Analog Devices ADRV9005 Reference Manual page 182

Reference Manual
RECEIVER GAIN CONTROL
Measuring External Gain Control Path Delay
To accurately apply the attenuation for different components in the datapath, the user needs to measure the external path delay associated with
the LNA and set the delay through the API command adi_adrv9001_Rx_ExternalLna_DigitalGainDelay_Set().
Properly load Rx gain table, set gain control and LNA configurations. Only LNA gain step 0 (0dB) and 1 (NdB) are needed for this
►
measurement. Set the delay value as 0.
Based on gain control and LNA configurations, the following info is known:
►
LNA gain step of N dB = LNAStepdB.
►
Min gain index allowed (when LNA gain is 0dB) = minGainIndex.
►
The adjacent gain index has a gain step size 0.5 dB.
►
With above info, compute the following:
►
Gain index start and end that correspond to an N dB ADRV9001 RF attenuator only gain change is:
►
attenStartIndex = minGainIndex.
►
attenEndIndex = minGainIndex + 2 × LNAStepdB.
►
Gain index start and end that correspond to an N dB LNA only gain change is:
►
lnaStartIndex = minGainIndex − 2 × LNAStepdB.
►
lnaEndIndex = minGainIndex.
►
Inject Rx input signal at desired level, through LNA into RX input port.
►
Initialize ADRV9001 with all initial calibrations properly performed including Rx Gain Delay Initial Calibrations.
►
Perform measurement 1—capture Rx data with attenuator only gain change.
►
Perform measurement 2—capture Rx data with LNA only gain change.
►
Analyze both captures to determine the timing when gain change starts (t1 for RF attenuator gain change and t2 for LNA gain change). The
►
LNA delay is calculated as (t2 − t1) × ADRV9001 AGC clock/4.
Multiple External Gain Control Components
As mentioned previously, the maximum gain of an external gain control component that the ADRV9001 can control is 29 dB due to the
ADRV9001 having a maximum of 30 dB linear attenuation.
The ADRV9001 can also support up to 3 different external gain control components e.g. LNAs. In gain correction mode, each LNA can provide
any gain up to 29 dB. This allows a maximum gain of −117 dB to be achieved using 3x 29 dB LNA's and the ADRV9001 i.e. N = 29 dB, M = 29
dB, L = 29 dB. The purpose of using multiple LNA's is to allow higher overall gain without compromising the ability to keep 0.5 dB step sizes.
In gain compensation mode, multiple LNAs are supported but the maximum external gain is limited to 29 dB total i.e. N + M + L <= 29 dB.
Customizing the Rx Gain Table
Recall the default Rx gain table
User may modify the default gain table. However, strict limitations apply to this modification:
Gain step sizes should always be decreasing from 255.
►
Each step size should be the same e.g. 255 = 0 dB gain, 254 = −2 dB gain, 253 = −4 dB gain.
►
To do this, remove unwanted gain steps in the table but ensure to keep the index column the same.
►
Table 74 shows an example gain table where the step sizes were increased from 0.5 dB to 20 dB. To create this table, the following procedure
was done:
Row 255 remained the same at gain of 0 dB.
►
Every 4th row from the default gain table was kept to create the equal 2 dB step sizes.
►
The index column was maintained but now the table ends at row 238, due to needing less rows for 2 dB step sizes.
►
analog.com
Table 71. This default table has rows from 187 to 255 representing approximately 0.5 dB step sizes.
ADRV9001
Rev. A | 182 of 377