ACE9030
27
TIME GATING
SET CURRENT
OVERFLOW
COMPENSATION
CHARGE PUMP
CHARGE TO
LOOP FILTER
(TO PRESCALER
MODULUS CONTROL)
COMPENSATION
DAC
ACCUMULATOR
MODULO-5 OR 8
FRACTION
REGISTER
NF
COMPARISON
FREQUENCY
Ico
PROPORTIONAL
CHARGE PUMP
FRACTIONAL-N
COMPENSATION
CHARGE PUMP
(INTEGRAL)
FRACTIONAL-N
COMPENSATION
CHARGE PUMP
(PROPORTIONAL)
EXTERNAL,
PASSIVE
LOOP FILTER
Cp, Ri, Ci
INTEGRAL
CHARGE PUMP
PDI
Cp
PDP
Ri
Ci
Fc
Fc
To VCO
VDD
Icomp(0) =
ACC.Ico
Icomp(1) =
x2, 4, 8, 16
Icomp2 =
K.Icomp(1)
lint =
K.lprop(1)
DOWN
UP
VDD
UP
Icomp(0) =
CN.lbo
Iprop(1) =
x 2, 4, 8, 16
DOWN
VSS (GND)
φ
φ
φ
φ
Fig. 25 Fractional-N Add-On To Main Synthesiser
and so increment the total division.
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 = I
RSC
/320.
In a typical system the required value of Ico is between
/
10
and
1
/
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
should be less than 5 pF. For a full description of this mode see
the later section “Detailed Operation of Fractional-N Mode”.
In fractional-N mode a zero fraction numerator, in both
1
/
’s and
1
/
’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
is also useful in operation by stopping all compensation pulses
when none are needed. Similarly in
1
/
’s mode if
1
/
( and
3
/
)
or
1
/
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.
Main Synthesiser - Fractional-N Mode with Speed-up
In Fractional-N mode with Speed-up the normal compen-
sation current is increased by the same factor (2
L+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-
ing charge by a current Icomp(2) set to Icomp(1) x K. This extra
compensation is included so that the loop will step to exactly
the desired frequency during the Speed-up phase and then be
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
Fig. 26 Main Synthesiser Charge Pumps