AD9857SLASHPCB datasheet(15/31 Pages) AD

15 / 31 page

AD9857

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FREQUENCY

Figure 23. Half-Band 1 and 2 Pass Band Detail; Frequency

Relative to HB1 Output Sample Rate

The usable bandwidth of the ﬁlter chain puts a limit on the

maximum data rate that can be propagated through the AD9857.

A look at the pass band detail of the half-band ﬁlter response

indicates that in order to maintain an amplitude error of no

more than 1 dB, we are restricted to signals having a bandwidth

of no more than about 90% of fNYQ. Thus, in order to keep the

bandwidth of the data in the flat portion of the ﬁlter pass band,

the user must oversample the baseband data by at least a factor

of two prior to presenting it to the AD9857. Note that without

oversampling, the Nyquist bandwidth of the baseband data cor-

responds to the fNYQ. Because of this, the upper end of the data

bandwidth will suffer 6 dB or more of attenuation due to the

frequency response of the half-band ﬁlters. Furthermore, if the

baseband data applied to the AD9857 has been pulse shaped

there is an additional concern.

Typically, pulse shaping is applied to the baseband data via a

ﬁlter having a raised cosine response. In such cases, an

α value

is used to modify the bandwidth of the data where the value of

is such that 0 <

α < 1. A value of 0 causes the data bandwidth

to correspond to the Nyquist bandwidth. A value of 1 causes the

data bandwidth to be extended to twice the Nyquist bandwidth.

Thus, with 2

× oversampling of the baseband data and α = 1, the

Nyquist bandwidth of the data will correspond with the I/Q

Nyquist bandwidth. As stated earlier, this results in problems

near the upper edge of the data bandwidth due to the roll-off

attenuation of the half-band ﬁlters. The following diagrams

illustrate the relationship between

α and the bandwidth of raised

cosine shaped pulses. The problem area is indicated by the shading

in the tail of the pulse with

α = 1, which extends into the roll-off

region of the half-band ﬁlter.

The effect of raised cosine ﬁltering on baseband pulse bandwidth,

and the relationship to the half-band ﬁlter response is shown in

Figure 24.

fNYQ

2*fNYQ

fIQ

fNYQ

2*fNYQ

fNYQ

2*fNYQ

HALF-BAND FILTER RESPONSE

OVERSAMPLE RATE

= 0

= 0.5

= 1

BANDWIDTH

of I or Q

DATA

Figure 24. Effect of Alpha

Programmable (2 –63 ) CIC Interpolating Filter

The Programmable Interpolator is implemented as a CIC

(Cascaded Integrator-Comb) ﬁlter. It is programmable by a

6-bit control word, giving a range of 2

× to 63× interpolation.

This interpolator has a low-pass frequency characteristic that is

compensated by the Inverse CIC ﬁlter.

The Programmable Interpolator can be bypassed to yield a 1

× (no

interpolation) conﬁguration by setting the bit in the appropri-

ate control register, per each proﬁle. Whenever the Programmable

Interpolator is bypassed (1

× CIC rate) power to the stage is

removed. If the Programmable Interpolator is bypassed, the

Inverse CIC ﬁlter (see above) is automatically bypassed, since

its compensation is not needed in this case.

The output of the Programmable Interpolator is the data from

the 4

× interpolator upsampled by an additional 2× to 63×, accord-

ing to the rate chosen by the user. This results in the input data

being upsampled by a factor of 8

× to 252×.

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