AD6652 Datasheet, PDF (40/76 Pages) Analog Devices – 12-Bit, 65 MSPS IF to Baseband Diversity Receiver

AD6652
INTERPOLATING HALF-BAND FILTERS
The AD6652 has two interpolating half-band FIR filters that
immediately precede the two digital AGCs and follow the four
RCF channel outputs. Each interpolating half-band takes
16-bit I and 16-bit Q data from the preceding RCF and outputs
16-bit I and 16-bit Q to the AGC. The half-band and AGC
operate independently of each other, so the AGC can be
bypassed, in which case the output of the half-band is sent
directly to the output data port. The half-band filters also
operate independently of each other—either one can be enabled
or disabled. The control register for Half-Band A is at Address
0x08 and for Half-Band B is at Address 0x09.
Half-band filters also perform the function of interleaving data
from various RCF channel outputs prior to the actual function
of interpolation. Interleaving of data is allowed even when the
half-band filter is bypassed. This allows the implementation of a
polyphase filter by combining the processing power of multiple
channels to act upon a single carrier. This is accomplished by
appropriate phasing of the processing channels using one of the
following methods:
• RCF phase decimation
• Start hold-off counter
For example, if two channels of the AD6652 are used to process
one CDMA2000 carrier, RCF filters for both the channels
should be 180° out of phase. This can be done using RCF phase
decimation or an appropriate start hold-off counter followed by
appropriate NCO phase offsets.
Half-band A can listen to all four channels: Channels 0, 1, 2, and
3; Channel 0 and 1; or only Channel 0. Half-band B can listen to
Channels 2 and 3, or only Channel 2. Each half-band interleaves
the channels specified in its control register and interpolates by
two on the combined data from those channels. For one channel
running at twice the chip rate, the half-band can be used to
output channel data at four times the chip rate. The frequency
response of the interpolating half-band FIR is shown in
Figure 50.
SPECTRUM OF HALF-BAND
0
–10
–20
–30
–40
–50
–60
–70
–80
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
i
N
3f_SAMP
f_SAMP
f_CHIP
f_CHIP
MULTIPLES OF CHIP RATE
Figure 50. Interpolating Half-Band Frequency Response
The SNR of the interpolating half-band is around −149.6 dB.
The highest error spurs due to fixed-point arithmetic are
around −172.9 dB. The coefficients of the 13-tap interpolating
half-band FIR are given in Table 18.
Table 18. Half-Band Coefficients
0 14 0 −66 0 309 512 309 0 −66 0 14 0
Rev. 0 | Page 40 of 76