ADF4154 Analog Devices, ADF4154 Datasheet - Page 16

• Manufacturer Part Number: ADF4154
• Description: Fractional-n Frequency Synthesizer
• Manufacturer: Analog Devices

ADF4154
RF Charge Pump Three-State
This bit puts the charge pump into three-state mode when
programmed to 1. It should be set to 0 for normal operation.
RF Power-Down
DB4 on the ADF4154 provides the programmable power-down
mode. Setting Bit DB4 to 1 powers down the device. Setting
Bit DB4 to 0 returns the synthesizer to normal operation. While
in software power-down mode, the part retains all information
in its registers. Only when supplies are removed are the register
contents lost.
When a power-down is activated, the following events occur:
1.
2.
3.
4.
5.
6.
Lock Detect Precision (LDP)
When the LDP bit is programmed to 0, 24 consecutive reference
cycles of 15 ns must occur before the digital lock detect is set.
When this bit is programmed to 1, 40 consecutive reference
cycles of 15 ns must occur before digital lock detect is set.
Phase Detector Polarity
DB6 in the ADF4154 sets the phase detector polarity. When the
VCO characteristics are positive, this should be set to 1. When
they are negative, it should be set to 0.
Charge Pump Current Setting
DB7, DB8, DB9, and DB10 set the charge pump current, which
should be set according to the loop filter design (see Table 10).
REF
Setting the REF
4-bit RF R counter, which disables the doubler. Setting the REF
bit to 1 multiplies the REF
feeding into the 4-bit R counter. When the doubler is disabled,
the REF
fractional synthesizer. When the doubler is enabled, both the
rising and falling edges of REF
PFD input.
When the doubler is enabled and the lowest spur mode is
chosen, the in-band phase noise performance is sensitive to the
REF
as 5 dB for the REF
The phase noise is insensitive to the REF
lowest noise mode and in the lowest noise and spur mode. The
phase noise is insensitive to the REF
doubler is disabled.
IN
IN
All active dc current paths are removed.
The synthesizer counters are forced to their load
state conditions.
The charge pump is forced into three-state mode.
The digital lock detect circuitry is reset.
The RF
The input register remains active and capable of loading
and latching data.
Doubler
duty cycle. The phase noise degradation can be as much
IN
falling edge is the active edge at the PFD input to the
IN
input is de-biased.
IN
bit to 0 feeds the REF
IN
duty cycles outside a 45% to 55% range.
IN
frequency by a factor of 2 before
IN
become active edges at the
IN
duty cycle when the
IN
IN
signal directly to the
duty cycle in the
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IN
NOISE AND SPUR REGISTER, R3
The on-chip noise and spur register is programmed by setting
R3[1, 0] to [1, 1]. Table 7 shows the input data format for
programming this register.
Noise and Spur Mode
Noise and spur mode allows the user to optimize a design either
for improved spurious performance or for improved phase
noise performance. When the lowest spur setting is chosen,
dither is enabled. This randomizes the fractional quantization
noise so that it looks more like white noise rather than spurious
noise. This means that the part is optimized for improved
spurious performance. This operation would normally be used
when the PLL closed-loop bandwidth is wide for fast-locking
applications. A wide-loop bandwidth is seen as a loop
bandwidth greater than 1/10 of the RF
resolution (f
to a level that a narrow-loop bandwidth would. When the low
noise and spur setting is enabled, dither is disabled. This
optimizes the synthesizer to operate with improved noise
performance. However, the spurious performance is degraded
in this mode compared to the lowest spurs setting. To further
improve noise performance, the lowest noise setting option can
be used, which reduces the phase noise. As well as disabling the
dither, it ensures that the charge pump operates in an optimum
region for noise performance. This setting is extremely useful
where a narrow-loop filter bandwidth is available. The
synthesizer ensures extremely low noise and the filter attenuates
the spurs. The typical performance characteristics give the user
an idea of the trade-off in a typical WCDMA setup for the
different noise and spur settings.
RESERVED BITS
These bits should be set to 0 for normal operation.
RF SYNTHESIZER: A WORKED EXAMPLE
This equation governs how the synthesizer should be
programmed.
where:
RF
INT is the integer division factor.
FRAC is the fractionality.
MOD is the modulus.
where:
REF
D is the RF REF
R is the RF reference division factor.
OUT
IN
RF
F
is the RF frequency output.
PFD
is the reference frequency input.
OUT
= [ REF
= [ INT + ( FRAC/MOD )] × [ F
RES
). A wide-loop filter does not attenuate the spurs
IN
IN
doubler bit.
× (1 = D)/R ]
OUT
PFD
channel step
]
(3)
(4)