Analog Devices ADRV9005 Reference Manual page 151

Reference Manual ADRV9001
RECEIVER/OBSERVATION RECEIVER SIGNAL CHAIN
device. The duplexer is to support both the FDD and TDD modes, which stands for a frequency duplexer in the FDD mode and an RF switch in
the TDD mode.
Figure 147 shows that each receiver, besides acting as a primary data channel for receiving RF signals, also serves as an observation channel,
which receives loopback signals from the transmitter. There are three loopback paths: internal loopback (ILB), external loopback type 1 (ELB1),
and external loopback type 2 (ELB2), as shown in Figure 147. When users are in full control of the loopback channel for running their own
algorithms, the receiver is renamed as the observation receiver (ORx). In such a case, use either ELB1 or ELB2 depending on the algorithm
requirements. For example, if running an external DPD, use the ELB2 path.
Figure 147. ADRV9001 Rx/ORx/Loopback Diagram
The ADRV9001 also uses the three loopback paths internally for two major purposes: transmitter initial calibrations and transmitter tracking
calibrations, including the integrated digital predistortion (DPD) operation. Transmitter initial calibrations configure the device properly based
on the system configurations during the initialization time. It can be done through either ILB, ELB1, or ELB2. The major advantage of using
ELB1 compared to ILB is to observe common-mode voltage. In addition, during the transmitter initial calibrations procedure, test tones are
generated and present at the transmitter output. Therefore, ensure an appropriate level of isolation from the ADRV9001 transmitter output
to the antenna to ensure that test tones are not transmitted by the system. Achieve this isolation by disabling the power amplifier during
transmitter initial calibration in ELB1. For ELB2, the calibration signal might be transmitted out through the antenna. Although the power level
of calibration signal is set as low as possible, make sure this does not cause any problem when using this option. For more details, see the
Transmitter/Receiver/Observation Receiver Signal Chain Calibrations section.
The transmitter tracking calibrations tweak the system on the fly during its normal operations for optimal performance. Similarly, it uses ILB,
ELB1, or ELB2. ILB and ELB1 are used when DPD is not required, while ELB2 is used when DPD is required, in which the transmit signal
is looped back to the receiver after power amplifier. Note that ELB1 and ELB2 share the same receiver input. So, they cannot be used
simultaneously.
No matter whether it is used by the user or internally by the device, the observation channel shares the same datapath as the regular receiver.
Therefore, the observation can only be performed when no regular reception is required at the same time. This is the case for transmitter initial
calibrations and be aware that receiver might not be idle, as it works as the observation channel during initialization internally by the device.
Different from transmitter initial calibrations, transmitter tracking calibrations are performed on the fly. So, they have to share time with the
regular receiver operations. For example, in a TDD system, when the transmitter is transmitting, the receiver should not be receiving. Therefore,
it can be used for transmitter tracking calibrations. For a system where the receiver is fully occupied all the time to receive RF signals, such
as a 2Tx2Rx FDD system, it is not possible to perform transmitter tracking calibrations, including DPD operations. However, in a 1Tx1Rx FDD
system, because one receiver is always idle, it can be used as an observation channel. For example, if Transmitter 1 is transmitting, then
Receiver 1 can be used for observation by receiving loop back signals from Transmitter 1 and Receiver 2 can be used as the main receive
path. The observation receiver must be on the same side of the transmitter it observes. So, observation channel 1 is always for Transmitter 1
and observation channel 2 is always for Transmitter 2. When in control of the observation channel, configure the ORx based on requirements
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