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25.9.2.1 AGC Algorithm
If PRF_PKT_CONF.AGC_EN is 1, an automatic gain control (AGC) algorithm is run while the receiver is
looking for sync. The AGC algorithm switches between two different front-end gain settings in the
LNAGAIN register. It is recommended to use AGC when running on 2 Mbps to improve the saturation
performance.
Parameters for control of the AGC algorithm are found in page 5 of the radio RAM; see Table 25-7. This
table lists reset values that the LLE sets for these parameters.
The LLE polls the RSSI value at every update and compares it to the values of PRFX_RSSI_LIM_LOWER
and PRFX_RSSI_LIM_UPPER. If the observed RSSI is below PRFX_RSSI_LIM_LOWER, the LNA gain is
set to the high gain setting. If the observed RSSI is above PRFX_RSSI_LIM_UPPER, the LNA gain is set
to the low gain setting. If the observed RSSI is between these limits, the LNA gain is not changed.
The high gain to use is the value found in the LNAGAIN register when the task is started. The low gain to
use is the value found in the PRFX_LNAGAIN_SAT RAM register.
The PRFX_RSSI_LIM_LOWER and PRFX_RSSI_LIM_UPPER values must differ in order to account for the
difference that is observed from the RSSI register when the LNA gain is changed and to have hysteresis
to avoid too-frequent gain changes.
When sync is obtained on the receiver, the AGC algorithm stops updating the LNA gain, which remains at
the value last set. When the receiver is switched off, the LNAGAIN register is set back to the value it had
when the task started, i.e., the high gain setting.
When the gain is reduced during the reception of a packet, the value found in the PRF_LAST_RSSI
register and (if configured) in the RSSI byte of the Rx FIFO is updated to reflect this. This update is done
by adding the value of the register PRFX_RSSI_DIFF to the value found in the RSSI register.
PRFX_RSSI_DIFF should therefore contain the difference between the RSSI offset for the two LNA gain
settings in use, available from the device data sheet. Note that the hardware RSSI register is not updated
this way.
For the AGC algorithm to operate correctly, it requires some signal, having the same power as the packet,
transmitted in the band in front of the packet. That signal can be extra preamble bytes or tone. The length
required for this signal depends on the RSSI accuracy setting in MDMTEST0.RSSI_ACC, see Section 25.7.
An average of n windows of length t
RSSI
requires the extra signal to last at least (n + 1) t
RSSI
. Extra
preamble bytes can be set up using MDMCTRL2.NUM_PREAM_BYTES. Note that the extra signal required
comes in addition to the 1 preamble byte always used in a packet. When adding extra preamble bytes,
this must be accounted for in PRF_TX_DELAY, PRF_RETRANS_DELAY and PRF_RX_TX_TIME. The Rx
requires n × t
RSSI
extra time to start when using the AGC. In the dc offset estimation, the extra signal must
be accounted for when setting the delay.
25.9.2.2 Tone in Front of Packet
In order to get the transmission format to resemble that of other vendors, a tone may be sent in front of
the preamble. This tone can be used by the AGC algorithm on the receiver side. If
PRF_PKT_CONF.START_TONE is 1, such a tone is transmitted as a replacement of the first preamble
bytes. This means that this feature must only be used in combination with increasing the number of
preamble bytes. The tone lasts for a time given by the RAM register PRFX_TONE_DURATION. In order to
get a smooth transition from tone to preamble, it is recommended to set PRFX_TONE_DURATION as given
in Table 25-15.
Tone transmission is allowed to coincide with the synthesizer stabilizing (this may, however, cause the
start of the tone to have larger frequency variations than the packet). For this reason, when
PRF_PKT_CONF.START_TONE is 1, the synthesizer startup time is reduced by the value of the register
PRFX_TONE_OFFSET. This should normally correspond to the time of the extra preambles, but it must not
be larger than 4096 (corresponding to 128 µs). PRFX_TONE_OFFSET can thus be used to compensate for
the extra time added by the extra preamble bytes used for tone generation. However, the duration of the
extra preamble bytes configured must be accounted for in PRF_TX_DELAY, PRF_RETRANS_DELAY, and
PRF_RX_TX_TIME.
The default values of PRFX_TONE_DURATION and PRFX_TONE_OFFSET correspond to 48 µs and are
tuned for using 12 extra preamble bytes (13 in total) on 2 Mbps. When using the reset values,
MDMCTRL2.NUM_PREAM_BYTES should thus be set to 0x0C.
318
CC2541 Proprietary Mode Radio SWRU191C– April 2009–Revised January 2012
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