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 is generally restricted to integer values, as the comparator will match when the waveform is in phase.

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 fo is required to be in the hundreds of MHz range.

Interposing a fixed prescaler that can operate at this frequency range with a division ratio M of, say, 40 drops the output frequency into the operating range of the programmable divider.

Alternatively, if fr is reduced by a factor of 40 to compensate; it becomes 312.5 Hz, which is much too low to give good filtering and lock performance characteristics.

So while we still have a factor of M being multiplied by N, we can add an additional count, A, which effectively gives us a divider with a fractional part.

The diagram below shows the elements and arrangement of a frequency synthesizer with a dual-modulus prescaler.

For this to work properly, A must be strictly less than M, as well as less than or equal to N. These restrictions on values of A imply that you can't get every division ratio V. If V falls below M(M - 1), some channels will be missing.

Today, most dual-modulus prescalers exist inside 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.

This circuit happens to use a Motorola MC145158 with a Fujitsu MB-501 dual-modulus prescaler operating in the 128/129 mode.

The PLL is locked at 917.94 MHz (fo) with a channel spacing frequency of 30 kHz (fr).

The result of this frequency choice is that the prescaler spends most of its time counting at 128 and just a brief period at 129.

This is shown by the upper purple trace, the modulus control, A, counter output.

It is plainly obvious that the modulus control is low for precisely 6 cycles of the prescaler output.

What is not obvious is the fact that the frequency changes by less than one percent between the two states of the modulus control.