The following diagram shows the on-chip VOP feature.
The variable output power is achieved by varying the full-scale current in fine steps defined in the Zynq UltraScale+ RFSoC Data Sheet: DC and AC Switching Characteristics (DS926). The full scale current of the RF-DAC can be derived as follows:
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RF-DAC full scale current (μA) =
Minimum VOP Current+N*VOP step size (μA)
Where N = 0, 1, 2, …, limited by the maximum full scale current.
To avoid the potential harm to the wireless transmitter signal chain when gain changes dramatically, the API is designed to divide a large jump to several small steps then update the VOP sequentially with certain time interval. The step is approximately ±10% of the current VOP value.
The default full scale current during start-up is set by the IP configuration.