NJ88C50

4

The main N divider is programmable so that it can

determine how many cycles of each division ratio the external

prescaler will perform.

The total division ratio of the output from the system VCO

and R1, R2, R3 and R4 are the available prescaler ratios and

N1, N2, N3 and N4 are the corresponding number of cycles for

each ratio selected, within one complete division cycle.

The divider is programmed via the serial data bus and the

values needed to be programmed for each of the possible

prescaler ratios are as follows:-

In 2 modulus mode (division ratios R1, R2)

Programmed values needed:

N1 - a 12 bit value giving the number of times R1 is to be used

N2 - a 8 bit value giving the number of times R2 is to be used

In 3 modulus mode (division ratios R1, R2, R3)

Programmed values needed:

N1 - a 12 bit value giving the number of times R1 is to be used

N2 - a 4 bit value giving the number of times R2 is to be used

N2+N3 - a 4 bit value where N3 is the number of times R3 is

to be used and (N2+N3) is modulo-16 addition

In 4 modulus mode (division ratios R1, R2, R3, R4)

Programmed values needed:

N1 - a 12 bit value giving the number of times R1 is to be used

N2 - a 4 bit value giving the number of times R2 is to be used

N2+N3 - a 4 bit value where N3 is the number of times R3 is

to be used.

N2+N3+N4 - a 4 bit value where N4 is the number of times R4

is to be used. (N2+N3) and (N2+N3+N4) are modulo-16

addition.

To facilitate the use of multimodulus prescalers the N

divider is based upon a twelve bit up/down counter which

functions as follows

The first value, N1, is loaded into the counter which then

counts down from N1 to zero. During this time, the modulus

ratio R1 is selected.

When the counter reaches zero modulus R2 is selected

and the counter then counts up to the N2 value. If 2 modulus

operation is chosen, the counter is then reloaded with N1 and

the count is repeated.

For operation with 3 or 4 modulus devices, the counter

continues to count up once it has reached the N2 value. The

count continues to the N2+N3 value and during this time the

R3 ratio is selected. In the 3 modulus case, when the N2+N3

value is reached the counter is then reloaded with the N1

value and the modulus ratio R1 is selected.

For 4 modulus operation the counter will continue its count

up to the N2+N3+N4 value before reloading the N1 value.

During this time the R4 modulus is selected.

If N2, N3, or N4 are set to zero this will give a full count of 16

for the corresponding modulus.

The N divider block also has a special control line from the

Fractional-N logic. When required this control will cause the

total division ratio to be increased from N to N+1. This is

achieved by forcing a cycle which would have normally used

a prescaler ratio R1 to use ratio R2 instead. R1 and R2 are

chosen so that R2 equals R1+1.

Further explanation of the operation of the synthesiser

when using 2, 3 or 4 modulus prescaler is given in the section

on multimodulus division (page 8).

The phase detector used on the main synthesiser is

similar to the type used on the auxiliary synthesiser (Figure.3).

In this case, however, the detector will drive two pairs of

complimentary charge pumps, one of which is intended to

drive the loop integrator capacitor to provide integral control,

whilst the other provides proportional control for the VCO.

This system is shown in Fig 5, and has applications where fast

locking of the loop is required.

MODES OF OPERATION

Normal Mode

The synthesiser will operate in normal mode while the

strobe line of the serial data bus is low. In this mode the

following current levels are produced. The charge pump

providing the proportional feedback term will have a normal

current level designated by Iprop(0), that is set by an external

bias resistor, RSM. Iprop(0) will vary when different N-divider

ratios are programmed, so that it is proportional to the total

division ratio. To avoid the necessity of computing the total

division ratio on chip, an eight bit number representing the

most significant bits of Ntot will be loaded via the serial data

bus. Iprop(0) is therefore given by

Iprop(0) = CN.Ibo

where CN is the loaded eight bit number and the value Ibo is

scaled from the external current setting resistor RSM where

Ibo = Irsm/32. Typically Ibo = 1µA ,and therefore Iprop(0) will

have a maximum value equal to 255µA.

Fig.5 Loop filter using both charge pumps