ADF4351BCPZ AD [Analog Devices], ADF4351BCPZ Datasheet - Page 22

• Manufacturer Part Number: ADF4351BCPZ
• Description: Wideband Synthesizer
• Manufacturer: AD [Analog Devices]

ADF4351
It is important that the PFD frequency remain constant (in this
example, 13 MHz). This allows the user to design one loop filter
for both setups without encountering stability issues. Note that
the ratio of the RF frequency to the PFD frequency principally
affects the loop filter design, not the actual channel spacing.
CYCLE SLIP REDUCTION FOR FASTER LOCK TIMES
As described in the Low Noise and Low Spur Modes section, the
ADF4351
for noise performance. However, in fast-locking applications,
the loop bandwidth generally needs to be wide and, therefore,
the filter does not provide much attenuation of the spurs. If the
cycle slip reduction feature is enabled, the narrow loop band-
width is maintained for spur attenuation, but faster lock times
are still possible.
Cycle Slips
Cycle slips occur in integer-N/fractional-N synthesizers when
the loop bandwidth is narrow compared to the PFD frequency.
The phase error at the PFD inputs accumulates too fast for the
PLL to correct, and the charge pump temporarily pumps in the
wrong direction. This slows down the lock time dramatically.
The
extends the linear range of the PFD, allowing faster lock times
without modifications to the loop filter circuitry.
When the circuitry detects that a cycle slip is about to occur, it
turns on an extra charge pump current cell. This cell outputs a
constant current to the loop filter or removes a constant current
from the loop filter (depending on whether the VCO tuning
voltage needs to increase or decrease to acquire the new
frequency). The effect is that the linear range of the PFD is
increased. Loop stability is maintained because the current is
constant and is not a pulsed current.
If the phase error increases again to a point where another cycle
slip is likely, the
This continues until the
quency has exceeded the desired frequency. The extra charge
pump cells are turned off one by one until all the extra charge
pump cells are disabled and the frequency settles to the original
loop filter bandwidth.
Up to seven extra charge pump cells can be turned on. In most
applications, seven cells are enough to eliminate cycle slips
altogether, providing much faster lock times.
Setting Bit DB18 in Register 3 to 1 enables cycle slip reduction.
Note that the PFD requires a 45% to 55% duty cycle for CSR to
operate correctly. If the REF
duty cycle, enabling the RDIV2 mode (Bit DB24 in Register 2)
ensures that the input to the PFD has a 50% duty cycle.
ADF4351
contains a number of features that allow optimization
contains a cycle slip reduction feature that
ADF4351
ADF4351
turns on another charge pump cell.
IN
frequency does not have a suitable
detects that the VCO fre-
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SPURIOUS OPTIMIZATION AND FAST LOCK
Narrow loop bandwidths can filter unwanted spurious signals,
but these bandwidths usually have a long lock time. A wider
loop bandwidth achieves faster lock times but may lead to
increased spurious signals inside the loop bandwidth.
The fast lock feature can achieve the same fast lock time as the
wider bandwidth but with the advantage of a narrow final loop
bandwidth to keep spurs low.
FAST LOCK TIMER AND REGISTER SEQUENCES
If the fast lock mode is used, a timer value must be loaded into
the PLL to determine the duration of the wide bandwidth mode.
When Bits[DB16:DB15] in Register 3 are set to 01 (fast lock
enable), the timer value is loaded by the 12-bit clock divider
value (Bits[DB14:DB3] in Register 3). The following sequence
must be programmed to use fast lock:
1.
2.
FAST LOCK EXAMPLE
If a PLL has a reference frequency of 13 MHz, f
and a required lock time of 60 µs, the PLL is set to wide bandwidth
mode for 20 µs. This example assumes a modulus of 65 for channel
spacing of 200 kHz. The VCO calibration time of 20 µs must also
be taken into account (achieved by programming the higher band
select clock mode using Bit DB23 of Register 3).
If the time set for the PLL lock time in wide bandwidth mode is
20 µs, then
Therefore, a value of 8 must be loaded into the clock divider
value in Register 3 (see Step 2 in the Fast Lock Timer and
Register Sequences section).
Start the initialization sequence (see the Register Initialization
Sequence section). This sequence occurs only once after
powering up the part.
Load Register 3 by setting Bits[DB16:DB15] to 01 and by
setting the selected fast lock timer value (Bits[DB14:DB3]).
The duration that the PLL remains in wide bandwidth mode
is equal to the fast lock timer/f
Fast Lock Timer Value = (VCO Band Select Time +
PLL Lock Time in Wide Bandwidth) × f
Fast Lock Timer Value = (20 µs + 20 µs) × 13 MHz/65 = 8
PFD
.
PFD
Data Sheet
/MOD
PFD
of 13 MHz,