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Output atenuator is set with 3 MACOM AT220. AT220 is programable atenuator having 2 dB steps up to -30 dB. It is placed after DDS output amplifier. Maximum atenuation is -90 dB. Schematics of one section:
PCB dimensions are 67 x 45 mm. There is 21 SMD bridges. -5 V for pin 7 is given on back side by wire bridge. PCB is doublesided. Grounds are connected by a piece of wire.
Components placement:
Photo:
For first two 74HC00 inputs are set to provide -10 and -20 dB atenuation. Third circuit is used to set -2, -4, -8 and -16 dB. This arrangement allows to set up to -90 dB in 2 dB steps. Resistors 1.1 Ohm are used (not critical).
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