The figure shows the principle of the measure that can be obtained from this software approach.

Basically the PC commands the variable frequency generator, a DDS unit (1), the output (2) of the generator is connected to the device under test (DUT). The output from the DUT (3) is detected and converted into a digital stream by an AD/C device The samples are sent back (4) to the program that plots the graph on the PC screen.

Two different hardware configurations can be interfaced by the program:

• Option A: the hardware is connected to the PC via the parallel port (LPT). The program directly drives the DDS unit and reads the output from the AD/C.
• Option B: the DDS unit is driven by a PIC. The program interfaces via the USB/serial port the PIC sending commands and receiving back the samples acquired by the PIC. The AD/C function is implemented into the PIC.