RF SYNTHESIZER: A WORKED EXAMPLE

The following equation governs how the synthesizer should be

programmed:

RFOUT = [INT + (FRAC/MOD)] × [FPFD]

(3)

where:

RFOUT is the RF frequency output.

INT is the integer division factor.

FRAC is the fractionality.

MOD is the modulus.

FPFD = REFIN × [(1 + D)/(R × (1+T))]

(4)

where:

REFIN is the reference frequency input.

D is the RF REFIN doubler bit.

T is the reference divide-by-2 Bit(0 or 1).

R is the RF reference division factor. For example, in a GSM

1800 system, where 1.8 GHz RF frequency output (RFOUT) is

required, a 13 MHz reference frequency input (REFIN) is

available, and a 200 kHz channel resolution (fRES) is required, on

the RF output.

MOD = REFIN/fRES

MOD = 13 MHz/200 kHz = 65

From Equation 4

FPFD = [13 MHz × (1 + 0)/1] = 13 MHz

(5)

1.8 GHz = 13 MHz × (INT + FRAC/65)

(6)

where INT = 138; FRAC = 30.

MODULUS

The choice of modulus (MOD) depends on the reference signal

(REFIN) available and the channel resolution (fRES) required at

the RF output. For example, a GSM system with 13 MHz REFIN sets

the modulus to 65. This means that the RF output resolution (fRES)

is the 200 kHz (13 MHz/65) necessary for GSM. With dither off,

the fractional spur interval depends on the modulus values chosen.

See Table 7 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.

The reference divide-by-2 divides the reference signal by 2,

resulting in a 50% duty cycle PFD frequency. This is necessary

for the correct operation of the cycle slip reduction (CSR)

ADF4156

function. See the Cycle Slip Reduction for Faster Lock Times

section for more information.

12-BIT PROGRAMMABLE MODULUS

Unlike most other fractional-N PLLs, the ADF4156 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 configurations

for the application, when combined with the reference doubler

and the 5-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 results 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 programming the modulus to divide by 130. This

also results in 200 kHz resolution and offers superior phase

noise performance over the previous setup.

The programmable modulus is also very useful for multi-

standard applications. If a dual-mode phone requires PDC

and GSM 1800 standards, the programmable modulus is a

great benefit. PDC requires 25 kHz channel step resolution,

whereas GSM 1800 requires 200 kHz channel step resolution.

A 13 MHz reference signal can be fed directly to the PFD and

the modulus can be programmed to 520 when in PDC mode

(13 MHz/520 = 25 kHz).

The modulus needs to be reprogrammed to 65 for GSM 1800

operation (13 MHz/65 = 200 kHz).

It is important that the PFD frequency remains constant (13 MHz).

This allows the user to design one loop filter that can be used in

both setups without running into stability issues. It is the ratio

of the RF frequency to the PFD frequency that affects the loop

design. By keeping this relationship constant, the same loop

filter can be used in both applications.

CYCLE SLIP REDUCTION FOR FASTER LOCK TIMES

As mentioned in the Noise and Spur Mode section, the

ADF4156 can be optimized for noise performance. However, in

fast-locking applications, the loop bandwidth needs to be wide,

and therefore, the filter does not provide much attenuation of

the spurs. The cycle slip reduction function on the ADF4156

can be used to get around this issue. Using cycle slip reduction,

the loop bandwidth can be kept narrow to attenuate spurs and

still obtain fast lock times.

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