Dual-modulus prescaler

The Dual modulus prescaler is an electronic circuit used in high frequency frequency synthesiser 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 division ratio. Dual-modulus means that it can divide by two different factors, usually M and M+1. Which factor is applied is changed using a control input. By careful arrangement, the system automatically operates to solve the problem at hand.

The Problem

A frequency synthesiser produces an output frequency f, which is the reference frequency f_r x the division ratio N, that is

f = Nf_r

Since N is an integer, the output frequency must therefore be restricted to whole multiples of f_r. Typically, these 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 divider N is only able to operate at a maximum clock frequency of 10 MHz, but the output f is in the hundreds of MHz range. If we interpose a fixed prescaler with a value M of say, 40, we can drop the output frequency easily into the operating range of the divider N. However, we have now introduced a factor of 40 into the equation, so the output frequency is now:

f = 40Nf_r

If f_r remains at 12.5 kHz, we can only obtain every 40th channel, not very useful. Alternatively if we reduce f_r by a factor of 40 to compensate, the comparison frequency becomes 312.5 Hz, which is much too low to give good filtering and lock performance characteristics. It also means that programming the divider becomes more complex, as we need to only use those ratios that give us true channels, not the 1/40th of a channel that are now available!

The solution

The solution is the dual modulus prescaler. The main divider is split into two parts, the main part N and an additional divider A which is much shorter than N. Both dividers are clocked from the output of the dual-modulus prescaler, but only the output of the N divider is fed back to the comparator. Initially, the prescaler is set to divide by M+1. Both N and A count down until A reaches zero, at which point the prescaler is switched to a division ratio of M. At this point, the divider N has completed A counts. Counting continues until N reaches zero, which is an additional N-A counts. At this point the cycle repeats.

Thus:

f = f_r(M(N-A) + A(M+1)) which reduces to f = f_r(MN+A)

So while we still have a factor of M being multiplied by N, we can ADD an additional factor A, which effectively gives us a divider with a fractional part. Only the prescaler needs to be constructed from high-speed parts, and the reference frequency can remain equal to the desired output frequency spacing.

The diagram below shows the elements and arrangement of a frequency synthesiser with dual-modulus prescaler. (Compare with diagram on main synthesiser page).