AD6653 Datasheet, PDF (33/80 Pages) Analog Devices – IF Diversity Receiver

AD6653
DECIMATING HALF-BAND FILTER AND FIR FILTER
The goal of the AD6653 half-band digital filter is to allow the
0
sample rate to be reduced by a factor of 2 while rejecting aliases
–10
that fall into the band of interest. This filter is designed to operate
–20
as either a low-pass or a high-pass filter and to provide >100 dB
–30
of alias protection for 11% of the input rate of the structure.
–40
Used in conjunction with the NCO and the FIR filter, the half-
–50
band filter can provide an effective band-pass. For an ADC
–60
sample rate of 150 MSPS, this provides a maximum usable
–70
bandwidth of 33 MHz.
–80
HALF-BAND FILTER COEFFICIENTS
–90
The 19-tap, symmetrical, fixed-coefficient half-band filter has low
power consumption due to its polyphase implementation. Table 15
lists the coefficients of the half-band filter. The normalized coeffi-
cients used in the implementation and the decimal equivalent
value of the coefficients are also listed. Coefficients not listed
in Table 15 are 0s.
Table 15. Fixed Coefficients for Half-Band Filter
Coefficient
Number
Normalized
Coefficient
Decimal Coefficient
(20-Bit)
C0, C18
0.0008049
844
C2, C16
−0.0059023
−6189
C4, C14
0.0239182
25080
C6, C12
−0.0755024
−79170
C8, C10
0.3066864
321584
C9
0.5
524287
HALF-BAND FILTER FEATURES
In the AD6653, the half-band filter cannot be disabled. The
filter can be set for a low-pass or high-pass response. For a high-
pass filter, Bit 1 of Register 0x103 should be set; for a low-pass
response, this bit should be cleared. The low-pass response of
the filter with respect to the normalized output rate is shown in
Figure 70, and the high-pass response is shown in Figure 71.
0
–10
–20
–100
–110
0
0.1
0.2
0.3
0.4
FRACTION OF INPUT SAMPLE RATE
Figure 71. Half-Band Filter, High-Pass Response
The half-band filter has a ripple of 0.000182 dB and a rejection
of 100 dB. For an alias rejection of 100 dB, the alias protected
bandwidth is 11% of the input sample rate. If both the I and the
Q paths are used, a complex bandwidth of 22% of the input rate
is available.
In the event of even Nyquist zone sampling, the half-band filter
can be configured to provide a spectral reversal. Setting Bit 2
high in Address 0x103 enables the spectral reversal feature.
The half-band decimation phase can be selected such that the
half-band filter starts on the first or second sample following
synchronization. This shifts the output from the half-band between
the two input sample clocks. The decimation phase can be set to
0 or 1, using Bit 3 of Register 0x103.
FIXED-COEFFICIENT FIR FILTER
Following the half-band filters is a 66-tap, fixed-coefficient FIR
filter. This filter is useful in providing extra alias protection for
the decimating half-band filter. It is a simple sum-of-products
FIR filter with 66 filter taps and 21-bit fixed coefficients. Note
that this filter does not decimate. The normalized coefficients
used in the implementation and the decimal equivalent value of
the coefficients are listed in Table 16.
–30
The user can either select or bypass this filter, but the FIR filter
–40
can be enabled only when the half-band filter is enabled. Writing
–50
Logic 0 to the enable FIR filter bit (Bit 0) in Register 0x102
–60
bypasses this fixed-coefficient filter. The filter is necessary when
–70
using the final NCO with a real output; bypassing it when using
–80
other configurations results in power savings.
–90
–100
–110
0
0.1
0.2
0.3
0.4
FRACTION OF INPUT SAMPLE RATE
Figure 70. Half-Band Filter, Low-Pass Response
Rev. 0 | Page 33 of 80