
Preliminary Technical Data
UG-1828
Rev. PrC | Page 167 of 338
Table 67. APD Attack and Recovery Step Sizes
Gain Change
Step Size
Gain Attack
apdGainStepAttack
Gain Recovery
apdGainStepRecovery
Step size refers to the number of indices of the gain table the gain is changed. As explained earlier, the gain table is programmed with the
largest gain in the Max Gain Index (typically index 255), with ever decreasing gain for decreasing gain index. Thus, if the APD gain attack
step size was programmed to 6, then this means that the gain index is reduced by 6 when the apdHighThresh has been exceeded more
than apdUpperThreshPeakExceededCnt times. For example: if the gain index had been 255 before this over range condition, then the
gain index would be reduced to 249. The amount of gain reduction this equates to is dependent on the gain table in use. The default table
has 0.5dB steps which in this example would equate to a 3dB gain reduction upon an APD over range condition.
The APD is held in reset for a configurable amount of time following a gain change to ensure that the receiver path is settled at the new
gain setting.
Half-Band Peak Detector
The HB peak detector is located in the digital domain at the output of the HB Filtering block. It can therefore also be referred to as the
Decimated Data Overload Detector because it works on decimated data. Like the APD detector, it functions by comparing the signal level
to programmable thresholds. It monitors the signal level by observing individual samples (I2 + Q2 or peak I/peak Q) over a period of
time and compares these samples to the threshold. If a sufficient number of samples exceed the threshold in the period of time, then the
threshold is noted as exceeded by the detector. The duration of the HB measurement is controlled by hbOverloadDurationCnt, while the
number of samples that should exceed the threshold in that period is controlled by hbOverloadThreshCnt.
Once the required number of samples exceed the threshold in the duration required, then the detector records that the threshold was
exceeded. Like the APD detector, the HB detector requires a programmable number of times for the threshold to be exceeded in a gain
update period before it will flag an over-range condition.
Figure 157 shows the two-level approach which is different from APD. It shows the gain update counter period, with the time being
broken into subsets of time based on the setting of hbOverloadDurationCount. Each of these periods of time is considered separately, and
hbOverloadThreshCount individual samples must exceed the threshold within hbOverloadDurationCount for an overload to be declared.
These individual samples greater than the threshold are shown in grey. Two examples are shown, one where the number of samples
exceeding the threshold is sufficient for the HB peak detector to declare an overload, and a second example where the number of samples
exceeding the threshold is not sufficient to declare an overload. The number of overloads is counted, and if the number of overloads of
the hbHighThresh exceed hbUpperThreshPeakExceededCount in a gain update counter period, then an over-range condition is called.
Likewise, if the number of overloads of the hbUnderRangeHighThresh does not exceed hbUnderRangeHigh-ThreshExceededCount, then
an under-range condition is called. Note that if hbOverloadDurationCount is set to equal to the time duration of 1 sample and
hbOverloadThreshCount is set to 1, the HB two-level approach becomes similar to the APD algorithm.
Figure 157. HB Detector, Two-Level Approach for an Overload Condition
hbOverloadDurationCount
hbOverloadDurationCount
hbOverloadThreshCount
EXCEEDED
hbOverloadThreshCount
NOT EXCEEDED
gainUpdateCounter
24159-
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