Datasheet

Data Sheet ADF4360-3
Rev. D | Page 9 of 24
CIRCUIT DESCRIPTION
REFERENCE INPUT SECTION
The reference input stage is shown in Figure 10. SW1 and SW2
are normally closed switches. SW3 is normally open. When
power-down is initiated, SW3 is closed, and SW1 and SW2 are
opened. This ensures that there is no loading of the REF
IN
pin
on power-down.
04437-010
BUFFER
TO R COUNTER
REF
IN
100kΩ
NC
SW2
SW3
NO
NC
SW1
POWER-DOWN
CONTROL
Figure 10. Reference Input Stage
PRESCALER (P/P + 1)
The dual-modulus prescaler (P/P + 1), along with the A and B
counters, enables the large division ratio, N, to be realized
(N = BP + A). The dual-modulus prescaler, operating at CML
levels, takes the clock from the VCO and divides it down to a
manageable frequency for the CMOS A and B counters. The
prescaler is programmable. It can be set in software to 8/9,
16/17, or 32/33 and is based on a synchronous 4/5 core. There is
a minimum divide ratio possible for fully contiguous output
frequencies; this minimum is determined by P, the prescaler
value, and is given by (P
2
− P).
A AND B COUNTERS
The A and B CMOS counters combine with the dual-modulus
prescaler to allow a wide range division ratio in the PLL feed-
back counter. The counters are specified to work when the pre-
scaler output is 300 MHz or less. Thus, with a VCO frequency
of 2.5 GHz, a prescaler value of 16/17 is valid, but a value of 8/9
is not valid.
Pulse Swallow Function
The A and B counters, in conjunction with the dual-modulus
prescaler, make it possible to generate output frequencies that
are spaced only by the reference frequency divided by R. The
VCO frequency equation is
( )
RfABPf
REFIN
VCO
/×]+×[=
where:
f
VCO
is the output frequency of the VCO.
P is the preset modulus of the dual-modulus prescaler (8/9,
16/17, and so on).
B is the preset divide ratio of the binary 13-bit counter (3 to 8191).
A is the preset divide ratio of the binary 5-bit swallow counter (0 to 31).
f
REFIN
is the external reference frequency oscillator.
N = BP + A
TO PFD
FROM VCO
N DIVIDER
MODULUS
CONTROL
LOAD
LOAD
13-BIT B
COUNTER
5-BIT A
COUNTER
PRESCALER
P/P+1
04437-011
Figure 11. A and B Counters
R COUNTER
The 14-bit R counter allows the input reference frequency to
be divided down to produce the reference clock to the phase
frequency detector (PFD). Division ratios from 1 to 16,383 are
allowed.
PFD AND CHARGE PUMP
The PFD takes inputs from the R counter and N counter
(N = BP + A) and produces an output proportional to the phase
and frequency difference between them. Figure 12 is a simpli-
fied schematic. The PFD includes a programmable delay ele-
ment that controls the width of the antibacklash pulse. This
pulse ensures that there is no dead zone in the PFD transfer
function and minimizes phase noise and reference spurs. Two
bits in the R counter latch, ABP2 and ABP1, control the width
of the pulse (see Table 9).
04437-012
PROGRAMMABLE
DELAY
U3
CLR2
Q2D2
U2
CLR1
Q1D1
CHARGE
PUMP
DOWN
UP
HI
HI
U1
ABP1 ABP2
R DIVIDER
N DIVIDER
CP OUTPUT
R DIVIDER
N DIVIDER
CP
CPGND
V
P
Figure 12. PFD Simplified Schematic and Timing (In Lock)