68HC11 Function Generator:

Circuit makes simple FSK modulator: 10/11/2001 EDN - Design Ideas / The need for a compact telemetry system poses a challenge for designing a small, light, low-component-count system. Interfacing serial data from the microprocessor is also difficult because most low-cost RF transmitters do not accept dc levels at the input. Commercial FSK (frequency-shift-keying) modulators are bulky and need many passive components. .