED ON THE N3ZI KITS
DDS 2 DIRECT DIGITAL SYNTHESIZER KIT.  IT FEATURES DIGITAL READOUT, 12
MEMORIES, AND FREQUENCY RESOLUTION TO 1 HZ.  IT ALSO MAKES A GREAT SIGNAL
GENERATOR.  VFO OVERVIEW MY EICO 720 TRANSMITTER HAS A SPECIAL INPUT FOR AN
EXTERNAL VFO.  IF YOUR TRANSMITTER LACKS SUCH AN INPUT, YOU CAN PROBABLY
FEED THE VFO SIGNAL DIRECTLY INTO THE CRYSTAL SOCKET WITHOUT MAKING ANY
CHANGES TO THE TRANSMITTER.  REGARDLESS, THE OUTPUT OF THE N3ZI SYNTHESIZER
MUST BE AMPLIFIED, AND SOME SORT OF KEYING CIRCUIT MUST BE ADDED.  I
MOUNTED ALL OF THE COMPONENTS FOR MY VFO EXCEPT THE KEYING CIRCUIT
COMPONENTS IN A STEEL RADIOSHACK PROJECT BOX SHOWN IN FIGURE 1.  FIGURE 2
SHOWS THE DDS BOARD, BUFFER AMPLIFIER, AND KEYING INPUT.  I WANTED TO USE
THE SYNTHESIZER BOTH AS A VFO AND AS A SIGNAL GENERATOR, SO I MADE
PROVISIONS FOR BYPASSING THE BUFFER AMPLIFIER AND KEYING CIRCUITS.  FIGURE
3 SHOWS THE INSIDE OF THE VFO PROJECT BOX.  THE DDS BOARD UPPER CENTER
‰  END OF 40 WPM TEXT  ‰  QST DE W1AW  ƒ