GC1115 Datasheet, PDF (5/80 Pages) Texas Instruments – Crest Factor Reduction Processor

GC1115
www.ti.com
SLWS144 – FEBRUARY 2005
How Peak Cancellation Works: Time Domain View
Figure 4 provides a time-domain example of GC1115 operation. Figure 4 shows the magnitude of the complex
input signal in blue, and the magnitude of the complex, peak-reduced signal (after GC1115 processing) in red.
[Note: for users who print this data sheet on a black-and-white printer, the input signal is a solid line, while the
output signal is a finely dotted line.] Notice that for most of the 130 samples in this example, the red output
waveform is identical to the blue input waveform. However, the GC1115 has reduced the magnitude of samples
around two peaks (one at sample 63, one at sample 95) that exceed the detection threshold. The aqua-colored
waveform above the gain threshold line demonstrates the magnitude of the cancellation pulse that was
subtracted from the complex input waveform. Notice that the GC1115’s PDC stages each have two independent
thresholds:
1. Detection threshold: interpolated peaks above the detection threshold are candidates for cancellation
2. Target peak level: detected peaks will be reduced to the target peak level, assuming cancellation resources
are available
Independent detection and target peak levels allow the GC1115 greater flexibility in peak reduction processing.
x 10⁴
7
Detection Threshold
6
5
Target Peak Level
4
3
2
1
0
0
20
40
60
80
100
120
140
Figure 4. Peak Cancellation Example: Time Domain
How Peak Cancellation Works: Frequency Domain View
Figure 5 demonstrates the frequency-domain effects of peak cancellation. In Figure 5, the original signal’s
frequency response is shown in blue. The output signal’s frequency response looks identical and is not shown.
The yellow (8 dB PAR), red (7 dB PAR), green (6 dB PAR), and black (5 dB PAR) curves show the spectra of
just the added signal (GC1115 input – GC1115 output) to achieve peak reduction. The difference between the
input and output signals is the signal distortion. [Note: for users who print this data sheet on a black-and-white
printer, the input spectrum is a solid line, while the output spectra are finely dotted lines.] Note that the distortion
is not significant in the spectral response:
1. The out-of-band energy of the distortion added by the GC1115 during peak cancellation is 70 to 80 dB below
the signal and hence is not an issue. The out-of-band energy is only a weak function of the target PAR level.
2. The amount of in-band distortion energy rises with decreasing output PAR. This is to be expected, because
lower PAR thresholds result in more peaks being canceled. Lower PAR levels require more energy to cancel
the peaks, thus increasing the distortion level.
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