AD6622AS Analog Devices Inc, AD6622AS Datasheet - Page 26

• Manufacturer Part Number: AD6622AS
• Manufacturer: Analog Devices Inc

Specifications of AD6622AS

• Lead Free Status / RoHS Status: Not Compliant

AD6622
MULTIPLE TSP OPERATION
Each of the four Transmit Signal Processors (TSPs) of the
AD6622 can adequately reject the interpolation images of nar-
row bandwidth carriers such as AMPS, IS-136, GSM, EDGE,
and PHS. Wider bandwidth carriers such as IS-95 and UMTS
require a coordinated effort of multiple processing channels.
This section demonstrates how to coordinate multiple TSPs
to create wider bandwidth channels without sacrificing image
rejection. As an example, a UMTS carrier is modulated using
four TSPs (an entire AD6622). The same principals can be
applied to different designs using more or fewer TSPs. This sec-
tion does not explore techniques for using multiple TSPs to
solve problems other than Serial Port or RCF throughput.
Designing filter coefficients and control settings for deinterleaved
TSPs is no harder than designing a filter for a single TSP. For
example, if four TSPs are to be used, simply divide the input data
rate by four and generate the filter as normal. For any design, a
better filter can always be realized by incrementing the number
of TSPs to be used. When it is time to program the TSPs, only
two small differences must be programmed. First each channel
is configured with exactly the same filter, scalars, modes and
NCO frequency. Since each channel receives data at 1/4 the
data rate and in a staggered fashion, the Start Hold-Off Counters
must also be staggered (see Programming Multiple TSPs section
below). Second, the phase offset of each NCO must be set to
match the demultiplexed ratio (1/4 in this example). Thus the
phase offset should be set to 90 degrees (16384, which is 1/4
of a 16-bit register).
Determining the Number of TSPs to Use
There are three limitations of a single TSP that can be over-
come by deinterleaving an input stream into multiple TSPs:
Serial Port bandwidth, the time restriction to the RCF impulse
response length (N
If the input sample rate is faster than the Serial Port can accept
data, the data can be deinterleaved into multiple Serial Ports.
Recalling from the Serial Port description, the SCLK frequency
(f
number of processing channels, SCLK
as low as possible to get the highest f
source can accept.
A minimum of 32 SCLK cycles are required to accept an input
sample, so the minimum number of TSPs (N
Serial Port bandwidth is a function of the input sample rate (f
as shown by the equation below.
For a sample UMTS system, we will assume f
and the serial data source can drive data at 30.72 MBPS
(SCLK
mum N
Multiple TSPs are also required if the RCF does not have enough
time or DMEM space to calculate the required RCF filter. Recall-
ing the maximum N
three restrictions to the RCF impulse response length, N
SCLK
N
f
) is determined by the equation below. To minimize the
SCLK
TSP
DIVIDER
TSP
≥
is 4. (This is TSP channels, not TSP ICs.)
=
ceil
2
= 0). To achieve f
×



32
(
RCF
SCLK
f
TAPS
SCLK
×
), and the DMEM restriction to N
f
f
IN
CLK
equation from the RCF description, are
DIVIDER



+
IN
1
)
= 3.84 MHz, the mini-
SCLK
DIVIDER
that the serial data
TSP
CLK
should be set
) due to limited
= 61.44 MHz,
RCF
RCF
IN
.
.
(23)
(24)
),
where:
Deinterleaving the input data into multiple TSPs will extend the
time restriction and may possibly extend the DMEM restriction,
but will not extend the CMEM restriction. Deinterleaving the
input stream to multiple TSPs divides the input sample rate to
each TSP by the number of TSPs used (N
put rate fixed, L must be increased by a factor of N
extends the time restriction. This increase in L may be achieved
by increasing any one or more of L
normal limits. Achieving a larger L by increasing L
of L
In a UMTS example, N
3.84 MHz, resulting in L = 64. Factoring L into L
8, and L
restriction. Figure 22 shows an example RCF impulse response
that has a frequency response as shown in Figure 23 from
0 Hz to 7.68 MHz (f
CIC frequency response is shown in Figure 24, on the same fre-
quency scale. This figure demonstrates a good approximation to
a root-raised-cosine with a roll-off factor of 0.22, a pass-band
ripple of 0.1 dB, and a stopband ripple better than –65 dB until
the lobe of the first image which peaks at –50 dB about 5.6 MHz
from the carrier center. This lobe could be reduced by shifting
more of the interpolation towards the RCF, but that would
sacrifice near-in performance. As shown, the first image can easily
be rejected by an analog filter further up the signal path.
Scaling must be considered as normal with an interpolation
factor of L, to guarantee no overflow in the RCF, CIC, or NCOs.
The output level at the summation port should be calculated
using an interpolation factor of L/N
Programming Multiple TSPs
Configuring the TSPs for deinterleaved operation is straight-
forward. All of the Channel Registers and CMEM of each TSP
are programmed identically, except the Start Hold-Off Counters
and NCO Phase Offset.
In order to separate the input timing to each TSP, the Hold-
Off Counters must be used to start each TSP successively in
response to a common Start SYNC. The Start SYNC may origi-
nate from the SYNC pin or the Microport. Each subsequent
TSP must have a Hold-Off Counter value L/N
predecessor’s. If the TSPs are located on cascaded AD6622s,
the Hold-Off Counters of the upstream device should be incre-
mented by an additional one.
In the UMTS example, L = 64 and N
respond as quickly as possible to a Start SYNC, the Hold-Off
Counter values should be 1, 17, 33, and 49.
CIC5
N
RCF
CIC2
or L
L
≤
= 1, results in a maximum N
CIC2
=
min
L
Restriction
, will relieve the DMEM restriction as well.
RCF
Time



↓
L
2
IN
×
,
Restriction
× L
16
L
DMEM
TSP
CIC
RCF
×
↑
= 4, f
5
L
×
/N
RCF
L
TSP
Restriction
CLK
RCF
CIC
CMEM
,
128
). The composite RCF and
TSP
2
, L
↓
= 61.44 MHz, and f
=
.
CIC5
TSP



RCF
N
TSP
TSP
, or L
= 4, so in order to
= 32 due to the time
). To keep the out-
f
TSP
IN
×
RCF
CIC2
f
CLK
larger than its
= 8, L
within their
RCF
CH
, which
instead
IN
CIC
(25)
=
=