AN827 Datasheet | Motorola Semiconductor

MOTOROLA SEMICONDUCTOR TECHNICAL DATA Order this document by AN827/D AN827 The Technique of Direct Programming by Using a Two-Modulus Prescaler Prepared by: PLL Applications The theory in this application note is still applicable, but some of the products referenced may be discontinued. INTRODUCTION The MC12009, MC12011, or MC12013 can be used as part of a variable modulus (divisor) prescaling subsystem used in certain Digital Phase- Locked Loops (PLL). More often than not, the feedback loop of any PLL contains a counter- divider. Many methods are available for building a divider, but not all are simple, economical, or convenient in a particular application. The technique and system described here offer a new approach to the construction of a phase- locked loop divider. In addition to using either the MC12009, MC12011, or the MC12013 variable modulus prescaler, this system requires an MC12014 Counter Control Logic Function, together with suitable programmable counters (e.g., MC4016s or SN74LS716s). Data sheets for these additional devices should be consulted for their particular functional descriptions. quency) to give a new reference frequency: channel spacing/P. Another solution is found by considering the defining Equation 1 for fout of Figure 1. From the equation it may be seen that only every P channel can be programmed simply, because N is always an integer. To obtain intermediate channels, P must be multiplied by an integer plus a fraction. This fraction would be of the form: A/P. If N is defined to be an integer number, NP, plus a fraction, A/P, N may be expressed as: N = NP + A/P. Substituting this expression for N in Equation 1 gives: or fout = (NP + A/P) - P - fref fout = (NP P + A) - fref (2) (3) fout = NP - P - fref + A - fref. (4) Equation 4 shows that all channels can be obtained directly if N can take on fractional values. Since it is difficult to multiply by a fractional number, Equation 4 must be synthesized by some other means. fout = N - P -...