GC4014 datasheet(11/46 Pages) TI

11 / 46 page

GRAYCHIP, INC.

- 6 -

APRIL 27, 1999

GC4014 QUAD RECEIVER CHIP

DATA SHEET REV 0.6

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

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.

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.

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:

. 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:

. 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

(a) Before Dithering

(b) After Dithering

-92 dB

-102 dB

-82 dB

UPPER

24 BITS

DATA

OUT

CLOCKED AT FULL RATE

CLOCKED AT 1/N RATE

24 BITS

16 BITS

GAIN

N42 SCALE

BIG_SCALE

–

()

SCALE

BIG_SCALE

()

4log

–

()

≤

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