Hi techie,
Reading in the Clayton Paul's book "Introduction to EMC", recently, I found one possible reason why somebody would want to have higher frequency instead lower. Say there are 2 bus lines on the board, both with 10 MHz freq. In that case there will be two harmonics from each line at 20MHz, 30MHz, etc., and the amplitudes of the harmonics will be adding (if they have same phase) increasing noise. Suppose none of the frequency could be decreased because of functional reasons. Then we would prefer to increase one of them to say 15 MHz, and have the harmonics not adding one to the other.
Other than this I thought, the target is to use the lowest frequency allowed by functional requirements.
nike