A transmitter shifts a baseband data signal having a bandwidth of 2.omega..sub.M using a tone signal having a frequency .omega..sub.M, which centers a sideband of the data signal at zero frequency. The transmitter then filters the shifted signal to filter out the non-centered sideband, and then adds the tone signal to form a composite signal. The transmitter upconverts the composite signal during a first operational mode using a RF signal having a frequency .omega..sub.C equal to a center frequency of a channel. In a second operational mode, the transmitter upconverts a DC level using the same RF signal. The upconverted DC level can be used as a continuous wave carrier signal having a frequency .omega..sub.C. The transmitter has a single RF oscillator to output both a SSB signal centered at frequency .omega..sub.C during the first mode and a carrier signal of frequency .omega..sub.C during the second mode.