Analog Devices ADRV9001 User Manual page 170

Reference Manual
RECEIVER GAIN CONTROL
It is possible to enable a fast attack mode, whereby the AGC is instructed to reduce gain immediately when an overrange condition occurs
instead of waiting until the next expiry of the gain update counter using changeGainIfThreshHigh. This parameter has independent controls for
the APD and HB detectors. Values from 0 to 3 are valid, as shown in
Table 74. changeGainIfThreshHigh Settings
changeGainIfThreshHigh[1:0]
00
01
10
11
Figure 153 shows how AGC reacts when the changeGainIfThreshHigh is set for the APD or HB detector. In this case, when the interferer
appears, the gain is updated as soon as the number of peaks exceeds the peak counter. It does not wait for the next expiry of the gain
update counter. Hence, a number of gain changes are made in quick succession, providing a much faster attack than the default operation.
The assumption here is that if the ADC is overloaded, then it is best to decrease the gain quickly rather than wait for a suitable moment in the
received signal to change the gain. This mode is referred to as the "fast-attack" mode.
Besides the "fast attack" mode, it is also possible to enable a "fast recovery" mode. This functionality is enabled with the enableFastRecovery-
Loop parameter.
This "fast recovery" mode only works with the HB detector.
using multiple low thresholds and step sizes as well as the update periods. When the signal level falls below hbUnderRangeLowThresh,
the lowest threshold, the gain is incremented the most by hbGainStepLowRecovery following the expiry of a gain update period. Note that
in the "fast recovery" mode, the agcUnderRangeLowInterval is used instead of the gain update counter to set the gain update period (this
also applies to the APD). After sufficient gain increases to bring the signal level above hbUnderRangeLowThresh, the gain is incremented
by hbGainStepMidRecovery after the expiry of agcUnderRangeMidInterval, which is a multiple of agcUnderRangeLowInterval. Finally, when
the signal level is increased above hbUnderRangeMidThresh, the gain is incremented by hbGainStepHighRecovery following the expiry of
agcUnderRangeHighInterval, which is a multiple of agcUnderRangeMidInterval.
The multiple thresholds and interval parameters allow for faster gain recovery. Typically, agcUnderRangeHighInterval is set to gain update
counter, as shown in Figure 154. Therefore, when the signal level is below the middle and low thresholds, the recovery happens multiple times
within a single gain update counter, making the recovery process much faster. Note that in the "fast recovery" mode, gain recovery might not
always happen at the expiry of the gain update counter, which is different from the mode without "fast recovery". If the gain update counter is
set to align with the frame or subframe boundary, it is possible that a fast recovery happens in the middle of a frame or subframe. Therefore, it
is recommended not to use the "fast recovery" mode when there is a stringent requirement for keeping a constant gain for an entire frame or
subframe.
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Table
Gain Change Following APD Overrange
After expiry of gainUpdateCounter
After expiry of gainUpdateCounter
Immediately
Immediately
Figure 153. APD/HB Gain Changes with Fast Attack Enabled
Figure 154 shows this operation. In this mode, achieve the "fast recovery"
74.
Gain Change Following HB Overrange
After expiry of gainUpdateCounter
Immediately
After expiry of gainUpdateCounter
Immediately
ADRV9001
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