Analog Devices ADRV9001 User Manual page 132

UG-1828
Table 57. APD Attack and Recovery Step Sizes
Gain Change
Gain Attack
Gain Recovery
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 114 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 purple, while those less than the threshold are shown in
green. Two examples are shown, one where the number of samples exceeding the threshold is sufficient for the HB peak detector to
declare an overload (this time period is shown as yellow in the gain update counter timeline), and a second example where the number of
samples exceeding the threshold is not sufficient to declare an overload (this time period is shown as blue in the gain update counter
timeline). 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 hbUnderRangeHighThreshExceededCount, 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.
Step Size
apdGainStepAttack
apdGainStepRecovery
hbOverloadDurationCnt
hbOverloadThreshCnt
EXCEEDED
agcGainUpdateCounter
Figure 114. HB Detector, Two-Level Approach for an Overload Condition
Rev. PrA | Page 132 of 253
Preliminary Technical Data
hbOverloadDurationCnt
hbOverloadThreshCnt
NOT EXCEEDED