Preliminary Technical Data
UG-1828
Rev. PrC | Page 163 of 338
The power detect provides the RMS power measurement of the receiver data at the output of HB Filtering block. In power detect mode,
the AGC compares the measured signal level to programmable thresholds which provide a 2nd order control loop, whereby gain can be
changed by larger amounts when the signal level is farther from the target level while make smaller gain changes when the signal is closer
to the target level. This could allow the gain change faster when the level is farther away from the targeted range.
Figure 154 shows the operation of the AGC when using the power detect. Considering the power detect in isolation from the peak detectors, the
AGC will not modify the gain when the signal level is between overRangeLowPowerThresh and underRangeHighPowerThresh. This range 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 underRangeHighPowerThresh, the AGC will increment 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 will decrease the gain by
overRangeLowPowerGainStepAttack. underRangeLowPowerThresh and overRangeHighPowerThresh are also called outer thresholds.
Figure 154. PMD Thresholds and Gain Changes for Under-range and Over-range Conditions
It is possible for the AGC to get contrasting requests from the power and peak detectors. An example would be an interferer that is visible
to the analog peak detector but is significantly attenuated at the power detector. In this case the APD could be requesting a gain
decrement, while the power detector could be requesting a gain increment. The AGC has the following priority scheme in peak/power
detect mode:
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 would be decremented because the APD over-range has a higher priority than the power measurement.
However, APD and HB lower level overloads act differently in peak detect and peak/power detect mode. In peak detect mode, the lower
level thresholds for these detectors are used to indicate an under-range condition which caused the AGC to increase the gain. In
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 is set, and if the signal level is exceeding a lower level threshold, the AGC is prevented from
increasing the gain regardless of the power measurement.
When a signal has higher than expected Peak to Average Power Ratio (PAR), the power detector could indicate a gain increase while the
peak detector low threshold could still be exceeded. In such a case, gain increase will be prevented to avoid an overloading possibility. In
addition, this could prevent an oscillation condition that could otherwise occur to an interferer visible to APD but filtered before the
power detect. In such a case, the peak detect could cause the AGC to decrease gain. It would do this until the interferer is no longer
exceeding the defined threshold. At this point, the power detect could request an increase in gain and would do so until the detector’s
peak threshold is exceeded. This might cause an oscillation condition. By using these lower level thresholds of peak detect, the AGC is
prevented from increasing gain as the signal level approaches an overload condition, providing a stable gain level for the receiver chain
under such a condition.
SIGNAL
LEVEL
NO GAIN CHANGE
GAIN
DECREMENT
GAIN
DECREMENT
GAIN UPDATE
PERIOD
GAIN
INCREMENT
RECEIVED
SIGNAL LEVEL
CHANGE
POWER
MEASUREMENT
DURATION
GAIN
INCREMENT
underRangeHighPowerTresh
underRangeLowPowerTresh
overRangeHighPowerTresh
overRangeLowPowerTresh
INCREMENT GAIN BY
underRangeHighPowerGainStepAttack
INCREMENT GAIN BY
underRangeLowPowerGainStepAttack
DECREMENT GAIN BY
overRangeLowPowerGainStepAttack
DECREMENT GAIN BY
overRangeHighPowerGainStepAttack
RECEIVED
SIGNAL LEVEL
CHANGE
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