AD9860 datasheet(20/32 Pages) AD

20 / 32 page

REV. 0

–20–

AD9860/AD9862

The second interpolation filter will provide an additional 2

inter-

polation for an overall 4

interpolation. The second filter is a

15 tap filter. It suppresses out-of-band signals by 60 dB or more.

The flat passband response (less than 0.1 dB attenuation) is 38%

of the Tx input data rate (9.5% of fDAC). The maximum input

data rate per channel is 32 MSPS per channel when using

interpolation.

The 2

and 4

Interpolation Filter Transfer function plots are

shown in Figure 4a and 4b, respectively.

NORMALIZED –

–10

–20

–30

–40

–50

–60

–70

–80

–100

0.1

–90

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

INTERPOLATION

FILTER

INCLUDUNG SIN (X)/X

NORMALIZED –

–10

–20

–30

–40

–50

–60

–70

–80

–100

0.1

–90

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

INTERPOLATION

FILTER

INCLUDUNG SIN (X)/X

Figure 4. Spectral Response of 2

Interpolation Filter

(top) and 4

Interpolation Filter (bottom)

Fine Modulation Stage

A digital fine modulation stage is available in the transmit path to

shift the complex Tx output spectrum using a 24-bit numerically

controlled oscillator (NCO). To utilize the Fine Modulation

Block, 4

interpolation is required. Therefore, the maximum

input date rate is 32 MSPS per channel, which generates a DAC

update rate, fDAC, of 128 MSPS. The NCO can tune up to 1/4 of

fDAC, providing a step resolution of fDAC/2

26. Since the Fine

Modulation Stage precedes the Interpolation Filters, care must

be taken to ensure the entire desired signal is placed within the

pass band of the Interpolation Filter.

By default, the Fine Modulation Block is bypassed. To enable it

to perform a complex mix of the Tx I and Q data, Register 2’s data

paths, Fine Mod and Fine, should be configured. The NCO

frequency tuning word is set in the three FTW registers.

Hilbert Filter

The Hilbert filter is available to provide a Hilbert transform of

“real” input data at a low intermediate frequency (IF) between

12.5% to 38% of the input data rate. The Hilbert filter essentially

transforms this “real,” single channel input data into a complex

representation (i.e., I and Q components) that can be used as

part of an image rejection architecture. The complex data can than

be processed further using the on-chip digital complex modulators.

The Hilbert filter requires 4

interpolation to be enabled and

accepts data at a maximum 32 MSPS. Figure 5 shows a spectral

plot of the Hilbert filter impulse response.

FREQUENCY – MHz

100

–20

–40

–60

–80

–100

–15

–10

–5

Figure 5. Tx Hilbert Filter, Keeping Positive

Frequencies Spectral Plot

Latch/Demultiplexer

The AD9860/AD9862 Tx path accepts dual or single channel

data. The dual channel data can represent two independent real

signals or a complex signal. Various input data latching schemes

relative to one of the output clocks, CLKOUT1 or CLKOUT2,

are allowed, including using any combination of rising and falling

clock edges.

Associated Tx timing is discussed in detail in the Clock Overview

section of the data sheet.

TRANSMIT APPLICATIONS SECTION

The AD9860/AD9862 transmit path (Tx) includes two, high speed,

high performance, 12-/14-bit TxDACs. Figure 3 shows a detailed

block diagram of the transmit data path and can be referred to

throughout the explanation of the various modes of operation.

The various Tx modes of operation are broken into three parts,

determined by the format of the input data. They are:

1. Single Channel DAC Data

2. Two Independent Real Signal DAC Data (diversity or dual

channel

3. Dual Channel Complex DAC Data (I and Q or Single Sideband)

Single Channel DAC Data

In this mode, 12-/14-bit single channel Tx data is provided to

the AD9860/AD9862 and latched using either CLKOUT1 or

CLKOUT2 edges as defined in the Clock Overview section of

the data sheet. All Tx digital signal processing blocks can be

utilized to address reconstruction filtering at the DAC output

and aid in frequency tuning.

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