AD9874ABST datasheet(22/40 Pages) AD

22 / 40 page

REV. A

AD9874

–22–

–25

–90

–100

–110

–120

–20

–15

–10

–5

FREQUENCY OFFSET – kHz

–80

–70

–60

–50

–40

–30

–20

–10

–130

–140

CP = 0

CP = 2

CP = 4

CP = 6

EXT CLK

Figure 7c. CLK Phase Noise vs. Charge Pump Setting Bias

(IF = 73.35 MHz, IF = 71.1 MHz, –31 dBm, fCLK = 18 MHz,

fREF = 16.8 MHz) (CLK SYN Settings: CKO Bias = 3, CKR = 56,

and CKN = 60 with fREF = 300 kHz)

Table VIII. SPI Registers Associated with CLK Synthesizer

Address

Bit

Default

(Hex)

Breakdown Width

Value

Name

0x00

(7:0)

0xFF

STBY

0x01

(3:2)

CKOB

0x10

(5:0)

CKR(13:8)

0x11

(7:0)

0x38

CKR(7:0)

0x12

(4:0)

0x00

CKN(12:8)

0x13

(7:0)

0x3C

CKN(7:0)

0x14

(6)

CKF

(5)

CKINV

(4:2)

CKI

(1:0)

CKTM

0x15

(3:0)

0x0

CKFA(13:8)

0x16

(7:0)

0x04

CKFA(7:0)

IF LNA/MIXER

The AD9874 contains a single-ended LNA followed by a Gil-

bert-type active mixer, shown in Figure 8 with the required

external components. The LNA uses negative shunt feedback to

set its input impedance at the IFIN pin, thus making it depen-

dent on the LNA bias setting and input frequency. It can be

modeled as approximately 370

Ω//1.4 pF (620%) for the higher

bias settings below 100 MHz. Figures 9a and 9b show the

equivalent input impedance versus frequency characteristics of

the AD9874 with all the LNA bias settings. The increase in shunt

resistance versus frequency can be attributed to the reduction in

bandwidth, thus the amount of negative feedback of the LNA.

Note that the input signal into IFIN should be ac-coupled via a

10 nF capacitor since the LNA input is self-biasing.

IFIN

RGAIN

RBIAS

VDDI

MXOP

LO INPUT =

0.3V p-p TO

1.0V p-p

DC SERVO

LOOP

MULTI-TANH

V–I STAGE

CXIF

MXON

CXVM

CXVL

2.7V TO 3.6V

Figure 8. Simplified Schematic of AD9874’s LNA/Mixer

FREQUENCY – MHz

500

100

400

600

200

550

450

150

350

300

350

300

250

LNA BIAS = 0

LNA BIAS = 1

LNA BIAS = 2

LNA BIAS = 3

Figure 9a. The Shunt Input Resistance vs. the

Frequency of the AD9874’s IF1 Input

FREQUENCY – MHz

1.5

100

0.5

2.5

200

2.0

1.0

150

350

300

250

LNA BIAS = 3

LNA BIAS = 2

LNA BIAS = 1

LNA BIAS = 0

Figure 9b. The Shunt Capacitance vs.

the Frequency of the AD9874’s IF1 Input

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