Most of the circuits we have built over the last few months have been used for radio reception or simple test equipment. This month, we will re-use some of the modules from previous projects to build an SSB exciter. SSB (single sideband) is a form of AM (amplitude modulation). Figure 1A shows the spectrum of a full carrier AM signal where the carrier is modulated by a 1 kHz tone. The carrier frequency is 3.700MHz. The upper and lower sideband signals are at +/- 1 kHz. If the modulation level is 100%, the power in either sideband is 25% of the carrier power. Figure 1B shows the spectrum of a voice-modulated AM transmitter. If a low pass filter is used to limit the bandwidth of the speech signal to a maximum of 3kHz, the transmitted AM signal will have a minimum bandwidth of 6kHz. This assumes that we are using an ideal AM transmitter that generates no distortion or noise. A real transmitter will always produce unwanted distortion that will increase the bandwidth of the transmitted signal.
展开▼
