Texas Instruments Incorporated

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GC1012B 3.3V DIGITAL TUNER

SLWS138B

This document contains information which may be changed at any time without notice

See Section 3.2 for more details on the frequency load modes.

The 28 bit frequency word is accumulated in the 28 bit frequency accumulator. The frequency

accumulator will normally free run, but can be synchronously cleared by either the system sync (SS) or the

accumulator sync (AS). The accumulator clear modes are controlled by bits in control register 4. See

Section 3.2 for details.

The upper 13 frequency accumulator bits are used to generate the oscillator’s sine and cosine

outputs. These sines and cosines are generated to 12 bit accuracy. The oscillator’s peak spur levels are

below -75 dB.

1.5

MIXER

The mixer multiplies the 12 bit input samples by the 12 bit sine and cosine values coming from the

digital oscillator. An input signal at the oscillator’s tuning frequency will be centered at zero frequency after

passing through the mixer. The mixer outputs are rounded to 13 bits using the “round-to-even” rounding

algorithm. The “round-to-even” algorithm prevents a DC rounding bias by detecting fractions which are

exactly equal to 0.5 and rounding them up half of the time and rounding them down half of the time. The

choice to round up or down is made so as to always give an even result.

1.6

PROGRAMMABLE LOW PASS FILTER

The mixer’s output is filtered using a programmable bandwidth low pass filter. The filter allows the

output sample rate to be reduced by a factor of D = 2, 4, 8, 16, 32, or 64. The value of D is set using control

register 5. The filter can be bypassed by setting D equal to 1. This allows the chip to be used as a mixer

without any output filtering.

The low pass filter is a finite impulse response (FIR) filter with linear phase, 0.13 dB peak to peak

bandwidth). The number of taps is equal to 20D.

The coefficients for the 40 tap decimate by 2 filter are:

-12

-42

-52

7

85

46

-110

-145

82

276

39

-396

-293

434

714

-273

-1400

-409

3115

6462

6462

3115

-409

-1400

-273

714

434

-293

-396

39

276

82

-145

-110

46

85

7

-52

-42

-12

The coefficients for the other filters are available from GRAYCHIP.

Figure 2 shows the spectral response of the decimate by 2 low pass filter. The decimate by 4, 8,

16, 32 and 64 filters are similar. Figure 2(a) shows the overall frequency response prior to decimation.Note

that the filter rolls off quickly to 60 dB and is down below 75 dB in the region which aliases back into the

passband. Figure 2(b) shows the 0.13 dB ripple in the passband.