GC4116 Datasheet, PDF (14/57 Pages) List of Unclassifed Manufacturers – MULTI-STANDARD QUAD DUC CHIP

GC4116 MULTI-STANDARD QUAD DUC CHIP
The NCO’s spur level is reduced to below -113 dB
through the use of phase dithering. The spectrums in Figure
9 show the NCO spurs for a worst case tuning frequency
before and after dithering has been turned on. Notice that the
spur level decreases from -105 dB to -116 dB. Dithering is
turned on or off using the DITHER_SYNC controls in
addresses 8 through 11.
The worst case NCO spurs at -113 to -116dB, such as
the one shown in Figure 9(b), are due to a few frequencies
that are related to the sampling frequency by multiples of
F_CK/96 and F_CK/124. 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). Setting the
initial phase to 4 for multiples of F_CK/96 or F_CK/124 (and to 0
for other frequencies) will result in spurs below -115 for all
frequencies.
DATA SHEET REV 1.0
Figure 10 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 -107 dB before dithering
and is -121 dB after dithering has been turned on and the
phase initialization described above has been used.
3.4 THE OVERALL INTERPOLATION
FILTER RESPONSE
The image rejection of the up-convert channel is equal
to the stop band rejection of the overall interpolation filter
response. The overall response is obtained by convolving
the interpolated responses of the PFIR and CFIR filters and
CIC filter response. The overall response and appropriate
transmit masks are shown in Figure 11 for four common
standards. Figure 11a shows the overall response for IS136
(also referred to as DAMPS). Figure 11b shows the response
for GSM (input at 2 samples per bit). Figures 11c & d shows
the response for IS95 and 3.84 MB UMTS.
IS136 Transmit Mask
GSM Transmit Mask
(a) IS136
TO BISE9G5 ETNraEnRsmATitEMDask
(b) GSM
(c) IS95
(d) 3.84 MB UMTS
Figure 11. Overall Filter Response
© 1999−2001 GRAYCHIP,INC.
-9-
APRIL 27, 2001
This document contains preliminary information which may be changed at any time without notice