Texas Instruments CC13 Series, Application Report

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Antenna Diversity Patch
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4 SWRA523B – September 2016 – Revised March 2017
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CC13xx Antenna Diversity
2.2 Automatic Gain Control (AGC)
The antenna diversity algorithm has a dedicated automatic gain control (AGC) (ANT_DIV AGC).
There is no gain adjustment during a preamble search. The AGC performs one front-end gain adjustment
when entering the Sync Search state (see
Figure 1 ) and then uses the RSSI from the selected antenna to
calculate the optimal RF gain setting.
The user can change the signal level where the AGC starts to reduce the front-end gain by programming
the AGC RSSI reference level (
AgcRssiRef ) (see Table 3 ). The AGC uses the measured RSSI and the
programmed RSSI reference level to calculate the AGC_ERROR and optimum RF gain level, as shown in
Equation 1 .
AGC_ERROR = RSSI – AgcRssiRef_dBm (1)
2.3 Recommended Operating Limits
The antenna diversity algorithm is flexible when it comes to data rate, but has a limitation related to the
minimum preamble length.
Equation 2 shows the calculation for the minimum preamble length.
Minimum Preamble Length = 4 × PreTimeout + 8 [symbols] (2)
The worst-case scenario occurs when a signal is received at the sensitivity level. For example, if the first
antenna misses the desired preamble and the signal level is too low for the second antenna, two preamble
time-outs have occurred without receiving a preamble hit. Then the receiver gets a preamble hit and must
check the other antenna. This is the theory behind the worst-case scenario of 4-preamble time-outs. In
addition, it is necessary to add the internal hardware delay.
Thus, for the default setting with PreTimeout = 10 symbols, the minimum preamble length is 6 bytes.
Because there is no AGC during antenna switching, the algorithm might not select the antenna with
highest RSSI if both antenna signals are above approximately –50 dBm.
2.4 Register Overrides
SmartRF™ Studio [2] should be used to generate the register settings. However, three of the radio
register settings from the SmartRF Studio must be removed (see Table 1 ) when using the antenna
diversity patch and one must be added (see Table 2 ).
Table 1. Register Overrides to be Removed from SmartRF™ Studio Code Export
Override Comment
HW_REG_OVERRIDE(0x6084,0xXXXX) Remove from SmartRF Studio code export
HW_REG_OVERRIDE(0x6088,0xXXXX) Remove from SmartRF Studio code export
HW_REG_OVERRIDE(0x608C,0xXXXX) Remove from SmartRF Studio code export
Table 2 lists the radio register settings from the SmartRF Studio that must be changed to use the antenna
diversity patch.
Table 2. Register Override to be Added to the SmartRF™ Studio Code Export
Override Comment
MCE_RFE_OVERRIDE(1, 0, 0, 1, 0, 0) Run the MCE and RFE patches
[4] contains a smartrf_settings.c file that has the complete override list to be used with the patch.