AD6684 Datasheet, PDF (52/107 Pages) Analog Devices – 135 MHz Quad IF Receiver

AD6684
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
0
1/8 f_S
3/8 f_S
1/2 f_S
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 Center
Edge
Frequency
Upper Band
Edge
2 (0x02)
0
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)
0
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
Data Sheet
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.
dBFS
–30
–1/2 f_S
0
1/8 f_S 3/8 f_S 1/2 f_S
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
Lower
Center
Tuning Word Band Edge Frequency
Upper Band
Edge
0 (0x00)
−1/8 fS
0
1/8 fS
1 (0x01)
−1/16 fS
1/16 fS
3/16 fS
2 (0x02)
0
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
fS
15 (0x0F)
13/16 fS
15/16 fS
17/16 fS
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