adf4153 Analog Devices, Inc., adf4153 Datasheet - Page 17

• Manufacturer Part Number: adf4153
• Description: Fractional-n Frequency Synthesizer
• Manufacturer: Analog Devices, Inc.

INITIALIZATION SEQUENCE
The following initialization sequence should be followed upon
powering up the part:
1.
2.
3.
4.
5.
6.
The part now locks to the set frequency.
If using the phase resync function, an extra step is needed after
Step 3. This involves loading the R divider register with load
control = 1 and the required delay interval in place of the MOD
value. The sequence above can then be followed ensuring that
in Step 4, the value of MOD is written to the R divider register
with load control = 0.
See the Spur Consistency and Phase Resync sections for more
information on the phase resync feature.
RF SYNTHESIZER: A WORKED EXAMPLE
The following 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
Write all zeros to the noise and spur register. This ensures
that all test modes are cleared.
Write again to the noise and spur register, this time
selecting which noise and spur mode is required. For
example, writing Hex 0003C7 to the part selects low noise
mode.
Enable the counter reset in the control register by writing a
1 to DB2; also select the required settings in the control
register. If using the phase resync function, set the resync
bits to the required settings.
Load the R divider register (with load control [DB23]
set to 0).
Load the N divider register.
Disable the counter reset by writing a 0 to DB2 in the
control register.
RF
F
PFD
is the RF frequency output.
is the reference frequency input.
OUT
= [REF
= [INT + (FRAC/MOD)] × [F
IN
IN
doubler bit.
× (1 + D)/R]
PFD
]
Rev. B | Page 17 of 24
(3)
(4)
For example, in a GSM 1,800 system, where 1.8 GHz RF
frequency output (RF
frequency input (REF
resolution (f
MODULUS
The choice of modulus (MOD) depends on the reference signal
(REF
the RF output. For example, a GSM system with 13 MHz REF
would set the modulus to 65. This means that the RF output
resolution (f
With dither off, the fractional spur interval depends on the
modulus values chosen. See Table 11 for more information.
REFERENCE DOUBLER AND REFERENCE DIVIDER
The reference doubler on-chip allows the input reference signal
to be doubled. This is useful for increasing the PFD comparison
frequency. Making the PFD frequency higher improves the
noise performance of the system. Doubling the PFD frequency
usually improves noise performance by 3 dB. It is important to
note that the PFD cannot be operated above 32 MHz due to a
limitation in the speed of the Σ-Δ circuit of the N divider.
12-BIT PROGRAMMABLE MODULUS
Unlike most other fractional-N PLLs, the ADF4153 allows the
user to program the modulus over a 12-bit range. This means
that the user can set up the part in many different configu-
rations for the application, when combined with the reference
doubler and the 4-bit R counter.
The following is an example of an application that requires
1.75 GHz RF and 200 kHz channel step resolution. The system
has a 13 MHz reference signal.
One possible setup is feeding the 13 MHz directly to the PFD
and programming the modulus to divide by 65. This would
result in the required 200 kHz resolution.
Another possible setup is using the reference doubler to create
26 MHz from the 13 MHz input signal. This 26 MHz is then fed
into the PFD. The modulus is now programmed to divide by
130. This also results in 200 kHz resolution and offers superior
phase noise performance over the previous setup.
MOD = REF
MOD = 13 MHz/200 kHz = 65
From Equation 4:
F
1.8 G = 13 MHz × (INT + FRAC/65)
where: INT = 138; FRAC = 30
IN
PFD
) available and the channel resolution (f
= [13 MHz × (1 + 0)/1] = 13 MHz
RES
RES
) is the 200 kHz (13 MHz/65) necessary for GSM.
) is required on the RF output.
IN
/f
RES
OUT
IN
) is available and a 200 kHz channel
) is required, a 13 MHz reference
RES
) required at
ADF4153
IN
(5)
(6)