GC1115 Datasheet, PDF(20/80 Page) Texas Instruments – Crest Factor Reduction Processor

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GC1115 Datasheet, PDF (20/80 Pages) Texas Instruments – Crest Factor Reduction Processor

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GC1115

SLWS144 â FEBRUARY 2005

www.ti.com

Setting Detection Thresholds and Target Peak Levels

The most obvious user-specified parameter for a CFR processor is the desired output PAR. The output PAR

determines the largest sample magnitude on the output ports of the GC1115. Ultimately, the PAR determines the

maximum Vin voltage of the PA. This maximum value is then adjusted to fit just under the 1 dB compression

point of the PA. The average power of the GC1115 input samples (0 dB point on the CCDF curve) can be

calculated from a group of input samples. TI provides a Matlab function that uses a group of input samples to

calculate the 16-bit threshold-squared values that are loaded into the GC1115âs DETECT_TSQD and

GAIN_TSQD registers (one pair of registers for each of four stages; total of eight TSQD registers). Upon request,

TI will provide the Matlab source code (m-file) for the threshold calculations based on input samples.

Alternately, GC1115 users can use the GC1115âs built-in hardware power measurement capability to determine

the average input power. The detection thresholds and target peak levels can then be calculated from the

average input power.

GC1115 threshold settings are based on a threshold SQUARED (power) value, not a threshold (magnitude)

value. The following example demonstrates how the GC1115âs threshold-squared values are calculated, using

the Matlab language:

% Calculate the average power of the complex input array x, and the desired fractional (magnitude)

% threshold, given the user-specified target peak level (in dB).

avgMag

= sqrt(mean(abs(x).^2));

frac = avgMag * 10 ^ (dB_target / 20);

Derive the following threshold-related values from the target PAR magnitude:

% thresh

signed, 14-bit threshold value (a magnitude!)

% threshSqd threshold set as a 16-bit mag-squared value (reduced from a 29-bit mag-squared value)

thresh

= floor (frac * 2^13);

% set the threshold relative to signed 14-bit samples

threshSqd = floor((thresh ^ 2)/ (2^13)); % mag-sqd of a 14-bit value = 28 bits plus an add = 29

bits

% Divide by 2^13 to convert to 16-bit thresh-sqd value

The GC1115 uses four sequential PDC stages to achieve the desired output PAR. For output PAR levels below

8 dB, both cEVM and PCDE performance is improved if the earlier stages have higher detection thresholds (set

to 8 dB or 7 dB), while later stages contain the final, desired threshold (7 dB, 6 dB, or 5 dB, for instance). In all

cases, the GAIN_TSQD registers should be set to the target peak level (desired output PAR). When the desired

output PAR is at or above 8 dB, the DETECT_TSQD and GAIN_TSQD registers are set as shown in Table 8:

STAGE

Table 8. Detection and Gain Thresholds, PAR â¥ 8 dB

DETECT_TSQD

my_tsqd

GAIN_TSQD

my_tsqd

When the desired output PAR is below 8 dB, the recommended DETECT_TSQD and GAIN_TSQD register

settings are set as shown in Table 9:

STAGE

Table 9. Detection and Gain Thresholds, PAR < 8 dB

DETECT_TSQD

min(8 dB, my_tsqd + 2 dB)

min(7 dB, my_tsqd+1 dB)

my_tsqd

GAIN_TSQD

my_tsqd

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