Stanford Research Systems DS345, Manual

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DS345 Circuitry
8-7
signal inputs) to help cancel out the 2f component in the output. The network
of resistors connected to the multiplier X inputs serves to offset the control
voltage so that the compliance of the multiplier inputs is not approached,
minimizing distortion.
For square-wave outputs, the DDS sine-wave output is buffered by Q700 and
Q701 and discriminated by comparator U704. This method is used to gener-
ate square-waves because the DDS output cannot contain frequency compo-
nents higher than half of the sampling clock frequency. The fast edges on
the square-wave output must contain very high frequency components in or-
der to maintain the square-wave pulse shape. The output of U704 is level-
shifted by differential pair Q702 and Q703. U703 serves as a reference for
Q705, the current source for Q702/Q703. Comparator U704 always runs to
provide a front panel sync output via U313F, which buffers the sync signal to
the bottom board. Current source Q705 is shut down to disable the square-
wave output when not in use via Q704, which also actuates relay U700 to de-
select the square-wave output.