ACE9030MSLASHIWSLASHFP2Q datasheet(32/39 Pages) MITEL

32 / 39 page

ACE9030

The value of SF can be found from any channel but to get

a quick estimate the highest frequency can be considered as

there is a fixed upper limit on CN of 256 so:

Putting suggested typical values into equation (7);

TOT(max) = 10000, CN(max) = 250, and MOD = 8, and then

assuming the current flows for the whole comparison period

the current to be multiplied by ACC is Ibo / 320. The typical Ibo

is only 1

µA so this is a very small current of around 3 nA and

would be too small to control accurately and certainly too small

for production testing.

The error signal to be cancelled is a narrow pulse at the

comparison frequency so the best cancellation of the whole

spectrum of the error is also a narrow pulse. It is not practical

to generate a variable width pulse to match the error pulse but

a fixed width variable amplitude pulse is possible and it can be

timed to approximately coincide with the error pulse to give

good cancellation.

The compensation current amplitude is also increased by

gating it with a small time window and in ACE9030 the gate is

set to two cycles of the reference clock which straddle the

active edge of the comparison frequency signal to the phase

comparator. The total reference division from reference clock

to comparison frequency is the programmable divider set by

NR in Word D multiplied by 1, 2, 4, or 8 as selected by the SM

bits also in Word D and this total may be called R

MAIN, so for the

compensation current the scaling is R

MAIN / 2.

The charge needed is still as in equation (7) but the

current can be defined as:

where, in ACE9030, the compensation reference current Ico

is set by an external resistor RSC such that:

but this Ico must be chosen to cancel the error charge in

equation (7), and the scaling effect of 2 reference cycles in

MAIN has been derived above, giving:

then removing SF to help evaluate the values needed:

this can then be further processed by replacing R

MAIN and

TOT(max) by the frequency ratios:

then when substituting these into equation (9) the f

COMP terms

cancel leaving:

RMAIN =

and NTOT (max) =

RMAIN

For a typical AMPS cellphone the f

VCO(max) for 45 MHz

I.F. is 938·97 MHz, f

CRYSTAL is 14·85 MHz, MOD is 8 and

CN(max) can be assumed to be chosen around 200, giving Ico

= 0·198 x Ibo.

Fractional-N Mode with Speed-Up

When Speed-up is active the main proportional charge

pumps are run at an increased current and the integral charge

pumps are switched on to move the loop filter voltage faster.

The phase errors due to Fractional-N mode will be the same

as normal once the loop is locked so the compensation pulses

must be increased to match the proportional and integral

charge pump currents in order to allow a smooth change over

to normal mode at the end of Speed-up time. The same 2L + 1

and K coefficients as used for the proportional and integral

charge pump currents are used on the compensation currents

so from equation (8):

Normal Mode:

Proportional Compensation Current:

Icomp(0) = ACC x Ico

Integral Compensation Current:

none = off

Speed-up Mode:

Proportional Compensation Current:

Icomp(1) = 2L + 1 x ACC x Ico

Integral Compensation Current:

Icomp(2) = K x 2L + 1 x ACC x Ico

Required Accuracy of Compensation

With the compensation scheme used in ACE9030 it is not

possible to get perfect cancellation of the loop disturbance by

the Fractional-N system due to the mis-match of the pulse

shapes leaving some high frequency terms, but if the areas

are matched there will be complete removal of the low fre-

quency components and the loop filter can be assumed able

to remove higher frequencies.

Typical timing waveforms for the phase error and its

compensation are shown in figure 28 for a loop operating in

8’s mode (hence MOD = 8), with a VCO at 1 GHz, and a

comparison frequency of 100 kHz (hence N

TOT = 10,000 and

COMP period = 10 µs) so that each phase error can be found

from equation (4) as:

(ACC x 10

µs) / (10,000 x 8) = ACC x 0·125 ns.

If the reference is a 12·8 MHz crystal, it gives a correction

pulse

duration,

two

reference

cycles,

2/(12·8 MHz) = 156 ns.

If the charge pump current of 250

µA is set by a CN value

of 250 the reference current Ibo from equation (6) is 1

µA and

the compensation step current Ico can be found from equation

(10) as 0·2 x Ibo = 0·2

µA.

Areas of the error and the compensation pulses, equa-

tions (4) and (8) must match to get good low frequency

cancellation. Although shown as a very narrow pulse on Ø

DOWN

the phase error will often appear as a change in size of the

pulses on either Ø

DOWN or ØUP which occur to maintain lock.

The following calculations would then apply to the changes

and give the same final result.

If there was no compensation the Ø

DOWN pulses would give

sidebands at a level set by the loop filter capacitor values and

the VCO gain.

In a typical system the filter proportional capacitor can be

6·8 nF and the VCO could cover 30 MHz in 3 V, giving

10 MHz/V. Assuming for the moment that all error pulses are

the same at the level of a mid-range ACC value, say 4, and do

SF =

CN(max)

NTOT(max)

Icomp (0) = ACC x Ico .....(8)

Ico =

IRSC

320

Ico =

x Ibo

MOD x NTOT(max)

Ico =

x Ibo ....(9)

CN (max)

RMAIN

fCRYSTAL

fCOMP

fvco (max)

fCOMP

MOD x fVOC (max)

Ico =

x Ibo ....(10)

CN (max)

fCRYSTAL

MOD

ACE9030M/IW/FP2Q Fiches technique(PDF) 32 Page - Mitel Networks Corporation

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