Block Schematic and Function Description
The figure shows the simplified block schematic of the system. For ease of portability the unit is powered and controlled from the USB port of a PC.
Two PIC18F14K50 microcontrollers are used to provide the functionality of the unit.
The first PIC microcontroller implements the following functions:
· Communicates with the host PC for enumeration as a USB to UART device
· Sets up the unit as a 500mA device
· Switches on power to the DC-DC converter
· Provides the complimentary square wave drive to the DC-DC converter
· Acts as a USB communication interface to the second PIC
· Provides the Amplitude reference for DAC2
· Provides the digital word for DAC1 giving the offset value.
The second PIC microcontroller implements the remaining DDS functions
· Direct Digital Synthesis (DDS) signal generation
· Sin/ triangle word output to DAC2
· TTL and 1 V square reference output
The DC-DC converter is a driven 1W, 5V to ± 12V operating at 100 kHz. The +12 V output is fed to a linear regulator to provide a variable +2 to 7 V output controlled by a variable potentiometer.
The DC-DC converter provides full protection to the host PC USB port as its outputs are short circuit and current limit protected.
The circuit under test can be rigged up on a standard breadboard and powered from the power sockets provided.