Specifications

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SLWS142J − JANUARY 2003 − REVISED AUGUST 2007
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NOTE:If overall-gain is not used, and if the gain is set too high, then the signal can overflow internally, which
causes CIC instability. A momentary high power noise spike is seen on the output before the autoflush forces
the CIC to zero. If the gain is set so high that the signal rapidly overflows internally, the output appears as
a pulsing signal as the CIC periodically overflows.
The CIC may be bypassed by setting bypass_cic or by setting cic_bypass bit, and setting cic_int = 1. This is
appropriate for small overall interpolation (< 6) where the CIC filter requirement for wide transition bandwidth would
be a problem, or when using the chip only for filtering operations. The CIC can by bypassed only in splitIQ mode.
14.5.1 CIC in SplitIQ Mode
In the standard configuration, a dual CIC is used in each channel, one for I and one for Q. In the splitIQ, non-double
rate configuration, the I portion of each channel’s dual CIC is used. The CIC outputs from channels A and B go to
channel A’s mixer, and the CIC outputs from channels C and D go to channel C’s mixer.
14.5.2 CIC in Double Rate Mode
In the double rate mode, each CIC channel outputs even and odd time samples rather than I and Q samples. The
CIC interpolators are configured to calculate two results with each clock cycle. In this configuration, the CIC
interpolation must be even value and the filters are in the split IQ mode . The software configures this mode if the
variable toutf_rate is set to 2.
+ + + + +
Data In
− − − − − −
Scale and Round
Data Out
+
Zero Pad by
cic_int*1
Figure 21. 6-Stage CIC Interpolate Filter
14.6 Numerically Controlled Oscillator (NCO) and Mixer
The DUC NCO is identical to the DDC NCO (see Figure 5). The mix_rev_sel, mixer_rcv_cmplx, mix_icos, mix_isin,
mix_qcos, and mix_qsin are selected by the cmd5016 software to get the proper IQ data into the DUC Mixer:
Mode Channel (Mixer) I Source Q Source
4 Channel A,B,C,D TI TQ
SplitIQ A,C TI XQ
SplitIQ B,D not used not used
DoubleRate A,C TI XQ
DoubleRate B,D XI TQ
The mixer equations are identical to the DDC mode, in that the inv_q_sin control is a ’1’ for subtraction:
IMixOut = selected(I) * cos(NCOphase) − selected(Q) * sin(NCOphase)
QMixOut = selected(I) * sin(NCOphase) + selected(Q) * cos(NCOphase)
The mixer can be configured in normal mode, splitiq mode, or double rate mode. Figure 22 shows the normal mode
where each FIR processes a complex data stream and feeds it to a dual CIC block. The dual CIC block is configured
to process two streams (I and Q) at the clock rate. The interpolated output is sent to the mixer.