SLWS132 Datasheet, PDF (11/46 Pages) Texas Instruments – QUAD RECEIVER CHIP

GC4014 QUAD RECEIVER CHIP
DATA SHEET REV 0.6
Figure 6 shows the maximum spur levels as the tuning frequency is scanned over a portion of the frequency
range with the peak hold function of the spectrum analyzer turned on. Notice that the peak spur level is -82 dB before
dithering and is between -92 and -102 after dithering has been turned on.
-82 dB
-102 dB ^{-92 dB}
(a) Before Dithering
(b) After Dithering
Figure 6. NCO Peak Spur Scan Plot
The worst case NCO spurs at -92 dB, such as the one shown in figure 6(b), are due to a few frequencies
that are related to the sampling frequency by small rational numbers (for example FREQ = 3/16 * Fck). In these
cases the rounding errors in the sine/cosine lookup table repeat in a regular fashion, thereby concentrating the error
power into a single frequency, rather than spreading it across the spectrum. These worst case spurs can be
eliminated by selecting an initial phase that minimizes the errors or by changing the tuning frequency by a small
amount (50 Hz). All spurs can be made to fall below -96 dB with the selection of a proper initial phase or tuning
frequency.
3.3.2 Four Stage CIC Filter
The mixer outputs are decimated by a factor of N in a four stage CIC filter, where N is any integer between
8 and 16,384. The programmable decimation allows the chip’s usable output bandwidth to range from less than a
kilo-Hertz to 1.5 MHz when the input rate (which is equal to the chip’s clock rate) is 62.5 MHz. A block diagram of
the CIC filter is shown in Figure 7.
DATA ^{16 BITS}
IN
CLOCKED AT FULL RATE
UPPER
24 BITS
CLOCKED AT 1/N RATE
24 BITS
DATA
OUT
Figure 7. Four Stage CIC Decimate by N Filter
The CIC filter has a gain equal to N⁴ which must be compensated for in the “CIC_SCALE” circuit shown in
Figure 7. This circuit has a gain equal to 2^{(SCALE+6*BIG_SCALE-56)}, where SCALE ranges from 0 to 5 and BIG_SCALE
ranges from 0 to 7. The gain of the CIC circuit is equal to: GAIN = N⁴2^{(SCALE + 6 × BIG_SCALE – 56)} . The user must select
values for SCALE and BIG_SCALE such that GAIN is less than one, i.e., SCALE and BIG_SCALE must be selected
such that: (SCALE + 6 × BIG_SCALE) ≤ (56 – 4log₂N) . Overflows due to improper gain settings will go undetected if this
relationship is violated. For example, if N is equal to 8, then this restriction means that BIG_SCALE and SCALE
GRAYCHIP, INC.
-6-
APRIL 27, 1999
This document contains information which may be changed at any time without notice