GC1115_06 Datasheet, PDF (32/82 Pages) Texas Instruments – Crest Factor Reduction Processor

GC1115
SLWS144C – FEBRUARY 2005 – REVISED JUNE 2006
www.ti.com
Power Measurement
The GC1115 can measure signal power of a complex I/Q signal at one of two locations in the GC1115 datapath:
• at the input to PDC Stage 1, and
• at the output of PDC Stage 4
The POWER_CTL and POWER_CNT registers configure the GC1115 to measure power at one of these two
locations. The total number of samples observed during power measurement is POWER_CNT ×
POWER_CTL[13:0]. After power measurement has completed, the resulting average power value is available in
the POWER register.
The total number of samples used for the power calculation is the product of an inner loop count (determined by
the POWER_CNT register) and an outer loop count (determined by bits [13:0] of POWER_CTL):
POWER_CNT
inner loop count (# samples)
---------
---------------------------
0x1
16
0x3
256
0x7
4096
0xF
65536
The outer loop count is specified by the lower 14 bits of the POWER_CTL register, but only the most-significant
"1" matters - the other bits are ignored:
POWER_CTL[13:0]
outer loop count (# samples)
---------------
----------------------------
1
1
2-
3
2
4-
7
4
8 - 15
8
16 - 31
16
32 - 63
32
64 - 127
64
128 - 255
128
256 - 511
256
512 - 1023
512
1024 - 2047
1024
2048 - 4095
2048
4096 - 8191
4096
8192 - 16383
8192
For example, to calculate the average power over about a million samples, the GC1115 registers would be
initialized as follows:
POWER_CNT = 0xF (inner loop count = 65536)
POWER_CTL = 0x10 (outer loop count = 16)
Total # samples used to calculate power: 16 × 65536 = 1 048 576 samples
To summarize, the GC1115 performs the following steps during power measurement:
1. Use 18 bits I and 18 bits Q from each sample to calculate a 37-bit power value
NOTE:
Input samples must be left-justified, i.e. the input sample’s most significant bit is
always aligned to bit 17 (MSB) of input ports IN_A and IN_B.
2. Add the upper 16 bits of each 37-bit power value to a 40-bit power accumulator
3. Accumulate N = POWER_CNT × POWER_CTL[13:0]) sample powers
4. Shift the final accumulator value by 4 to 18 bits to calculate the average power over N samples, depending
on how POWER_CNT and POWER_CTL are set. The GC1115 does this shifting (divide by N) automatically.
5. Transfer the resulting 16 bits to the POWER register.
GC1115 users will also need to set the two MSBs of POWER_CTL properly, to specify whether the input or
output power is to be measured, and (in two-channel configuration) the channel number (channel 0 or channel
1) whose power is to be measured.
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