Trying to make a more complete signal generator from a AD9850 DSS module:

One weakness is the absence of a tone, or tones.  Here is one possible solution ...

"... voltage control or amplitude modulate the output current of the AD9850 DDS using an enhancement mode MOSFET to replace the fixed RSET resistor; and a broadband RF transformer to combine the DDS DAC outputs to  produce a  symmetrical AM modulation envelope. Modulation with reasonable  linearity is possible at rates exceeding 50 kHz. The AD9850 DDS output current (20 mA maximum) is normally set with a fixed resistor from the RSET (Pin 12) input to ground. The DAC outputs are unipolar and complementary (180 degrees out of phase) of each other."

Source: Unknown