A simple microcontroller based building block for digital communication applications

This paper describes the design of a simple building block for applications in the area of digital communication. The simple building block functions mainly as a digitally programmable oscillator (DPO) extendable to work as a BFSK modulator. The DPO is designed using an LM 565 PLL chip, the centre frequency of the VCO within which is controlled by an 8031 microcontroller using binary weighted switched programmable resistor array (BWSPRA) implemented by CMOS CD 4066 switches and binary weighted resistors. Good quality square and triangular waves were obtained from the block whose frequencies in the range of 90 kHz to 680 kHz were varied digitally. The incorporation of a sine shaper with the simple building block along with a buffer and amplifier provided sinusoidal DPO. In the design extension of the simple sinusoidal DPO to BFSK modulator the mark and the space frequencies were obtained by effectively switching a single resistor. The digital base band signal was a PN sequence of length 7 generated by the 8031 microcontroller. An assembly language program had been developed. Good quality BFSK output was obtained with good phase continuity.