A dual modulus prescaler is an electronic circuit used in high-frequency synthesizer designs to overcome the problem of generating narrowly-spaced frequencies that are nevertheless too high to be passed directly through the feedback loop of the system. The modulus of a prescaler is its frequency divisor. A dual-modulus prescaler has two separate frequency divisors, usually M and M+1.
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... Today, most dual-modulus prescalers exist inside of PLL chips, making it impossible to probe actual signals during operation ... The first dual-modulus prescalers were discrete ECL devices, separate from the PLL chips ... Here is an example of a dual-modulus prescaler in use ...