‰  NOW 40 WPM  ‰  TEXT IS FROM MAY 2015  QST  PAGE 37 ‰
BUILDER AND HOMEBREWER AND ENJOYS EXPERIMENTING WITH APPLYING NEW
TECHNOLOGY TO AMATEUR RADIO PROJECTS.  JIM HOLDS AN ASSOCIATES DEGREE IN
ELECTRONIC TECHNOLOGY AS WELL AS A BACHELOR OF SCIENCE IN ELECTRICAL
ENGINEERING.  HE HAS SPENT HIS PROFESSIONAL CAREER ENGINEERING AIR GROUND
HF/VHF COMMUNICATIONS SITES FOR AERONAUTICAL RADIO, DIRECTION FINDING
SYSTEMS FOR L3 COMMUNICATIONS AND LATELY SOFTWARE DEFINED RADIOS FOR
LOCKHEED MARTIN.  JIM CAN BE REACHED AT 

MHZ.  A LOW PASS FILTER REMOVES THE CLOCK AND ALIAS SIGNALS.  THE AD603
VARIABLE GAIN AMPLIFIER PROVIDES A 0 TO 40 DBM OUTPUT DEPENDING ON THE DC
VOLTAGE PRESENT ON THE CONTROL PIN, WHICH IN THIS CASE IS DRIVEN BY A SPI
CONTROLLED DIGITAL POTENTIOMETER.  THE POTENTIOMETER IS CONFIGURED TO
PROVIDE 200 STEPS FOR THE ENTIRE RANGE, WHICH TRANSLATES TO 0R2 DB STEPS.
TWO 40 DB SWITCHED FIXED ATTENUATORS FOLLOW THE AD603 VARIABLE GAIN
AMPLIFIER VGA TO PROVIDE A 0 TO 120 DBM RANGE.  VERY FEW PARTS ARE REQUIRED
FOR THIS PROJECT FIGURE 2 BECAUSE OF THE HIGH LEVEL OF INTEGRATION IN THE
VARIOUS MODULES.  SINCE THE UPPER FREQUENCY IS ONLY 10 MHZ, THE LAYOUT IS
NOT PARTICULARLY CRITICAL, BUT YOU SHOULD TAKE CARE TO KEEP THE LEADS OF
THE RESISTORS THAT MAKE UP THE 40 DB ATTENUATORS AS SHORT AS POSSIBLE.  USE
SHIELDED CABLE BETWEEN THE OUTPUT JACK AND THE FINAL ATTENUATOR TO PREVENT
COUPLING OF UNWANTED NOISE AND SIGNALS.  YOU CAN BUILD THIS USB CONTROLLED
GENERATOR IN TWO VERSIONS.  YOU CAN BUILD EITHER VERSION OF THE GENERATOR
ON PERFBOARD FIGURE 3, OR ON THE AVAILABLE CIRCUIT BOARD FIGURE 4.  THE USB
TO SPI MODULE PLUGS INTO THE UPPER LEFT OF THE PCB.  FIGURE 5 SHOWS THE

A 100 HZ TO 10 MHZ SIGNAL GENERATOR  HERE IS A COMPACT GENERATOR WITH A
TOUCHSCREEN AND A COMPUTER INTERFACE.  JIM VEATCH, WA2EUJ  A SIGNAL
GENERATOR IS ONE OF THE MOST USEFUL PIECES OF TEST EQUIPMENT FOR WORKING
WITH AMATEUR RADIO PROJECTS.  EVERY WORKBENCH NEEDS A SIGNAL GENERATOR FOR
INJECTING AN AUDIO SIGNAL TO SET LEVELS IN A TRANSMITTER, OR FOR EVALUATING
RECEIVER SENSITIVITY AND DYNAMIC RANGE, AND SO ON.  THIS ARTICLE DESCRIBES
A DIRECT DIGITAL SYNTHESIS DDS BASED GENERATOR SEE THE LEAD PHOTO, WHICH
FEEDS A VARIABLE GAIN AMPLIFIER AND SWITCHED ATTENUATORS TO GENERATE
SIGNALS FROM 100 HZ TO 10R8 MHZ IN 1 HZ STEPS, AND AT POWER LEVELS FROM 120
DBM TO 0 DBM IN 0R2 DB STEPS.  YOU CAN BUILD THE GENERATOR WITH A COLOR LCD
TOUCHSCREEN INTERFACE, OR WITH A USB INTERFACE FOR COMPUTER CONTROL.  IN
EITHER CONFIGURATION, CONSTRUCTION USES ALL THROUGH HOLE COMPONENTS.  A
PRINTED CIRCUIT BOARD PCB IS AVAILABLE TO MAKE ASSEMBLY EASY.  ALL
COMPONENTS ARE READILY AVAILABLE FROM ONLINE RETAILERS.  THE PC CONTROLLED
VERSION CAN BE BUILT FOR ABOUT 100, AND THE LCD VERSION FOR ABOUT 50 MORE.
THE DESIGN  THE BLOCK DIAGRAM FIGURE 1 SHOWS THE SIGNAL FLOW THROUGH THE
GENERATOR.  THE DDS 25 MODULE HAS A 25 MHZ OSCILLATOR AND AN ANALOG DEVICES
AD9833 DDS CHIP ALREADY MOUNTED AND TESTED.  A SERIAL PERIPHERAL INTERFACE
SPI BUS CONTROLS THE DDS, WHICH CAN GENERATE SIGNALS FROM 100 HZ TO 10R8
‰  END OF 40 WPM TEXT  ‰  QST DE W1AW  ƒ