So here's an update on the design of an Arduino Shield SDR RX.
First the AD9850 is no good, as it has not got 50:50 ratio output square waves. I need 50:50 to make my clock generator, which only divides by /2 not /4 of other designs - thus he VFO is half the frequency... and the Si570 is no good as it does not have complementary outputs. But the Si514 does, and this seems a good choice.
This is my circuit. The 7474 flip-flops and driven by complementary clock inputs and give quadrature output for the SDR switches (FST3253).
The complete circuit is this: