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Qorvo 1800MHz Small Cell RF Card Hardware User’s Guide
Hardware User’s Guide
5.14 Directional Coupler
In the Ch1 Observation (DPD) path, the Directional Coupler (U11) has the following functions:
It isolates the observation path from the main transmit path.
This is a waveguide so either port 1 or port 2 could have been used as the input
As configured, the chip provides "forward power" at port 3, while port 4 is isolated,
seeing onl
y small amounts of “back power”.
5.15 Circulator
After the Directional Coupler, the Ch1 DPD signal passes through U16, the Circular Isolator (Circulator).
It ensures any reflected signal due to impedance mismatch at U17 is circulated to
ground instead of coming back towards the transmitter.
The RF circulator is "behind" the duplexer to maximize the Noise Figure of the receiver
chain. In other words, putting the circulator at the antenna would make the Rx signal
traverse the circulator path (port 2 to port 3) and then the duplexer. This topology
removes the circulator from the Rx path altogether.
Figure 22
– DPD path Directional Coupler and Circulator
