Direct-Conversion Tuner ICs for

Digital DBS Applications

______________________________________________________________________________________

15

MHz

DESIRED

CHANNEL

(20Msps)

dBc

AT SIGNAL

POINT NO. 2

0

0

10

PASSBAND FILTER REQUIREMENTS:

(1) <20% OF BAUD PERIOD DELAY RIPPLE

(2) <0.1dB GAIN RIPPLE

TRANSITION BAND FILTER REQUIREMENTS:

< 2dB GAIN RIPPLE

FILTER CUTOFF

12

20

28

40

MHz

17.16

-30

ADC SAMPLING

TRANSPONDER SPACING = 29.16 MHz

LOWPASS FILTER RESPONSE

DESIRED

CHANNEL

(20Msps)

UNDESIRED ADJACENT CHANNEL

TRANSPONDER BW = 24MHz

dBc

AT SIGNAL

POINT NO. 1

0

0

10

DATA NYQUIST

NYQUIST FOR ADC

α = 0.2

12

20

29.16

40 41.16

17.16

-30

ADC SAMPLING

MHz

DESIRED

CHANNEL

(20Msps)

ADJ

CHANNEL

FOLDOVER

dBc

AT SIGNAL

POINT NO.3

0

0

10

ADC NYQUIST

12

20

40

17.16

-30

ADC SAMPLING

MHz

DESIRED

CHANNEL

(20Msps)

DIGITAL MATCHED FILTER

dBc

AT SIGNAL

POINT NO.4

0

10 12

40

-30

ADC SAMPLING

Figure 5. Lowpass Filtering Example. Note: Data Rate = 40Mbps, Transponder BW = 24MHz, Transponder Spacing = 29.16MHz,

Nyquist Filter:

α = 0.2