AD-DPGIOZ datasheet(38/61 Pages) AD

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AD9961/AD9963

Data Sheet

Rev. A | Page 38 of 60

TRANSMIT PATH

Tx Path General Description

The transmit section consists of two complete paths of

interpolation filters stages, each followed by a high speed

current output DAC. A data assembler receives interleaved data

from one of two digital interface ports, and de-interleaves and

buffers the data before supplying the data samples into the two

datapaths. The interpolation filter bank consists of three stages

that can be completely bypassed or cascaded to provide 2×, 4×,

or 8× interpolation. The supported clock rates for each of the

interpolation filters and the transmit DACs are listed in Table 1.

TX PORT

TRX PORT

DATA

ASSEMBLER

AND FIFO

I DAC

I SCALE

Q SCALE

I GAIN

TXVDD

Q GAIN

Q DAC

RFSADJ

LPF

1/2/4/8×

LPF

1/2/4/8×

TXIP

TXIN

TXQP

TXQN

TXCML

REFIO

Figure 47. Transmit Path Block Diagram

INTERPOLATION FILTERS

The I and Q transmit paths contain three interpolation filters

designated as INT0, INT1, and SRRC. Each of the interpolation

filters provides a 2× increase in output data rate. The filters can

be completely bypassed or cascaded to provide 2×, 4×, or 8×

upsampling ratios. The interpolation filters effectively increase

the DAC update rate while suppressing the images at the input

date rate. This reduces the requirements on the analog output

reconstruction filter.

INT1

INT0

SRRC

FROM

FIFO

DAC

SRRC_BP

0x30[2]

INT0_BP

0x30[3]

INT0_SCALE

0x36[4:0]

SRRC_SCALE

0x35[4:0]

INT1_SCALE

0x37[4:0]

INT1_BP

0x30[4]

Figure 48. Block Diagram of Transmit Datapath

The digital filters should be cascaded such that INT0 is enabled

for an interpolation factor of 2×, INT0 and INT1 should be

enabled for an interpolation factor of 4×, and INT0, INT1, and

the SRRC should be enabled for an interpolation factor of 8×.

The INT0 and INT1 filters have bandwidths of 40% of the input

data rate. Over their usable bandwidth, the filters have a passband

ripple of less than 0.1 dB. The SRRC has a roll-off factor of 0.22

with a 60 dB stop-band attenuation. In 2× and 4× interpolation

modes, the interpolation filters have an image rejection of greater

than 70 dB. In 8× interpolation mode, the image rejection is

greater than 65 dB. The usable bandwidth of the filters is

typically limited by the stop-band attenuation they provide,

rather than the passband flatness. The transfer functions of the

interpolation filters configured for 2×, 4×, and 8× interpolation

ratios are shown in Figure 49 through Figure 51.

–80

–70

–60

–50

–40

–30

–20

–10

NORMALIZED FREQUENCY (Relative to fDAC)

Figure 49. Digital Filter Transfer Function for 2× Interpolation

–80

–70

–60

–50

–40

–30

–20

–10

NORMALIZED FREQUENCY (Relative to fDAC)

Figure 50. Digital Filter Transfer Function for 4× Interpolation

–80

–70

–60

–50

–40

–30

–20

–10

NORMALIZED FREQUENCY (Relative to fDAC)

Figure 51. Digital Filter Transfer Function for 8× Interpolation

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