See the circuit! DSB transmitter has no need of a crystal filter. It does not need a frequency transverter. It is easy to make. And it can make QSO with normal SSB stations. Therefore DSB is best modulation for the amateur radio builder. The mic amplifier makes the audio signal stronger. The VXO oscillates a 10.6MHz signal. This is doubled by the amplifier of the TA7320 IC. These two signals are modulated by the DBM of the TA7320. There comes the 21MHz DSB signal. HF amplifier makes this signal to 250mW.