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| przy fractional N nie musimy trafiać dokładnie w raster bo i N nie jest całkowite. tak naprawdę nie wyskakując poza pętlę n zmienia się pomiędzy N a N+1 dzięki temu mamy niecałkowity podział "frctional-N" Tak działają te nowe syntezy na ADF'ach i Si5351 http://www.ti.com/lit/an/swra029/swra029.pdf Fractional-N architecture is very similar to integer-N, with the addition of an accumulator.The accumulator is a simple state machine that changes the main divider value (betweenN and N+1) during a locked condition. The accumulator varies the divide numberbetween N and N+1 dynamically in such a way as to provide an averaged divide ratiothat is a fractional number between N and N+1. This function enables the generation ofan average division that is a fractional number.For example: If N = 2000 and Fr = .48 MHz, the output frequency is as follows Fvco = 2000·(.48) = 960 MHz.In order to achieve 30 kHz resolution (generate 960.03 MHz for example), theaccumulator must dynamically change N from 2000 to 2001 one out of every 16reference cycles. Thus the division will be set to 2000 for 15 cycles of Fr and set to 2001for one cycle of Fr. Therefore, the average division will be 2000+1/16.(15·2000+1·2001)/16 = 2000+1/16. Here F = 16 and K = 1.So far, we have created a fractional divider. However, rather than generating a smoothN+K/F ratio, it was done in an abrupt manner. This will cause spurious signals in theoutput. A spurious compensation circuit is added to reduce these spurious signals to apractical minimum. |