AD9789BBCZ datasheet(44/76 Pages) AD

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AD9789

Rev. A | Page 44 of 76

Table 54 shows recommended sum scale values for each QAM

mapper mode. The criteria used to determine the recommended

sum scale values were MER/EVM measurements and spectral

purity. Because clipping results in impulsive noise, it can be

observed in the output spectrum as a transient increase in the

output noise floor. These sum scale values were chosen such

that the transient increases in the noise floor were minimal.

These tests were completed for one, two, three, and four carrier

outputs at approximately 850 MHz. Because clipping can occur

in the RF chain following the DAC, further verification of these

values should be performed at the system level by adding BER

tests to the sum scale selection criteria.

Table 54. Recommended Sum Scale Values for all QAM

Mapper Modes and Channel Count

QAM

Mode

Sum Scale Value (Decimal)

1 Channel

2 Channels

3 Channels

4 Channels

DVB-C

16-QAM

DVB-C

32-QAM

DVB-C

64-QAM

DVB-C

128-QAM

DVB-C

256-QAM

DOCSIS

64-QAM

DOCSIS

256-QAM

Digital 16× Tunable Band-Pass Filter

The digital band-pass filter works in conjunction with a fixed

16× interpolator (see Figure 88). The 16× interpolation filter

creates 16 images of the baseband signal in the Nyquist band

of the DAC. The digital band-pass filter must then be tuned to

reject the 15 undesired images. The center frequency of the

band-pass filter can be placed anywhere from dc to fDAC/2. The

tuning word for the band-pass filter center frequency can be

calculated as follows:

1







DAC

CENTER

Freq

Center

BPF

The resulting tuning word is a 16-bit value where the most

significant byte is written to Register 0x1D[7:0] and the least

significant byte is written to Register 0x1C[7:0].

Figure 88. Conceptual Block Diagram of 16× Tunable Band-Pass Filter

The width of the filter’s stop band is fixed at approximately

fDAC/64. The effective FLAT pass band is fDAC/64. As can be

inferred from Figure 89 to Figure 91, mistuning of the BPF

center frequency can result in unwanted images appearing. Care

should be taken to appropriately filter the desired signal with

the interpolation filters prior to the input of the BPF..

–20

–40

–60

–80

–100

0.5

1.0

1.5

2.0

FREQUENCY (GHz)

Figure 89. Band-Pass Filter Response at 200 MHz, fDAC = 2.4 GHz

–20

–40

–60

–80

–100

0.5

1.0

1.5

2.0

FREQUENCY (GHz)

Figure 90. Band-Pass Filter Response at 1 GHz, fDAC = 2.4 GHz

–2

–4

–6

–8

–10

020

FREQUENCY (MHz)

Figure 91. Band-Pass Filter Pass-Band Detail, fDAC = 2.4 GHz

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