Dual-modulus prescaler

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.

A frequency synthesizer produces an output frequency, f_o, which divided by the modulus is the reference frequency, f_r:

${\displaystyle {\frac {f_{o}}{N}}=f_{r}\Rightarrow f=Nf_{r}}$

The modulus, N, is generally restricted to integer values, as the comparator will match when the waveform is in phase. Typically, the possible frequency multiples will be the channels for which the radio equipment is designed for, so f_r will usually be equal to the channel spacing. For example, on narrow-band radiotelephones, a channel spacing of 12.5 kHz is typical.

Suppose that the programmable divider, using N, is only able to operate at a maximum clock frequency of 10 MHz, but the output f_o is in the hundreds of MHz range; . Interposing a fixed prescaler, which can operate at this frequency range, with a value M of say, 40, drops the output frequency into the operating range of the programmable divider. However, a factor of 40 has been introduced into the equation, so the output frequency is now:

${\displaystyle f_{o}=40Nf_{r}}$

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