Datasheet

Data Sheet ADF4154
Rev. C | Page 19 of 24
bandwidth can lead to notable spurious signals, which cannot
be reduced significantly by the loop filter.
Using the fast-lock feature can achieve the same fast-lock time
as the noise and spur register, but with the advantage of lower
spurious signals because the final loop bandwidth is reduced by
a quarter.
FAST-LOCK TIMER AND REGISTER SEQUENCES
If the fast-lock mode is used, a timer value needs to be loaded
into the PLL to determine the time spent in wide bandwidth
mode.
When the load control bit is set to 1, the timer value is loaded
via the 12-bit modulus value. To use fast lock, the PLL must be
written to in the following sequence:
1. Load the R-divider register with DB23 = 1 and the chosen
fast-lock timer value (DB13 to DB2) instead of the
modulus. Note that the duration that the PLL remains in
wide bandwidth is equal to the fast-lock timer/f
PFD
.
2. Load the noise and spur register.
3. Load the control register.
4. Load the R-divider register with DB23 = 0 and MUXOUT
= 110 (DB22 to DB20). This sets the fast-lock switch to
appear at the MUXOUT pin. All the other needed
parameters, including the modulus, also need to be loaded.
5. Load the N-divider register, including fast lock = 1 (DB23),
to activate fast-lock mode.
After this procedure is complete, the user need only repeat
Step 5 to invoke fast lock for subsequent frequency jumps.
FAST LOCK: AN EXAMPLE
If a PLL has reference frequencies of 13 MHz and f
PFD
= 13 MHz
and a required lock time of 50 µs, the PLL is set to wide bandwidth
for 40 µs.
If the time period set for the wide bandwidth is 40 µs, then
Fast-Lock Timer Value = Time in Wide Bandwidth × f
PFD
Fast-Lock Timer Value = 40 µs × 13 MHz = 520
Therefore, 520 must be loaded into the R-divider register in
Step 1 of the sequence described in the Fast-Lock Timer and
Register Sequences section.
FAST LOCK: LOOP FILTER TOPOLOGY
To use fast-lock mode, an extra connection from the PLL to the
loop filter is needed. The damping resistor in the loop filter
must be reduced to ¼ of its value while in wide bandwidth
mode. This is required because the charge pump current is
increased by 16 while in wide bandwidth mode, and stability
must be ensured. During fast lock, the MUXOUT pin is shorted
to ground (the fast-lock switch must be programmed to appear
at the MUXOUT pin). The following two topologies can be used:
• Divide the damping resistor (R1) into two values (R1 and
R1A) that have a ratio of 1:3 (see Figure 19).
• Connect an extra resistor (R1A) directly from MUXOUT,
as shown in Figure 19. The extra resistor must be chosen
such that the parallel combination of an extra resistor and
the damping resistor (R1) is reduced to ¼ of the original
value of R1 (see Figure 20).
ADF4154
CP
MUXOUT
C1
C2
R2
R1
R1A
C3
VCO
04833-029
Figure 19. Fast-Lock Loop Filter Topology—Topology 1
ADF4154
CP
MUXOUT
C1
C2
R2
R1R1A
C3
VCO
04833-030
Figure 20. Fast-Lock Loop Filter Topology—Topology 2
SPUR MECHANISMS
The following section describes three spur mechanisms that can
arise when using a fractional-N synthesizer and how to minimize
them in the ADF4154.
Fractional Spurs
The fractional interpolator in the ADF4154 is a third-order Σ-Δ
modulator (SDM) with a modulus MOD that is programmable
to an integer value between 2 and 4095. In low spur mode
(dither enabled), the minimum allowed value of MOD is 50.
The SDM is clocked at the PFD reference rate (f
PFD
) that allows
PLL output frequencies to be synthesized at a channel step
resolution of f
PFD
/MOD.
In low noise mode and low noise and spur mode (dither off),
the quantization noise from the Σ-Δ modulator appears as frac-
tional spurs. The interval between spurs is f
PFD
/L, where L is the
repeat length of the code sequence in the digital Σ-Δ modulator.
For the third-order modulator used in the ADF4154, the repeat
length depends on the value of MOD, as shown in Table 11.
Table 11. Fractional Spurs with Dither Off
Condition (Dither Off)
Repeat
Length
Spur Interval
If MOD is divisible by 2, but not 3 2 × MOD Channel step/2
If MOD is divisible by 3, but not 2 3 × MOD Channel step/3
If MOD is divisible by 6 6 × MOD Channel step/6
Otherwise MOD Channel step