AD9148BPCZ datasheet(54/73 Pages) AD

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AD9148

Preliminary Technical Data

Rev. PrA | Page 54 of 73

Figure 68 shows the pass-band filter response for HB3. In most

applications, the usable bandwidth of the filter is limited by the

image suppression provided by the stop-band rejection and not

by the pass-band flatness. Table 23 shows the pass-band flatness

and stop-band rejection the HB3 filter supports at different

bandwidths.

0.02

–0.02

–0.04

–0.06

–0.08

–0.10

00.28

0.24

0.20

0.16

0.12

0.08

0.04

(×

fIN3)

Figure 68. Pass-Band Detail of HB3

Table 23. HB3 Pass-Band and Stop-Band Performance by

Bandwidth

Bandwidth (% of fIN3)

Pass-Band

Flatness (dB)

Stop-Band

Rejection (dB)

0.001

40.8

0.0014

42.4

0.002

45.6

0.0093

49.8

0.03

55.6

0.1

The maximum bandwidth can be achieved if the signal carrier

frequency is placed directly at the center of one of the filter pass

bands. In this case, the entire quadrature bandwidth of the

interpolation filter (0.8 × fDATA) is available. The available signal

bandwidth decreases as the carrier frequency of the signal moves

away from the center frequency of the filter. The worst-case

carrier frequency is one that falls directly between the center

frequency of two adjacent filters. Figure 69 shows how the

signal bandwidth changes as a function of placement in the

spectrum and interpolation rate.

0.4

0.3

0.2

0.1

–1/2

1/2

–3/8

3/8

–1/4

1/4

–1/8

1/8

fC (×fDAC)

×2 MODE

×4 MODE

×8 MODE

CARRIER FREQUENCY

0.075

0.0375

0.15

Figure 69. Complex Signal Bandwidth as a Function of Output Frequency

FINE MODULATION

The fine modulation makes use of a numerically controlled oscillator,

a phase shifter, and a complex modulator to provide a means for

modulating the signal by a programmable carrier signal. A block

diagram of the fine modulator is shown in Figure 70. The fine

modulator allows the signal to be placed anywhere in the output

spectrum with very fine frequency resolution.

INTERPOLATION

NCO

–1

COSINE

SINE

I DATA

Q DATA

FTW[31:0]

SPECTRAL

INVERSION

OUT_I

OUT_Q

–

NCO PHASE OFFSET

WORD [15:0]

Figure 70. Fine Modulator Block Diagram

The quadrature modulator is used to mix the carrier signal

generated by the NCO with the I and Q signal. The NCO produces

a quadrature carrier signal to translate the input signal to a new

center frequency. A complex carrier signal is a pair of sinusoidal

waveforms of the same frequency, offset 90° from each other.

The frequency of the complex carrier signal is set via the

FTW[31:0] value in Register 0x54 through Register 0x57.

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