ADF4002BCPZ datasheet(8/24 Pages) AD

8 / 24 page

ADF4002

Rev. 0 | Page 8 of 24

THEORY OF OPERATION

REFERENCE INPUT SECTION

The reference input stage is shown in Figure 11. SW1 and SW2

are normally closed switches. SW3 is normally open. When

power-down is initiated, SW3 is closed and SW1 and SW2 are

opened. This ensures that there is no loading of the REFIN pin

on power-down.

100kΩ

REFIN

SW1

SW2

BUFFER

SW3

TO R COUNTER

POWER-DOWN

CONTROL

Figure 11. Reference Input Stage

RF INPUT STAGE

The RF input stage is shown in Figure 12. It is followed by a

2-stage limiting amplifier to generate the CML clock levels

needed for the N counter.

500Ω

1.6V

500Ω

AGND

BIAS

GENERATOR

RFINA

RFINB

AVDD

Figure 12. RF Input Stage

N COUNTER

The N CMOS counter allows a wide ranging division ratio in

the PLL feedback counter. Division ratios from 1 to 8191 are

allowed.

N and R Relationship

The N counter makes it possible to generate output frequencies

that are spaced only by the reference frequency divided by R.

The equation for the VCO frequency is

REFIN

VCO

where:

fVCO is the output frequency of external voltage controlled

oscillator (VCO).

N is the preset divide ratio of binary 13-bit counter (1 to 8191).

fREFIN is the external reference frequency oscillator.

TO PFD

FROM RF

INPUT STAGE

FROM N

COUNTER LATCH

13-BIT N

COUNTER

Figure 13. N Counter

R COUNTER

The 14-bit R counter allows the input reference frequency to be

divided down to produce the reference clock to the phase

frequency detector (PFD). Division ratios from 1 to 16,383 are

allowed.

PHASE FREQUENCY DETECTOR (PFD) AND

CHARGE PUMP

The PFD takes inputs from the R counter and N counter and

produces an output proportional to the phase and frequency

difference between them. Figure 14 is a simplified schematic.

The PFD includes a programmable delay element that controls

the width of the antibacklash pulse. This pulse ensures that

there is no dead zone in the PFD transfer function, and

minimizes phase noise and reference spurs. Two bits in the

reference counter latch (ABP2 and ABP1) control the width of

the pulse. See Figure 17 for details. The smallest antibacklash

pulse width (1.3 ns) should be used if the desired PFD exceeds

104 MHz.

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