GC1012B Datasheet, PDF (8/30 Pages) Texas Instruments – 3.3V DIGITAL TUNER CHIP

GC1012B 3.3V DIGITAL TUNER
SLWS138B
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
ripple and over 75 dB of out of band rejection. The 2 dB output bandwidth is +/- 0.4F_S (80% usable
bandwidth) where F_S is the complex output rate. The 0.1 dB bandwidth is +/- 0.36 F_S (72% usable
bandwidth). The number of taps is equal to 20D.
The coefficients for the 40 tap decimate by 2 filter are:
-12
39
6462
276
-42
-396
3115
82
-52
-293
-409
-145
7
434
-1400
-110
85
714
-273
46
46
-273
714
85
-110
-1400
434
7
-145
-409
-293
-52
82
3115
-396
-42
276
6462
39
-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.
Texas Instruments Incorporated
-4-
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