AD6652BBC datasheet(33/76 Pages) AD

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AD6652

Rev. 0 | Page 33 of 76

TROLLED OSCILLATOR

two multipliers, I and Q, and a 32-bit complex numerically

contr lled oscillator (NCO). Each channel of the AD6652 has

pendent NCO. The NCO serves as a quadrature local

r capable of producing an NCO frequency between

86 is

signed integer. Use the following

e NCO frequency:

NUMERICALLY CON

FREQUENCY TRANSLATION TO BASEBAND

This processing stage comprises a digital tuner consisting

an inde

oscillato

−CLK

/2 and +CLK/2 with a resolution of CLK/232 in the complex

mode. The worst-case spurious signal from the NCO is better

than −100 dBc for all output frequencies.

The NCO frequency programmed in Registers 0x85 and 0x

interpreted as a 32-bit un

equation to calculate th

⎟

⎠

⎞

⎜

⎝

⎛

CLK

FREQ

NCO

where:

NCO_FREQ is a decimal number equal to the 32-bit binary

ed at 0x85 and 0x86.

CLK is the AD6652 DDC master clock rate (in Hz).

A shadow register generally precedes an active register. It holds

the next number to be used by the active egister whenever that

function’s hold-off counter causes the active register to be

ort.

ly from

’s

lly

)

arts counting down at the DDC CLK rate and, when it reaches

one, the new frequency value in the shadow register is written to

the active NCO frequency register.

etting

cy u

es.

e pha

et r

gra

le o

e phas

f t

O. This 16-b

ter

eted as a 16-bit

r. A 0x0000 in this reg

rrespo

, an

res

2π ra

ster

nchro

e o

stan

e dif

ces.

O C

con

gis

igu

ures

ich

han

atu

ollo

sect

ass

To bypass the NCO of the AD6652, set Bit 0 of 0x88 high. When

the NCO is bypassed, down-con ersion is not performed, and

lex

digitized signal has already

been converted to baseband in prior analog stages or by other

f the NCO. To enable phase dither, set

e noise floor and spurious free

dynamic range is increased at the expense of slight decreases in

stem

urs are

or, then phase

ther

Amplitude dither can also be used to improve spurious

performance of the NCO. To enable amplitude dither, set Bit 2

of 0x88, which causes amplitude quantization errors to be

randomized within the angular-to-Cartesian conversion stage

of the NCO. This option reduces spurs at the expense of a

slightly raised noise floor and slightly reduced SNR. Amplitude

dither and phase dither can be used together, separately, or not

at all.

number to be programm

f is the desired NCO output frequency in Hz.

NCO SHADOW REGISTER

updated with the new value. Active registers are also updated

with the contents of a shadow register any time the channel is

brought out of sleep mode.

The NCO shadow register is updated during normal program

ming of the registers through the microport or serial input p

The active frequency register can receive update data on

the NCO shadow register. When software reads back an NCO

frequency, it is reading back the active frequency register and

not the shadow register.

NCO FREQUENCY HOLD-OFF REGIST

When the NCO frequency registers are written, data is actua

passed to a shadow register. Data can be moved to the active

sleep mode or when a SYNC hop occurs. As a result of either

event, a count-down counter is loaded with an NCO frequen

hold-off value. The 16-bit unsigned integer counter (0x84

The NCO can be set up to update its frequency immediately

upon receipt of a HOP_SYNC or START_SYNC, with no

hold-off count, by setting the hold-off count value to 1. S

the hold-off count to zero prevents any frequen

pdat

PHAS OFFS T

se offs

egister

x87) a

s a pro

mmab

ffset t

e accu

lator o he NC

it regis

is inte

unsign

intege

ister

nds to

offset

d a 0x

FF cor

ponds

an offs

dians.

is regi

allows

ultiple

Os to

nized to roduc

utputs

th con

t and k own

phas

feren

ONTR L REG TER

Use t

NCO

trol re

ter loca d at 0x

to conf

re the

feat

of the

O, wh

are co rolled o a per c

nel

basis.

ese fe

res are described i the f

wing

ions.

Byp

the AD6652 channel functions simply as a real filter on comp

data. This feature is useful for baseband sampling applications,

where the A input is connected to the I signal path within the

filter and the B input is connected to the Q signal path. Bypass

ing the NCO might be desired, if the

digital preprocessing.

Phase Dither

The AD6652 provides a phase dither option for improving the

spurious performance o

Bit 1 of Register 0x88, which causes discrete spurs due to phase

truncation in the NCO to be randomized. The energy from

these spurs is spread into th

the SNR. The choice of whether to use phase dither in a sy

depends ultimately on the system goals. If lower sp

desired at the expense of a slightly raised noise flo

dither should be employed. If the lowest noise floor is desired

and higher spurs can be tolerated or filtered by subsequen

stages, then phase dither is not needed.

Amplitude Di

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