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AD6684

Data Sheet

Rev. 0 | Page 52 of 107

VDR REAL MODE

The real mode of VDR works over a bandwidth of 25% of the

sample rate (50% of the Nyquist band). The output bandwidth

of the AD6684 can be 25% only when operating in real mode.

Figure 85 shows the frequency zones for the 25% bandwidth

real output VDR mode tuned to a center frequency (fVDR) of fS/4

(tuning word = 0x04). The frequency zones where the amplitude

cannot exceed −30 dBFS are the upper and lower portions of

the Nyquist band signified by the gray shaded areas.

dBFS

–30

01/8

3/8

1/2

Figure 85. 25% VDR Bandwidth, Real Mode

The center frequency (fVDR) of the VDR function can be tuned

within the Nyquist band from 1/8 fS to 3/8 fS in 1/16 fS steps. In

real mode, Tuning Word 2 (0x02) through Tuning Word 6

(0x06) are valid. Table 26 shows the relative frequency values,

and Table 27 shows the absolute frequency values based on a

sample rate of 491.52 MSPS.

Table 26. VDR Tuning Words and Relative Frequency

Values, 25% BW, Real Mode

Tuning

Word

Lower Band

Edge

Center

Frequency

Upper Band

Edge

2 (0x02)

1/8 fS

1/4 fS

3 (0x03)

1/16 fS

3/16 fS

5/16 fS

4 (0x04)

1/8 fS

1/4 fS

3/8 fS

5 (0x05)

3/16 fS

5/16 fS

7/16 fS

6 (0x06)

1/4 fS

3/8 fS

1/2 fS

Table 27. VDR Tuning Words and Absolute Frequency

Values, 25% BW, Real Mode (fS = 491.52 MSPS)

Tuning

Word

Lower Band

Edge (MHz)

Center

Frequency

(MHz)

Upper Band

Edge (MHz)

2 (0x02)

61.44

122.88

3 (0x03)

30.72

92.16

153.6

4 (0x04)

61.44

122.88

184.32

5 (0x05)

92.16

153.6

215.04

6 (0x06)

122.88

184.32

245.76

VDR COMPLEX MODE

The complex mode of VDR works with selectable bandwidths

of 25% of the sample rate (50% of the Nyquist band) and 43% of

the sample rate (86% of the Nyquist band). Figure 86 and Figure 87

show the frequency zones for VDR in the complex mode. When

operating VDR in complex mode, place in-phase (I) input signal

data in Channel A and place quadrature (Q) signal data in

Channel B.

Figure 86 shows the frequency zones for the 25% bandwidth

VDR mode with a center frequency of fS/4 (tuning word =

0x04). The frequency zones where the amplitude may not

exceed −30 dBFS are the upper and lower portions of the

Nyquist band extending into the complex domain.

–30

dBFS

1/8

3/8

1/2

–1/2

Figure 86. 25% VDR Bandwidth, Complex Mode

The center frequency (fVDR) of the VDR function can be tuned

within the Nyquist band from 0 to 15/16 fS in 1/16 fS steps. In

complex mode, Tuning Word 0 (0x00) through Tuning Word 15

(0x0F) are valid. Table 28 and Table 29 show the tuning words

and frequency values for the 25% complex mode. Table 28

shows the relative frequency values, and Table 29 shows the

absolute frequency values based on a sample rate of 491.52 MSPS.

Table 28. VDR Tuning Words and Relative Frequency

Values, 25% BW, Complex Mode

Tuning Word

Lower

Band Edge

Center

Frequency

Upper Band

Edge

0 (0x00)

−1/8 fS

1/8 fS

1 (0x01)

−1/16 fS

1/16 fS

3/16 fS

2 (0x02)

1/8 fS

1/4 fS

3 (0x03)

1/16 fS

3/16 fS

5/16 fS

4 (0x04)

1/8 fS

1/4 fS

3/8 fS

5 (0x05)

3/16 fS

5/16 fS

7/16 fS

6 (0x06)

1/4 fS

3/8 fS

1/2 fS

7 (0x07)

5/16 fS

7/16 fS

9/16 fS

8 (0x08)

3/8 fS

1/2 fS

5/8 fS

9 (0x09)

7/16 fS

9/16 fS

11/16 fS

10 (0x0A)

1/2 fS

5/8 fS

3/4 fS

11 (0x0B)

9/16 fS

11/16 fS

13/16 fS

12 (0x0C)

5/8 fS

3/4 fS

7/8 fS

13 (0x0D)

11/16 fS

13/16 fS

15/16 fS

14 (0x0E)

3/4 fS

7/8 fS

15 (0x0F)

13/16 fS

15/16 fS

17/16 fS

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