28

ACE9030

larger current. The charge pumps on PDI are only used in

speed-up and then drive the integrating capacitor Ci directly.

The control signals Ø

phase detector and have a duration proportional to the phase

error, the signal Fc is the Fractional-N compensation gating

pulse and has a fixed duration set by the crystal oscillator.

Nominal Charge Pump Currents

Shown in table 5, charge pump currents are set by the

resistors RSMA and RSC and by scaling coefficients loaded

from the serial bus. CN is an 8 bit number, L is 2 bit and K is

4 bit. Fractional-N compensation also depends on the instan-

taneous value ACC in the fractional accumulator, a 3 bit

number.

Two Modulus Prescaler Control

The Main ACE9030 synthesiser is designed to operate

with the two-modulus prescaler section of the ACE9020. This

allows channels to be spaced at the comparison frequency

while keeping the clock rates within the range possible in

CMOS. ACE9030 can also be used with standard two-modu-

lus prescalers such as SP8715.

The prescaler will have two division ratios, R1 and R2,

selected by a Modulus Control signal and designed to switch

quickly from one ratio to the other so that a sequence of R1 and

R2 can be used to give the required total division. It is usual to

have R2 = R1 + 1 and to select R1 by setting Modulus Control

to the HIGH state and R2 by a LOW state.

To reduce interference the Modulus Control output from

ACE9030 is a pair of differential signals MODMP and MODMN

each with a limited voltage swing but still able to drive selected

Pin

Charge pump

Normal

Speed-up

Maximum setting*

PDP

Main

Iprop(0) = CN x I

1·0 mA

PDI

Main

Off

Iint = K x Iprop(1)

5 mA

PDP

Fractional-N

Icomp(0) = ACC x I

12

µA

PDI

Fractional-N

Off

Icomp(2) = K x Icomp(1)

180

µA

PDA

Auxiliary

Iauxil = 8 x I

RSMA

no auxiliary speed-up

512

µA

* Larger values of current can be set by the programming numbers and the resistor values, but the circuit is not then guaranteed to give the

calculated current.

Table 5

standard prescalers from GEC-Plessey Semiconductors if

MODMP is used alone. Even if a prescaler with non-differen-

tial control input is used there could still be some benefit in

running a MODMN track on the circuit board beside the

MODMP track to partly cancel capacitive coupling to sensitive

nodes. Simplified waveforms are shown in figure 27.

Unlike many conventional synthesisers that use two

separate counters, to give both the total count, M, and the

portion for which the prescaler is at its higher ratio, A, there is

only one counter in the ACE9030 for both these functions. This

counter is loaded with the value N1, counts down to zero,

changes direction, counts up to N2, is again loaded with the

value N1 and changes direction to down, and so the cycle is

repeated indefinitely.

Fig. 27 Modulus Control Timing Diagram

N1 + N2

N1

...R1...

R1

R1

R2

...R2 ...

R2

R1

PRESCALER RATIO

FIM

FIMB

MODMP

MODMN

COUNTER OUTPUT TO

PHASE COMPARATOR

DELAY

T

SET-UP

T

N2