NJ88C50SLASHMASLASHNP datasheet(6/17 Pages) ZARLINK

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NJ88C50

However the alternation between the N and N+1 values

causes a ripple in the output frequency. This ripple is not

desirable in radio frequency synthesisers. This ripple or jitter

waveform is predictable from the pattern of N and N+1 values

and so can be cancelled.

The instantaneous accumulator value, A, is proportional to

the cumulative frequency error caused by ignoring the

fractional part during the periods of the divide by N. The

accumulator value, A, may therefore be used to generate a

waveform corresponding to the jitter waveform, that is then

used to cancel the jitter out of the phase detector. This jitter

compensation current pulse is equal to A.Icomp where Icomp

represents the step size as A is incremented.

Corresponding to the two alternative values of Iprop,

Iprop(0) and Iprop(1), Icomp will take the values Icomp(0) and

Icomp(1). Icomp is always pull-up, and the magnitude of its

steps for perfect jitter compensation are related to the value

of Iprop by the factors

0 , 1/Q.Ntot , 2/Q.Ntot , 3/Q.Ntot ........ Q-1/Q.Ntot

where Q = accumulator modulus in use (5 or 8)

Since

Iprop(0) = CN.Ibo

and CN is an approximation to Ntot apart from a scaling factor,

the value of Icomp(0) required becomes independent of Ntot

and its steps are

0 , 1/Q , 2/Q , 3/Q ........ Q-1/Q times Ibo.(scaling factor)

where scaling factor = Max. value of CN to be used

Corresponding max. value of Ntot

therefore

Ico = 1 x CN(max)

x Ibo

Ntot(max)

and

Icomp(0) = A.Ico

where Ico is scaled from the external current setting resistor

RSC.

Ico = Irsc/128.

Typically Ntot(max) might be 10000, with CN(max)=250

and Q=8, so the current step will be of magnitude Ibo/320.

Since Ibo is only 1 uA, this is a very small value; however this

value only applies if Icomp is a continuous current. Icomp

however will be a short current pulse coincident with the Iprop

pulse, in order to cancel jitter components over the widest

possible frequency range.

When the duty factor of Icomp is taken into account, its pulse

value may be increased accordingly. Icomp is therefore

generated as a pulse of fixed width equal to two periods of the

input reference clock frequency, with a timing that straddles

the active edge of the reference divider output pulse supplied

to the main phase detector, as shown below: (Fig 6).

Since the duty factor of Icomp is 2/M and depends on the

value of M programmed, it is possible to set the peak pulse

value of Icomp(0) by means of the external current setting

resistor RSC to correspond with the value of M intended, the

value of ‘scaling factor’ defined above, the accumulator

modulus Q and the value of Ibo set by the other current setting

resistor.

therefore

Ico = 1 x Nmax

x M x Ibo

Ntot(max)

This gives a typical value for Ico of 0.1

µA.

The two values of Icomp, Icomp(0) and Icomp(1) are related

Icomp(1) = 2L+1 .Icomp(0)

Icomp(0) occuring when the strobe line is low and Icomp(1)

occuring when the strobe line is high loading either WORDA

or WORDA2 (see programming section, page 8 and 9) .

Corresponding to the pull-up pulse Icomp(1) that is added

to the proportional charge pump pulse Iprop(1), there is also

a pull-up current pulse Icomp2 which is added to the integral

charge pump pulse Iint. This pulse Icomp2 only applies when

the stobe line is high (loading either WORDA or WORDA2).

When the strobe line is low there will be no Iint or Icomp2

pulses. The value of Icomp2 is given by

Icomp2 = Icomp(1).K

where K is a four bit number entered as part of the serial

programming data.

Figure 6

M cycles of the reference input frequency

Active edge

Reference to phase detector

Timing of Icomp pulse

2 reference frequency cycles

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