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
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