AD6622SSLASHPCB datasheet(26/28 Pages) AD

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AD6622

–26–

REV. 0

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 (NRCF), and the DMEM restriction to NRCF.

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

(fSCLK) is determined by the equation below. To minimize the

number of processing channels, SCLKDIVIDER should be set

as low as possible to get the highest fSCLK that the serial data

source can accept.

SCLK

CLK

DIVIDER

×+

()

(23)

A minimum of 32 SCLK cycles are required to accept an input

sample, so the minimum number of TSPs (NTSP) due to limited

Serial Port bandwidth is a function of the input sample rate (fIN),

as shown by the equation below.

ceil

TSP

SCLK

≥









(24)

For a sample UMTS system, we will assume fCLK = 61.44 MHz,

and the serial data source can drive data at 30.72 MBPS

(SCLKDIVIDER = 0). To achieve fIN = 3.84 MHz, the mini-

mum NTSP is 4. (This is TSP channels, not TSP ICs.)

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 NTAPS equation from the RCF description, are

three restrictions to the RCF impulse response length, NRCF.

Time

CMEM

Restriction

↓↓

RCF

≤×









min

128

(25)

↑

DMEM

Restriction

where:

RCF

CIC

TSP

CLK

=×

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 (NTSP). To keep the out-

put rate ﬁxed, L must be increased by a factor of NCH, which

extends the time restriction. This increase in L may be achieved

by increasing any one or more of LRCF, LCIC5, or LCIC2 within their

normal limits. Achieving a larger L by increasing LRCF instead

of LCIC5 or LCIC2, will relieve the DMEM restriction as well.

In a UMTS example, NTSP = 4, fCLK = 61.44 MHz, and fIN =

3.84 MHz, resulting in L = 64. Factoring L into LRCF = 8, LCIC =

8, and LCIC2 = 1, results in a maximum NRCF = 32 due to the time

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 (fIN

× L

RCF/NTSP). The composite RCF and

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/NTSP.

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/NTSP larger than its

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 NTSP = 4, so in order to

respond as quickly as possible to a Start SYNC, the Hold-Off

Counter values should be 1, 17, 33, and 49.

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