# Fractional-N Frequency Synthesizer with Dual-modulus control

Discussion in 'General Electronics Chat' started by lkgan, Feb 4, 2010.

1. ### lkgan Thread Starter Member

Dec 18, 2009
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0
Hi everyone,

Figure 2.6 shows a basic fractional-N synthesizer which involves a dual-modulus prescalar, a circuit block that could divide by N or N+1 according to external control. If the prescaler divides by N for A output pulses of the VCO and N+1 for B output pulses, then the equivalent divide ratio is equal to:

A+B
____________
A/N + B/(N+1)

The value of expression can vary between N and N+1 in fine steps by proper choice of A and B, which results to the "fractional" division ratio N.f.

My question from the above explanation is:

It mention that the circuit block could only divide by N or N+1 according to external control, why is the formula seems like a combination of input A and B that's output from VCO which both divide by different ratio? How does it actually works. How a multiple modulus divider could result in a fractional frequency? I have find lots of information about Fractional-pll, but it seems not easy to understand. Can someone please explain to me in an easier manner? I really need a discussion with the members in this forum. Thanks......

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Dec 18, 2009
117
0