‰  NOW 30 WPM  ‰  TEXT IS FROM MAY 2015  QST  PAGE 37 ‰

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
‰  END OF 30 WPM TEXT  ‰  QST DE W1AW  ƒ