Simple FSK modulator enables data transmission over low-speed link

摘要

FSK (frequency-shift keying) is a type of signal modulation for transmitting digital data over an analog communication link. An FSK modulator comprises a digitally controlled sine-wave generator whose frequency shifts between two predetermined frequencies in response to the two logic levels of the digital data. The circuit in Figure 1 generates a sine wave by continuously sampling a single sine cycle. The output of IC_(2A) is proportional to the currents through R_1, R_2, and R_3, These resistors connect together at one end to the inverting input of IC_(2A), which is biased at V_(CC)/2. Thernoutputs GP0, GP1, and GP2 of microcontroller IC_1 produce nonoverlapping pulse trains. When you set either output high or low, the others are off-that is, at high impedance. When you set an output high, the voltage across the resistor that connects to it is V_(CC)/2. When you set the output low, the volt-rnage across the resistor is -V_(CC)/2.