AD9856SLASHPCB datasheet(17/32 Pages) AD

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AD9856

–17–

REV. B

HALF-BAND FILTERS (HBFs)

Before presenting a detailed description of the HBFs, recall that

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

AD9856 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 AD9856

(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, which hereafter will be referred to as fNYQ.

HBF 1 is a 47-tap filter that provides a factor-of-two increase in

sampling rate. HBF 2 is a 15-tap filter offering an additional

factor-of-two increase in sampling rate. Together, HBF 1 and 2

provide a factor-of-four increase in the sampling rate (4

× f

IQ or

× f

NYQ). Their combined insertion loss is a mere 0.01 dB, so

virtually no loss of signal level occurs through the first two HBFs.

HBF 3 is an 11-tap filter and, if selected, increases the sampling

rate by an additional factor of two. Thus, the output sample rate

of HBF 3 is 8

× f

IQ or 16

× f

NYQ. HBF 3 exhibits 0.03 dB of

signal level loss. As such, the loss in signal level through all

three HBFs is only 0.04 dB and may be ignored for all practi-

cal purposes.

In relation to phase response, all three HBFs are linear phase

filters. As such, virtually no phase distortion is introduced within

the passband of the filters. This is an important feature as phase

distortion is generally intolerable in a data transmission system.

In addition to knowledge of the insertion loss and phase re-

sponse of the HBFs, some knowledge of the frequency response

of the HBFs is useful as well. The combined frequency response

of HBF 1 and 2 is shown in Figure 29.

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

mum data rate that can be propagated through the AD9856. A

look at the passband detail of the HBF 1 and 2 response indi-

cates 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 band-

width of the data in the flat portion of the filter passband, the

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

two prior to presenting it to the AD9856. Note that without

oversampling, the Nyquist bandwidth of the baseband data

corresponds to the fNYQ. As such, the upper end of the data

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

frequency response of HBF 1 and 2. Furthermore, if the base-

band data applied to the AD9856 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

α is such that 0 ≤ α ≤ 1. A value of 0

causes the data bandwidth to correspond to the Nyquist band-

width. 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 frequency response of HBF 1 and 2.

3.5

DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW

–30

–100

0.5

1.0

1.5

2.0

2.5

3.0

4.0

–40

–50

–60

–70

–80

–90

–10

–20

a. Half-Band 1 and 2 Frequency Response

DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW

–6

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

–1

–2

–3

–4

–5

0.9

1.0

b. Passband Detail

Figure 29. Combined Frequency Response of HBF 1 and 2

To reiterate, the user must oversample their baseband data by at

least a factor of two (2). In addition, there is a further restriction

on pulse shaping. That is, the maximum value of

α that can be

implemented is 0.8. This is because the data bandwidth be-

comes: 1/2(1 +

α) f

NYQ = 0.9 fNYQ, which puts the data band-

width at the extreme edge of the flat portion of the filter response.

If a particular application requires an

α value between 0.8 and

1, then the user must oversample the baseband data by at least a

factor of four (4).

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