AD9874EB datasheet(21/40 Pages) AD

21 / 40 page

REV. A

AD9874

–21–

CLOCK SYNTHESIZER

The clock synthesizer is a fully programmable integer-N PLL

capable of 2.2 kHz resolution at clock input frequencies up to

18 MHz and reference frequencies up to 25 MHz. It is similar

to the LO synthesizer described in Figure 5 with the following

exceptions:

•

It does not include an 8/9 prescaler nor an A counter.

•

It includes a negative-resistance core that, when used in conjunc-

tion with an external LC tank and varactor, serves as the VCO.

The 14-bit reference counter and 13-bit N-divider counter can

be programmed via registers CKR and CKN. The clock

frequency, fCLK, is related to the reference frequency by the

equation

CKN CKR

CLK

REF

() ×

(5)

The charge pump current is programmable via the CKI register

from 0.625 mA to 5.0 mA using the equation:

CKI

PUMP

() ×

10 625

(6)

The fast acquire subcircuit of the charge pump is controlled by

the CKFA register in the same manner as the LO synthesizer is

controlled by the LOFA register. An on-chip lock detect func-

tion (enabled by the CKF bit) automatically increases the

output current for faster settling during channel changes. The

synthesizer may also be disabled using the CK standby bit

located in the STBY register.

CLK OSC. BIAS

IBIAS = 0.15 mA, 0.25 mA,

0.40 mA, OR 0.65 mA

VDDC = 3.0 V

IOUTC

LOSC

0.1 F

RBIAS

COSC

LOOP

FILTER

CVAR

AD9874

CLKN

CLKP

VCM = VDDC – RBIAS

IBIAS > 1.6V

OSC > 1/{2

(LOSC (CVARACTOR//COSC))1/2}

Figure 7a. External Loop Filter, Varactor, and LC

Tank Are Required to Realize a Complete Clock

Synthesizer

The AD9874 clock synthesizer circuitry includes a negative-

resistance core so that only an external LC tank circuit with a

varactor is needed to realize a voltage controlled clock oscillator

(VCO). Figure 7a shows the external components required to

complete the clock synthesizer along with the equivalent input

circuitry of the CLK input. The resonant frequency of the VCO

is approximately determined by LOSC and the series equivalent

capacitance of COSC and CVAR. As a result, LOSC, COSC, and

CVAR should be selected to provide a sufficient tuning range to

ensure proper locking of the clock synthesizer.

The bias, IBIAS, of the negative-resistance core has four pro-

grammable settings. Lower equivalent Q of the LC tank circuit

may require a higher bias setting of the negative-resistance core

to ensure proper oscillation. RBIAS should be selected so the

common-mode voltage at CLKP and CLKN is approximately

1.6 V. The synthesizer may be disabled via the CK standby bit

to allow the user to employ an external synthesizer and/or VCO

in place of those resident on the IC. Note that if an external

CLK source or VCO is used, the clock oscillator must be dis-

abled via the CKO standby bit.

The phase noise performance of the clock synthesizer is depen-

dent on several factors, including the CLK oscillator IBIAS

setting, charge pump setting, loop filter component values, and

internal fREF setting. Figures 7b and 7c show how the measured

phase noise attributed to the clock synthesizer varies (relative to

an external fCLK) as a function of the IBIAS setting and charge

pump setting for a –31 dBm IFIN signal at 73.35 MHz with an

external LO signal at 71.1 MHz. Figure 7b shows that the opti-

mum phase noise is achieved with the highest IBIAS (CKO)

setting, while Figure 7c shows that the higher charge pump

values provide the optimum performance for the given loop

filter configuration. The AD9874 clock synthesizer and oscilla-

tor were set up to provide an fCLK of 18 MHz from an external

fREF of 16.8 MHz. The following external component values

were selected for the synthesizer: RF = 390

Ω, RD = 2 kΩ,

CZ = 0.68

µF, C

P = 0.1

µF, C

OSC = 91 pF, LOSC = 1.2

µH, and

CVAR = Toshiba 1SV228 Varactor.

–25

–90

–100

–110

–120

–20

–15

–10

–5

FREQUENCY OFFSET – kHz

–80

–70

–60

–50

–40

–30

–20

–10

–130

–140

CKO = 1

CKO = 0

CKO = 2

CKO = 3

EXT CLK

Figure 7b. CLK Phase Noise vs. IBIAS Setting (CKO)

(IF = 73.35 MHz, IF = 71.1 MHz, IFIN = –31 dBm,

fCLK = 18 MHz, fREF = 16.8 MHz) (CLK SYN Settings:

CKI = 7, CLR = 56, and CLN = 60 with fREF = 300 kHz)

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