SLWS132 Datasheet, PDF (10/46 Pages) Texas Instruments – QUAD RECEIVER CHIP

GC4014 QUAD RECEIVER CHIP
DATA SHEET REV 0.6
The PFIR will also, if desired, convert the complex output data to real. The complex to real conversion also
doubles the output sample rate so that the PFIR decimation is 1 or 2 in the real mode.
The PFIR filter is followed by a gain and output format circuit. The gain circuit allows the user to add an
additional 18 dB of gain in 0.14 dB steps. The output format circuit can also delete every other sample without
filtering. Anti-aliasing filtering must have already been performed (in the second stage filter). This is useful to achieve
deeper far-band rejection since the stopband performance of the CIC filter is a function of the decimation that follows
it.
3.3.1 The Numerically Controlled Oscillator (NCO)
The tuning frequency of each down converter is specified as a 32 bit word and the phase offset is specified
as a 16 bit word. The NCOs can be synchronized with NCOs on other chips. This allows multiple down converter
outputs to be coherently combined, each with a unique phase and amplitude. A block diagram of the NCO circuit is
shown in Figure 4.
FREQUENCY ^{32 BITS}
WORD
PHASE
DITHER
OFFSET ^GENERATOR
32 BITS
16 BITS
5 BITS
18 MSBs
14 MSBs
SINE/COSINE
LOOKUP
TABLE
15 BITS _SINE/COSINE
OUT
Figure 4. NCO Circuit
The tuning frequency is set to FREQ according to the formula FREQ = 2³²F/F_CK, where F is the desired
tuning frequency and F_CK is the chip’s clock rate. The 16 bit phase offset setting is PHASE = 2¹⁶P/2π, where P is
the desired phase in radians ranging between 0 and 2π. Note that a positive tuning frequency is used to
downconvert the signal. A negative tuning frequency can be used to upconvert the negative image of a real signal
(inverting the spectrum).
The NCO’s spur level is reduced to below -92 dBc through the use of phase dithering. The spectrums in
Figure 5 show the NCO spurs for an example tuning frequency before and after dithering has been turned on. Notice
that the spur level decreases from -82 dB to -105 dB.
-82 dB
-105 dB
(a) Before Dithering
(b) After Dithering
Figure 5. Example NCO Spurs
GRAYCHIP, INC.
-5-
APRIL 27, 1999
This document contains information which may be changed at any time without notice