What stage in a transmitter would change a 5.3-MHz input signal to 14.3 MHz?

A mixer is a circuit that combines two input frequencies to produce output frequencies equal to their sum and difference. Since 14.3 MHz is not an exact integer multiple of 5.3 MHz, a frequency multiplier cannot be used. Instead, a mixer combines the 5.3 MHz input with a 9.0 MHz local oscillator signal to produce the desired sum of 14.3 MHz. A frequency multiplier only produces exact harmonics, such as 10.6 MHz (x2) or 15.9 MHz (x3). A beat frequency oscillator (BFO) is a receiver component used to demodulate CW or SSB signals by creating an audio beat note, not a transmitter stage used for large frequency conversions.