The RF output is filtered using a 7 pole low pass filter then buffered with an ERA-3 amplifier. The output amplitude is adjustable by varying the DDS output current - this is controlled by varying the Rset value using a potentiometer.
The CPU and optical circuitry was mounted on vero-board attached to the back of the LCD. This is held in place with double-sided foam tape. Surface mount resistors and capacitors were fitted to the copper side of the board, leaving only the semiconductors to be mounted on the top side.
The step and lock switches are conventional through-hole push button switches that have been mounted to vero board using a surface mount technique. The board was then glued to the back of the enclosure. This allows the switch shaft to protrude slightly through the front panel. By coating the front panel in a clear plastic, these switches then function as membrane switches - pressing lightly on the panel activates the switch.
The DDS chip was soldered to a custom board and mounted inside a small enclosure made from double-sided PCB material. The DDS board is double-sided, with the crystal oscillator module and power supply mounted on the bottom side, whilst the DDS IC, low pass filter, and ERA amplifier are mounted on the top layer. The photo shows the internal view before the top lid was soldered in place.
Last update: 04 June 2008
© Steven J. Merrifield