GC5018_06 Datasheet, PDF (17/134 Pages) Texas Instruments – 8-CHANNEL WIDEBAND RECEIVER

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GC5018
8-CHANNEL WIDEBAND RECEIVER
SLWS169A – MAY 2005 – REVISED NOVEMBER 2005
Each GC5018 downconversion block can process two CDMA carriers or a single UMTS carrier. Signal
data is selected from one of four ports for real inputs, or two of four ports for complex inputs. Data from
the selected port(s) is multiplied with a complex, programmable numerically controlled oscillator (NCO)
which tunes the signal of interest to baseband. The delay adjust and zero pad blocks permits adjustment
of the delay in the end-to-end channel. Zero padding interpolates the signal to the rxclk rate. Filtering
consists of a six stage CIC filter which decimates the tuned data by a factor from 4 to 32, a compensating
FIR filter (CFIR) which decimates by a factor of two, followed by a programmable FIR filter (PFIR) which
does not decimate. The output interface block can be programmed to decimate by 2 if desired.
The RMS power meter measures the power within the channel’s bandwidth. The AGC automatically drives
the gain and keeps the magnitude of the signal at a user-specified level. This allows fewer bits to
represent the signal. The serial output interface formats and rounds the output data. Each of the above
blocks is described in greater detail in the following sections.
3.2.2 DDC Mixer
18
from
18
rx_distribution ₁₈
bus
18
4 to 2 ¹⁸
Select ¹⁸
18
Demux ₁₈
and
18
Round
18
20
20
cos
sin
mixer_gain
from NCO
Figure 3-15. Mixer Functional Block Diagram
I_A
Q_A
to
channel
I_B
delay
Q_B
The receive mixer translates the input (from one of the input signal sources) to baseband where
subsequent filtering is performed to isolate the signal of interest. The mixer is a complex multiplier that
accepts 18 bit I and 18 bit Q signal data from the receive input interface and 20 bit Sine and Cosine
sequences from the NCO. The NCO generates a mixing frequency (sometimes referred to as a local
oscillator, or LO) specified by the user so that the desired signal of interest is tuned to 0 Hertz.
A DDC channel can support one UMTS signal directly, or two CDMA channels at half the input rate. When
in CDMA mode, each channel may be set independently; the path selection and the mixer tuning and
phase. The mixer output produces two complex streams; one representing the signal path for the A-side
DDC, the other the B-side. Each of these streams drives a channel delay and zero pad block.
The maximum input rate for UMTS is rxclk for either real or complex input data.
The maximum input rate in CDMA mode with real inputs is rxclk (remix_only is set, see below).
The maximum input rate in CDMA mode with complex inputs is rxclk/2 due to sharing of multiplier
resources.
RECEIVE DIGITAL SIGNAL PROCESSING
17