How can a double-sideband phone signal be produced?

A double-sideband phone signal is produced primarily through the process of amplitude modulation (AM), where both the upper and lower sidebands carry the same information. The choice related to modulating the supply voltage to a class C amplifier is correct because class C amplifiers are designed to operate efficiently in applications such as AM transmissions. By changing the voltage applied to the amplifier, you can effectively modulate the amplitude of the RF carrier wave, allowing for the generation of a double-sideband modulated signal.

This method takes advantage of the characteristics of class C amplifiers, which produce a signal that is inherently capable of generating tones at frequencies that can be transmitted effectively in double-sideband mode. Thus, the modulation process using a class C amplifier leads to the creation of a double-sideband signal necessary for voice communications in amateur radio.

The other options suggest methods that do not inherently generate double-sideband signals or confuse modulation techniques. For instance, using a crystal oscillator mainly provides a stable frequency but does not directly produce double-sideband modulation. Frequency modulation techniques, while effective for certain kinds of signals, produce a different type of transmission known as frequency modulation (FM), which does not result in double-sideband amplitude modulation. Lastly, switching between AM and FM modes does