ADF4157 Data Sheet
Rev. D | Page 10 of 24
PHASE FREQUENCY DETECTOR (PFD) AND
CHARGE PUMP
The PFD takes inputs from the R counter and the N counter
and produces an output proportional to the phase and fre-
quency difference between them. Figure 14 is a simplified
schematic of the phase frequency detector. The PFD includes
a fixed delay element that sets the width of the antibacklash
pulse, which is typically 3 ns. This pulse ensures that there is no
dead zone in the PFD transfer function and gives a consistent
reference spur level.
U3
CLR2
Q2D2
U2
DOWN
UP
HI
HI
CP
–IN
+IN
CHARGE
PUMP
DELAY
CLR1
Q1D1
U1
05874-008
Figure 14. PFD Simplified Schematic
MUXOUT AND LOCK DETECT
The output multiplexer on the ADF4157 allows the user to access
various internal points on the chip. The state of MUXOUT is
controlled by M4, M3, M2, and M1 (see Figure 17). Figure 15
shows the MUXOUT section in block diagram form.
05874-009
ANALOG LOCK DETECT
MUXOUT
DV
DD
THREE-STATE OUTPUT
N DIVIDER OUTPUT
DV
DD
DGND
DGND
R DIVIDER OUTPUT
DIGITAL LOCK DETECT
SERIAL DATA OUTPUT
CLK DIVIDER OUTPUT
R DIVIDER/2
N DIVIDER/2
CONTROL
MUX
FASTLOCK SWITCH
Figure 15. MUXOUT Schematic
INPUT SHIFT REGISTER
The ADF4157 digital section includes a 5-bit RF R counter, a
12-bit RF N counter, and a 25-bit FRAC counter. Data is clocked
into the 32-bit input shift register on each rising edge of CLK.
The data is clocked in MSB first. Data is transferred from the
input shift register to one of five latches on the rising edge of
LE. The destination latch is determined by the state of the three
control bits (C3, C2, and C1) in the input shift register. These
are the three LSBs, DB2, DB1, and DB0, as shown in Figure 2.
The truth table for these bits is shown in Table 6. Figure 16
shows a summary of how the latches are programmed.
PROGRAM MODES
Table 6 and Figure 16 through Figure 21 show how to set up
the program modes in the ADF4157.
Several settings in the ADF4157 are double-buffered. These
include the LSB FRAC value, R counter value, reference doubler,
and current setting. This means that two events have to occur
before the part uses a new value of any of the double-buffered
settings. First, the new value is latched into the device by writing to
the appropriate register. Second, a new write must be performed
on Register 0, R0.
For example, updating the fractional value can involve a write
to the 13 LSB bits in R1 and the 12 MSB bits in R0. R1 should
be written to first, followed by the write to R0. The frequency
change begins after the write to R0. Double buffering ensures
that the bits written to in R1 do not take effect until after the
write to R0.
Table 6. C3, C2, and C1 Truth Table
C3 C2 C1 Register
0 0 0 Register 0 (R0)
0 0 1 Register 1 (R1)
0 1 0 Register 2 (R2)
0 1 1 Register 3 (R3)
1 0 0 Register 4 (R4)
