Andy,

Not being able to control the amplitude of the AD9850/51 DDS chip output
waveform makes it difficult to generate decent signalling waveforms.
However, this is possible by changing the current into the Rset, pin 12,
which sets the full scale amplitude of the D/A output.  On my DDS board

this

current is set by an innocuous 3.9k resistor.  Replacing this with a FET
acting as a variable resistor allows a DC level to control the amplitude

for

non critical applications - such as bandwidth limiting a signal by

envelope

shaping.

Note also that this makes a good place to key the output on and off without
resorting to software. I have a FET in series with the original 3.9k
resistor, and keying that eliminated a "backwave" problem when I keyed
subsequent stages.

John Andrews, W1TAG