AD9857SLASHPCB datasheet(14/31 Pages) AD

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AD9857

–14–

REV. 0

Fixed Interpolator (4 )

This block is a ﬁxed 4

× interpolator. It is implemented as two

half-band ﬁlters. The output of this stage is the original data

upsampled by 4

×.

Before presenting a detailed description of the half-band ﬁlters,

recall that in the case of the Quadrature Modulation Mode the

input data stream is representative of complex data; i.e., two

input samples are required to produce one I/Q data pair. The

I/Q sample rate is one-half the input data rate. The I/Q sample

rate (the rate at which I or Q samples are presented to the input

of the ﬁrst half-band ﬁlter) will be referred to as fIQ. Since the

AD9857 is a quadrature modulator, fIQ represents the baseband

of the internal I/Q sample pairs. It should be emphasized here

that fIQ is not the same as the baseband of the user’s symbol rate

data, which must be upsampled before presentation to the AD9857

(as will be explained later). The I/Q sample rate (fIQ) puts a

limit on the minimum bandwidth necessary to transmit the fIQ

spectrum. This is the familiar Nyquist limit and is equal to one-half

fIQ, hereafter referred to as fNYQ.

Together, the two half-band ﬁlters provide a factor-of-four increase

in the sampling rate (4

× f

IQ or 8

× f

NYQ). Their combined insertion

loss is 0.01 dB, so virtually no loss of signal level occurs through

the two half-band ﬁlters. Both half-band ﬁlters are linear phase ﬁl-

ters, so that virtually no phase distortion is introduced within the

pass band of the ﬁlters. This is an important feature as phase

distortion is generally intolerable in a data transmission system.

The half-band ﬁlters are designed so that their composite perfor-

mance yields a usable pass band of 80% of the baseband Nyquist

frequency (0.2 on the frequency scale below). Within that pass

band the ripple will not exceed 0.002 dB. The stopband extends

from 120% to 400% of the baseband Nyquist frequency (0.3

to 1.0 on the frequency scale below) and offers a minimum of

85 dB attenuation. The composite response of the two half-

band ﬁlters together are shown in Figures 22 and 23.

FREQUENCY

0.2

0.4

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

–130

–140

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.3

0.2

–85

Figure 22. Half-Band 1 and 2 Frequency Response;

Frequency Relative to HB1 Output Sample Rate

Inverse CIC Filter

The Inverse CIC Filter precompensates the data in order to off-

set the slight attenuation gradient imposed by the CIC ﬁlter (see

the Programmable (2

×–63×) CIC Interpolating Filter section).

The I (or Q) data entering the ﬁrst half-band ﬁlter occupies a

maximum bandwidth of one-half fDATA as deﬁned by Nyquist

(where fDATA is the sample rate at the input of the ﬁrst half-band

ﬁlter). This is shown graphically in Figure 21.

INBAND

ATTENUATION

GRADIENT

CIC FILTER RESPONSE

fDATA/2

fDATA

4fDATA

Figure 21. CIC Filter Response

Table I. Parallel Data Bus Timing

Symbol

Definition

Min

tDS

Data Setup Time

4 ns

tDH

Data Hold Time

0 ns

If the CIC ﬁlter is employed, the inband attenuation gradient

could pose a problem for those applications requiring an extremely

flat pass band. For example, if the spectrum of the data as supplied

to the AD9857 I or Q path occupies a signiﬁcant portion of the

one-half fDATA region, the higher frequencies of the data spectrum

will receive slightly more attenuation than the lower frequencies

(the worst-case overall droop from f = 0 to one-half fDATA is

< 0.8 dB). This may not be acceptable in certain applications. The

Inverse CIC ﬁlter has a response characteristic that is the inverse of

the CIC ﬁlter response over the one-half fDATA region.

The net result is that the product of the two responses yields in

an extremely flat pass band, thereby eliminating the inband

attenuation gradient introduced by the CIC ﬁlter. The price to

be paid is a slight attenuation of the input signal of approximately

0.5 dB for a CIC interpolation rate of 2 dB and 0.8 dB for inter-

polation rates of 3 to 63.

The Inverse CIC Filter is implemented as a digital FIR ﬁlter

with a response characteristic that is the inverse of the Program-

mable CIC Interpolator. The product of the two responses yields a

nearly flat response over the baseband Nyquist bandwidth. The

Inverse CIC ﬁlter provides frequency compensation that yields a

response flatness of

±0.05 dB over the baseband Nyquist band-

width, allowing the AD9857 to provide excellent SNR over its

performance range.

The Inverse CIC Filter can be bypassed by setting Control

interpolation rate is 1

×. Whenever this stage is bypassed, power

to the stage is shut off, thereby reducing power dissipation.

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