ADF4351BCPZ AD [Analog Devices], ADF4351BCPZ Datasheet - Page 23

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

Data Sheet
FAST LOCK LOOP FILTER TOPOLOGY
To use fast lock mode, the damping resistor in the loop filter is
reduced to one-fourth its value while in wide bandwidth mode.
To achieve the wider loop filter bandwidth, the charge pump
current increases by a factor of 16; to maintain loop stability,
the damping resistor must be reduced by a factor of one-fourth.
To enable fast lock, the SW pin is shorted to the AGND pin by
setting Bits[DB16:DB15] in Register 3 to 01. The following two
topologies are available:


SPUR MECHANISMS
This section describes the three different spur mechanisms that
arise with a fractional-N synthesizer and how to minimize them
in the ADF4351.
Fractional Spurs
The fractional interpolator in the
Σ-Δ modulator with a modulus (MOD) that is programmable
to any integer value from 2 to 4095. In low spur mode (dither
on), the minimum allowable value of MOD is 50. The Σ-Δ
modulator is clocked at the PFD reference rate (f
allows PLL output frequencies to be synthesized at a channel
step resolution of f
The damping resistor (R1) is divided into two values
(R1 and R1A) that have a ratio of 1:3 (see Figure 31).
An extra resistor (R1A) is connected directly from SW, as
shown in Figure 32. The extra resistor is calculated such
that the parallel combination of the extra resistor and the
damping resistor (R1) is reduced to one-fourth the original
value of R1 (see Figure 32).
ADF4351
ADF4351
Figure 31. Fast Lock Loop Filter Topology 1
Figure 32. Fast Lock Loop Filter Topology 2
CP
CP
PFD
OUT
OUT
SW
SW
/MOD.
R1A
C1
C1
ADF4351
R1
C2
R1
R1A
C2
R2
R2
is a third-order
C3
C3
PFD
VCO
VCO
), which
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In low noise mode (dither off), the quantization noise from the
Σ-Δ modulator appears as fractional spurs. The interval between
spurs is f
in the digital Σ-Δ modulator. For the third-order Σ-Δ modulator
used in the ADF4351, the repeat length depends on the value of
MOD (see Table 7).
Table 7. Fractional Spurs with Dither Off (Low Noise Mode)
MOD Value (Dither Off)
MOD is divisible by 2, but not by 3
MOD is divisible by 3, but not by 2
MOD is divisible by 6
MOD is not divisible by 2, 3, or 6
In low spur mode (dither on), the repeat length is extended
to 2
quantization error spectrum look like broadband noise. This
may degrade the in-band phase noise at the PLL output by as
much as 10 dB. For lowest noise, dither off is a better choice,
particularly when the final loop bandwidth is low enough to
attenuate even the lowest frequency fractional spur.
Integer Boundary Spurs
Another mechanism for fractional spur creation is the inter-
actions between the RF VCO frequency and the reference
frequency. When these frequencies are not integer related (the
purpose of a fractional-N synthesizer), spur sidebands appear
on the VCO output spectrum at an offset frequency that corre-
sponds to the beat note, or difference frequency, between an
integer multiple of the reference and the VCO frequency. These
spurs are attenuated by the loop filter and are more noticeable
on channels close to integer multiples of the reference, where
the difference frequency can be inside the loop bandwidth (thus
the name integer boundary spurs).
Reference Spurs
Reference spurs are generally not a problem in fractional-N
synthesizers because the reference offset is far outside the loop
bandwidth. However, any reference feedthrough mechanism
that bypasses the loop may cause a problem. Feedthrough of
low levels of on-chip reference switching noise, coupling to the
VCO, can result in reference spur levels as high as −80 dBc. The
PCB layout must ensure adequate isolation between VCO circuitry
and the input reference to avoid a possible feedthrough path on
the board.
21
cycles, regardless of the value of MOD, which makes the
PFD
/L, where L is the repeat length of the code sequence
Repeat
Length
2 × MOD
3 × MOD
6 × MOD
MOD
Spur Interval
Channel step/2
Channel step/3
Channel step/6
Channel step
ADF4351