Analog Devices ADRV9001 User Manual page 173

Reference Manual ADRV9001
RECEIVER GAIN CONTROL
Figure 156 shows the operation of the AGC when using the power detector. Considering the power detected in isolation from the peak
detectors, the AGC does not modify the gain when the signal level is between overRangeLowPowerThresh and underRangeHighPowerThresh.
This is the target range for the power measurement. The associated thresholds are also called inner thresholds.
When the signal level goes below underRangeLowPowerThresh, the AGC waits for the next gain update counter expiry and then increments
the gain by underRangeLowPowerGainStepRecovery. When the signal level is greater than underRangeLowPowerThresh but below under-
RangeHighPowerThresh, the AGC increments the gain by underRangeHighPowerGainStepRecovery. Likewise, when the signal level goes
above overRangeHighPowerThresh, the AGC decreases the gain by overRangeHighPowerGainStepAttack, and when the signal level is
between overRangeHighPowerThresh and overRangeLowPowerThresh, the AGC decreases the gain by overRangeLowPowerGainStepAttack.
underRangeLowPowerThresh and overRangeHighPowerThresh are also called outer thresholds.
Figure 156. Power Detector Thresholds and Gain Changes for Underrange and Overrange Conditions
It is possible for the AGC to get contrasting requests from the power and peak detectors. An example is an interferer visible to the analog peak
detector but significantly attenuated at the power detector. In this case, the APD is requesting a gain decrement, while the power detector is
requesting a gain increment. The AGC has the following priority scheme in peak/power detect modes:
1. APD overrange
2. HB overrange
3. APD lower level peak exceeded
4. HB lower level peak exceeded
5. Power measurement
In this example, the gain is decremented because the APD overrange has a higher priority than the power measurement. However, the
APD and HB lower level overload act differently in the peak detector and peak/power detect modes. In the peak detect mode, the lower
level thresholds for these detectors are used to indicate an underrange condition, which causes the AGC to increase the gain. In the
peak/power detect mode, these detectors are not used for gain recovery but used to control gain recovery by setting the API parameter
lowThreshPreventGainInc. If this parameter sets and the signal leveling exceeds a lower level threshold, the AGC is prevented from increasing
gain regardless of the power measurement.
When a signal has higher than the expected PAR, the power detector can indicate the gain increase while the peak detector's low threshold
can still be exceeded. In such a case, the gain increase is prevented to avoid an overloading possibility. In addition, this can prevent an
oscillation condition that can otherwise occur to an interferer visible to the APD but filtered before the power detects. In such a case, the
peak detect can cause the AGC to decrease the gain. It does this until the interferer is no longer exceeding the defined threshold. At this
point, the power detector can request an increase in the gain and does so until the detector's low threshold is exceeded. This might cause an
oscillation condition. By using these lower level thresholds of peak detection, the AGC is prevented from increasing the gain as the signal level
approaches an overload condition, providing a stable gain level for the receiver chain under such a condition.
Comparing the Peak Detect and Peak/Power Detect Modes
Among the two detection modes, peak detector offers the quickest response time to overload signals by employing "fast attack" mode. It allows
the AGC to respond within hundreds of nanoseconds in overload scenarios. In addition, the peak detector also provides a "fast recovery" option
to increase the gain of the desired signal quickly when an interferer disappears. It can also avoid the possible gain index oscillation issue of
peak/power detect when the signal has higher than expected peak-to-average ratio (PAR).
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