GC5316 datasheet(5/75 Pages) TI

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GC5316

SLWS154A − JANUARY 2004 − REVISED MARCH 2004

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Next the signal is sent to an 8-stage FIFO. This allows an arbitrary phase relationship between adcclk and rxclk. The

frequency relationship is fixed by the configuration. The FIFO can be bypassed, clocking the input data directly on

rxclk. Note that if the FIFO is bypassed, the hold times are longer than usual. If the input rate is a fraction (1/2,

1/4,

or 1/8) of rxclk then ssel_rxin determines which of the multiple rising clock edges are used to sample the data.

Table 1. Programming

VARIABLE

DESCRIPTION

rduz_sens_ena

When enabled adds noise to the ADC input using nz_pwr_mask.

nz_pwr_mask

Selects the noise bits to be added to the ADC input sample when rduz_sens_ena is one.

adc_fifo_bypass

When asserted bypasses the input FIFO. Data is latched directly using the rxclk input when FIFO is

bypassed. Should set this to 0 when input data is to be latched using the adcclk input. Most applications

should not bypass the fifo.

2.1.2 Resampler Block

Two of the four 16-bit data input ports, rxin_a, and rxin_c have resampler blocks. The data is clocked from an external

clock signal adcclk. The real input signal is downconverted by adcclk/4 and is then low-pass filtered and decimated.

Decimation can be either by 1.5 or by 2. Both these decimation modes support up to 24 CDMA DDC channels, or

12 UMTS DDC channels.

Resam p ler D ecim ate by 1.5

LPF

24 tap

adcclk

rxclk

bus b (or d)

bus a (or c)

at rxclk/2

rxin_a (or c)

at adcclk

rx_distribution

Figure 3. Resampler Decimate by 1.5 Mode

The decimate by 1.5 mode requires the input data rate to be 3/4 of the receive clock rate (adcclk frequency is 3/4

rxclk frequency). A 24-tap low-pass decimation filter with programmable 18-bit coefficients removes alias images that

would fold into the passband prior to decimation. The following table shows the performance of filters designed for

various bandwidths when the resampler is decimating by 1.5. The table also shows the resulting passband

frequencies assuming the input data rate is 92.16 MSPS. Each horizontal row is a unique 24-tap filter which is

available on the web.

Table 2. Resampler Filter Performance in the Decimate by 1.5 Mode

GENERAL APPLICATION

EXAMPLE APPLICATION

adclk: 92.16 MHz

PASSBAND

RIPPLE

STOPBAND

BANDWIDTH

F LOWER

F CENTER

F UPPER

of clk

MHz

0.05

−101.5

18.4

13.8

32.3

0.06

−98

22.1

34.1

0.07

−91.7

25.8

10.1

35.9

0.08

0.01

−84.1

29.5

8.3

37.8

0.09

0.03

−75.6

33.2

6.5

39.6

0.1

0.07

−67.1

36.9

4.6

41.5

0.11

0.18

−59.5

40.6

2.8

43.3

0.05

−101.5

18.4

13.8

32.3

0.06

−98

22.1

34.1

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