NJ88C50
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
to the synthesiser's phase detector may be expressed as N
TOT
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)
N
TOT
= N1.R1 + N2.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)
N
TOT
= N1.R1 + N2.R2 + N3.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)
N
TOT
= N1.R1 + N2.R2 + N3.R3 + N4.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.
Fig.5 Loop filter using both charge pumps
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.
4
MITEL [ MITEL NETWORKS CORPORATION ]
