Analog Devices ADRV9005 Reference Manual page 147

Reference Manual ADRV9001
TRANSMITTER SIGNAL CHAIN
Figure 144. Basic Block Diagram of the Slew Rate Limiter
Transmitter QEC
In the analog circuitry of a direct-conversion transmitter, there are three major non-idealities: gain variation between the I and Q datapath,
phase imbalance (non-90 degrees between LO driving I and Q mixers), and differences in the LPF such as group-delay variations. Without
properly correcting them, the output spectrum of the transmitter can significantly degrade due to the undesired images.
Transmitter QEC is designed to estimate and correct the non-idealities through initial and tracking calibrations. The initial calibration is
performed by generating a tone through the NCO and inserting it into the transmitter datapath. Note that this tone is visible at the transmitter
output; therefore, ensure the antenna is isolated from the transmitter (power amplifier is off) during initial transmit calibration. Internally in
the device, the output from the transmit upconverter is looped to the ORx through the internal loopback (ILB) path. The ORx output and
the transmitted tone estimate the mismatches. The tables are generated to record the initial calibration results, which can be further refined
through tracking calibrations on-the-fly. During signal transmission, the mismatch estimations are applied in the transmitter datapath so that the
non-idealities are compensated. For more details, see the Transmitter/Receiver/Observation Receiver Signal Chain Calibrations section.
Transmitter LOL
In the transmitter, any coupling of the LO to the RF output or the baseband DC offset generates an undesired tone at the LO frequency. Without
proper correction, it can cause a negative impact on the system performance.
Transmitter LOL is designed in the transmit signal chain to handle this problem. Similarly, it estimates the DC offset through initial and tracking
calibrations, and then applies the estimation in the baseband to cancel the undesired tone. For more details, see the Transmitter/Receiver/Ob-
servation Receiver Signal Chain Calibrations section.
Digital Predistortion (DPD)
DPD is an optional feature available on the ADRV9001 device to achieve higher power amplifier efficiency, while still meeting the error
vector magnitude (EVM) and adjacent channel leakage ratio (ACLR) requirements in the transmitter signal chain in compliance with standard
requirements. DPD works on the principle of predistorting the transmitter data to cancel distortion caused by power amplifier compression. It
uses the tracking calibration to capture the transmitted data samples and the data samples looped back through ORx to estimate the distortion
parameters. By applying the estimations in real time, the transmitted signal is predistorted to compensate for the power amplifier nonlinearity.
For more details, see the Digital Predistortion (DPD) section.
Transmitter NCO Internal Signal Source
The ADRV9001 has an internal quadrature numerically-controlled oscillator (NCO). It serves two major purposes. First, it generates the
calibration tones for the initial calibrations, such as the transmitter QEC. Second, it generates test tones through an API command to disable
the data port interface and simplify the design for specific use cases or testing. In all cases, as shown in Figure 138, the transmitter
preprocessor takes the input data from the NCO instead of the data port interface.
The internal signal source cannot be used with frequency hopping or TDD because these modes of operation require strict timing control by the
FPGA or BBIC. For example, based on the TDD timing parameters, the FPGA sets Tx_enable to high and aligns the time of sending the Tx
data at the Tx_enable rising edge, which cannot be achieved by the NCO.
Transmitter Frequency Offset Correction
The ADRV9001 provides user capability to correct small deviations in the transmit LO frequency through an API command. Through this API,
the user can provide the desired frequency offset in Hz, and change the frequency immediately or update it at the start of the next available
frame.
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