It is possible to get 9dB more power out of the AD9850 and AD9851 DDS chips
by using a 2+2:1 centre tapped transformer from both Iout pins with the
centre tap to ground to complete a DC path. The output from the transformer
is then at 50 ohms making filter design rather easier. This way 7dBm in the
HF to VHF range is available - enough to drive a diode ring mixer without
any extra gain stage. The push-pull drive also reduces some spurious levels
- particularly clock frequency leakage, which makes bandpass filtering the
image products more feasible.
The circuit details are given in the AD9851 data sheet available at
www.analog.com
Andy G4JNT
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