Preliminary Technical Data
UG-1828
Rev. PrC | Page 201 of 338
Satisfactory DPD performance also depends on successful completion some operations inside the device such as ADC and DAC
calibrations. Most of those operations are guaranteed internally in the device but the following operations are user configurable. It is
important to perform all those optional operations to achieve the optimal DPD performance.
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Time alignment between transmit x(t) and loopback y(t) for data capture
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Transmit closed loop gain control (CLGC) tracking calibration
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Transmit LOL calibration
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Transmit and Receive QEC calibration
CLOSED LOOP GAIN CONTROL (CLGC)
The ADRV9001 provides CLGC as one of the transmit tracking calibration algorithms. The sole purpose of CLGC is to maintain a fixed
gain between the PA output and transmit input digital amplitude. The main source of gain variation comes from the changes in the gain
of the PA, which could vary due to transistor temperature change in response to power output in the short term, RF carrier frequency
change within the bandwidth of the amplifier, and slow hardware degradation in the long term. Maintaining a fixed gain in the transmit
path is important to maintain a precise calibrated transmit power at the antenna port.
CLGC helps to achieve a fixed gain target by adjusting the transmit attenuation automatically through monitoring the gain variation of
PA. It can work with or without the DPD algorithm. When DPD is not activated, CLGC is an optional feature. When DPD is activated,
CLGC should always be activated. Note ADRV9001 allows to enable DPD without CLGC but it is not recommended. As discussed earlier,
DPD compensates the instantaneous compression of peak signal by expanding the input signal so that the peak signals are linearized.
When linearization is achieved, the gain in the compression region is adjusted (increased) to match the gain of the lower linear region, so
that the overall gain is independent of the input or output average power level. This has the advantage of having CLGC to focus on
compensating only the PA gain variation mainly due to temperature change.
By setting a proper gain target, CLGC could also help to
monitor and limit the transmit power level to keep the amplifier output power from rising beyond the linearization capable power limit of
the PA. Therefore, it is crucial to always enable CLGC while DPD is active.
As the first step of CLGC, user should set up a target transmit gain. This could be measured through ADRV9001 by using the “CLGC
Loop Open” method. The detailed steps of measuring target gain will be discussed later. After the measurement, ADRV9001 provides
user both an unfiltered and a filtered transmit gain value. Based on those, user could further adjust the value and set a proper gain target,
then close the loop and start the ADRV9001 CLGC algorithm to continuously track the gain variation based on the determined gain
target.
When DPD is active, the PA gain is defined as the gain in the linear region of the AM-AM curve as shown in Figure 185. To estimate this
reference gain, data samples are usually selected in the upper linear region and below the compression region, as indicated in red in
Figure 185. Note this same gain plus relative phase is used by DPD to scale the y(t) loopback data to match the pre-distorted transmit x(t)
data. The PA gain derived from data between these bounds represents the real gain in the linear region of the amplifier, while excluding
the distorted gain at the upper end due to compression. However, after DPD has converged, the gain in the compression region will
increase to match the gain in the lower region. When DPD is off, compression at the top region will not be corrected by DPD, hence it is
necessary to include all samples for integration to estimate the total power, including the compressed region, to define the transmit gain.
User could define the region for calculating the gain through API configurations.
Similar as the DPD algorithm, CLGC algorithm requires the time alignment between transmit x(t) and loopback y(t) for data capture.
User should measure the delay and provide it to ADRV9001 which is especially important for WB profiles. When DPD is enabled, the
same delay measurement serves both DPD and CLGC algorithm.
Note that ADRV9001 CLGC algorithm has a limit to track gain variations not exceeding ±3dB (this should be able to accommodate most
types of PA) and for each CLGC iteration, the maximum gain adjustment is limited to ±0.5 dB to prevent DPD algorithm becoming
unstable.
DPD/CLGC CONFIGURATION
To use the integrated DPD/CLGC properly and ensure optimal performance, user must configure DPD/CLGC parameters properly. This
could be done through ADRV9001 Transceiver Evaluation Software (TES) or Software Development Kit (SDK). The configuration
consists of 2 sets of DPD/CLGC parameters. The first set of DPD/CLGC parameters is “pre initial calibration” parameters since they
should be configured before performing initial calibration when the device is at the “STANDBY” state. The second set of DPD/CLGC
parameters is “post initial calibration” parameters since they should be configured after performing initial calibration when the device is
at the “CALIBRATED” state. These DPD/CLGC parameters will be explained in details in the next two subsections.
DPD/CLGC Pre Initial Calibration Parameters Configuration
In order to properly set the pre initial calibration parameters of DPD/CLGC, the user should have a general understanding of the DPD
model used in the device. The DPD model is described by the following equations:
