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Al's picture

The 60MHz limit is due to the maximum frequency of the AD9850 clock (125MHz @5v and 110MHz @3.3v). At 60MHz the output would be very distorted and therefore it's probably better (apart from cost) to use a better specified device.

I'm guessing the output filter does roll off high frequencies but it has the advantage of making the signal look less like a square wave at this point.