ACE9030
COMPENSATION
CHARGE PUMP
TIME GATING
SET CURRENT
Ico
COMPENSATION
DAC
CHARGE TO
LOOP FILTER
COMPARISON
FREQUENCY
ACCUMULATOR
MODULO-5 OR 8
OVERFLOW
(TO PRESCALER
MODULUS CONTROL)
FRACTION
NF
REGISTER
Fig. 25 Fractional-N Add-On To Main Synthesiser
and so increment the total division.
when none are needed. Similarly in 1/8’s mode if 1/4 ( and 3/4 )
or /2 fractions are set then the LSB or two LSB’s in the
accumulator will be forced to zero to relax the tolerance on the
compensation.
1
To avoid loop modulation due to the accumulating phase
shift at the fractional frequency, a compensation charge must
also be driven onto the loop filter to track the accumulated
phase error so a current Icomp(0) is output on PDP for a fixed
short duration. Icomp(0) is given by Icomp(0) = ACC x Ico,
where ACC is the value of the phase accumulator and Ico is
a current set by a resistor on pin RSC such that Ico = IRSC/320.
InatypicalsystemtherequiredvalueofIcoisbetween1/10 and
1/3 of Ibo and thus the resistor on RSMA has a value between
one and three times the value of the resistor on RSC. As with
RSMA, the pin RSC does not need any decoupling and to
avoid all possibilities of oscillation the external capacitance
shouldbelessthan5pF. Forafulldescriptionofthismodesee
the later section “Detailed Operation of Fractional-N Mode”.
In fractional-N mode a zero fraction numerator, in both
1/5’s and 1/8’s mode, will force the accumulator to zero and not
simply leave it at an arbitrary fixed value. This feature makes
testing easier by setting the accumulator to a known state, but
isalsousefulinoperationbystoppingallcompensationpulses
MainSynthesiser-Fractional-NModewithSpeed-up
In Fractional-N mode with Speed-up the normal compen-
sation current is increased by the same factor (2L+1) as the
main charge pump current to give Icomp(1), and at the same
time the integrating capacitor is also driven with a compensat-
ingchargebyacurrentIcomp(2)settoIcomp(1)xK.Thisextra
compensation is included so that the loop will step to exactly
thedesiredfrequencyduringtheSpeed-upphaseandthenbe
on channel when normal Fractional-N begins.
Main Synthesiser Charge Pumps
The charge pumps for all modes are shown together in
figure 26. The charge pumps on pin PDP are used normally to
hold a channel and are also used in speed-up modes at a
To VCO
VDD
VDD
Icomp(0) =
ACC.Ico
Icomp(1) =
x2, 4, 8, 16
φ
Icomp2 =
K.Icomp(1)
Fc
UP
Icomp(0) =
CN.lbo
Iprop(1) =
x 2, 4, 8, 16
φ
Fc
UP
PDP
Ri
lint =
K.lprop(1)
φ
DOWN
Cp
PDI
φ
Ci
DOWN
VSS (GND)
INTEGRAL
CHARGE PUMP
EXTERNAL,
PASSIVE
LOOP FILTER
Cp, Ri, Ci
FRACTIONAL-N
FRACTIONAL-N
COMPENSATION
CHARGE PUMP
(PROPORTIONAL)
PROPORTIONAL
CHARGE PUMP
COMPENSATION
CHARGE PUMP
(INTEGRAL)
Fig. 26 Main Synthesiser Charge Pumps
27
MITEL [ MITEL NETWORKS CORPORATION ]
