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NJ88C50IG Datasheet(PDF) 6 Page - Mitel Networks Corporation |
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NJ88C50IG Datasheet(HTML) 6 Page - Mitel Networks Corporation |
6 / 15 page NJ88C50 6 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 Q 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 Q Ntot(max) 2 This gives a typical value for Ico of 0.1µA. The two values of Icomp, Icomp(0) and Icomp(1) are related by 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. Fig.6 |
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