Data Sheet ADF41020
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 and
produces an output proportional to the phase and frequency
difference between them. Figure 13 is a simplified schematic.
The PFD includes a fixed delay element 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. The charge pump converts the
PFD output to current pulses, which are integrated by the PLL
loop filter.
MUXOUT AND LOCK DETECT
The output multiplexer on the ADF41020 allows the user
to access various internal points on the chip. The state of
MUXOUT is controlled by M3, M2, and M1 in the function
latch. Figure 17 shows the full truth table. Figure 12 shows
the MUXOUT section in block diagram form.
Lock Detect
MUXOUT can be programmed with digital lock detect.
Digital lock detect is active high. Digital lock detect is set high
when the phase error on five consecutive phase detector cycles
is less than 15 ns. It stays set high until a phase error of greater
than 25 ns is detected on any subsequent PD cycle.
Figure 12. MUXOUT Circuit
INPUT SHIFT REGISTER
The ADF41020 digital section includes a 24-bit input shift
register, a 14-bit R counter, and a 19-bit N counter, comprising
a 6-bit A counter and a 13-bit B counter. Data is clocked into
the 24-bit shift register on each rising edge of CLK. The data is
clocked in MSB first. Data is transferred from the shift register
to one of three latches on the rising edge of LE. The destination
latch is determined by the state of the two control bits (C2, C1)
in the shift register. C2 and C1 are the two LSBs, DB1 and DB0,
as shown in the timing diagram of Figure 2. The truth table for
these bits is shown in Table 5. Table 5 shows a summary of
how the latches are programmed. The SPI is both 1.8 V and
3 V compatible.
Table 5. C1, C2 Truth Table
Control Bits
Data Latch
C2 C1
0 1 N counter (A and B)
1 0 Function latch (including prescaler)
Figure 13. PFD Simplified Schematic
10304-013
GND
DV
DD
CONTROL
MUX
DIGITAL LOCK DETECT
R COUNTER OUTPUT
N COUNTER OUTPUT
SDOUT
MUXOUT
HIGH
HIGH
D1
D2
Q1
Q2
CLR2
CP
U1
U2
UP
DOWN
GND
U3
R DIVIDER
FIXED
DELAY
N DIVIDER
V
P
CHARGE
PUMP
CLR1
10304-012
Rev. C | Page 9 of 16
