GC1115_06 Datasheet, PDF (27/82 Pages) Texas Instruments – Crest Factor Reduction Processor

GC1115
www.ti.com
SLWS144C – FEBRUARY 2005 – REVISED JUNE 2006
Contact TI to receive an appropriate set of CRC initialization and test parameters.
Snapshot RAM Operation
The GC1115 contains two independent, identically operating snapshot RAMs for signal capture and histogram
generation. The snapshot RAMs operate in one of three modes:
• No operation (OFF)
• Capture mode (RAM is configured as a 2k × 16-bit memory, with I and Q samples interleaved)
• Histogram mode (RAM is configured with even addresses storing the 16 MSBs, and odd addresses storing
the 16 LSBs, of 32-bit counters)
These RAMs can capture or histogram sample streams at five independent locations within the GC1115:
• At the GC1115’s input (after the decimator and prior to Stage 1)
• After PDC Stage 1
• After PDC Stage 2
• After PDC Stage 3
• After PDC Stage 4
In capture mode, the 16 MSBs of each 18-bit sample are stored in the snapshot RAM as follows:
ADDRESS
i=0,2,4,…
j=1,3,5,…
VALUE
I[17:2] (sixteen MSBs of 18-bit I value)
Q[17:2] (sixteen MSBs of 18-bit Q value)
In histogram mode, each snapshot RAM monitors the output of a function generator that generates one of the
following values from its I & Q source samples:
• I sample
• Q sample
• Magnitude squared (I*I + Q*Q)
The arithmetic processing of samples in histogram mode is shown in Figure 11. First, the user-selected function
of the complex input sample I + jQ is calculated. Second, the calculated value is compared to SNAP_MINVAL
and SNAP_MAXVAL. If the calculated value is not within the desired range, no further action occurs for this
sample. If the calculated value is within the desired range, then SNAP_MINVAL is subtracted from the
calculated value, and the result is multiplied by SNAP_SCALE to generate a 10-bit histogram index (between 0
and 1023).
Let us consider a histogram example in which users are interested in monitoring the power of samples above
the average power (to calculate CCDF, for instance). Assume that the average power corresponds to 16-bit
samples with mag-squared values of 2 × (10000²), or 2 × 10⁸. For 16-bit samples, the largest possible
mag-squared value is 2 × 32767² = 2.14x 10⁹. The GC1115 removes the lower 16 bits from each 32-bit
mag-squared values, so that the function generator output is always in the signed 16-bit range. The initial values
for SNAP_MINVAL, SNAP_MAXVAL, and SNAP_SCALE are calculated as follows:
• SNAP_MINVAL = 3052 (2 × [10000²] / 2¹⁶)
• SNAP_MAXVAL = 32767 (2 × [32767²] / 2¹⁶)
• SNAP_SCALE = 1024 / (32767 – 3052) = 0.03446 (= 1129 as a signed 16-bit integer)
If users wanted to monitor the input signal for 30 seconds (about 1.8 × 10⁹ samples – plenty of statistical
significance), SNAP_HISTCOUNT = 27465 (1.8 × 10⁹ / 65536).
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