Determining Gain-Change Latency - 2.6 English

Zynq UltraScale+ RFSoC RF Data Converter Gen 1/2/3/DFE LogiCORE IP Product Guide (PG269)

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To apply the digital gain compensation at the correct moment, the relative latency between the propagation of the gain change through the analog path must be aligned with the propagation of the digital gain update code through the digital path. Because the external VGA communication and response time is specific to the user application, these relative delays must be measured in-system. For the digital path, each tile has a dedicated input called adcXY_pl_event that can be used to apply a setting, such as gain, at a deterministic time. To determine the relative delay between the digital and analog paths, a test signal of constant amplitude should be applied during the design phase, and digital and analog gain adjustments applied. Looking at the output data, the delta time between the digital and analog gain application moments can be observed as a hump/through in the output signal amplitude. This delta time should be added to the adcXY_pl_event assertion in by counting periods of the RF-ADC output clock (clk_adcX). Using this predetermined delay during normal operation of the design ensures that the gain application moments are aligned.

The following figure illustrates the stages of responding to a threshold event for an example AGC application. The middle region of the diagram represents signals related to the analog/VGA path, while the lower region represents signals related to the digital path.

Figure 1. Example AGC Threshold Event Response

As shown in this figure, the AGC update is initiated by the assertion of a threshold. In response to this the AGC algorithm in the PL calculates gain adjustments to be made for both the analog and digital paths. The propagation of the delay through the analog path is shown by the VGA update write, VGA settling response, and RF-ADC latency. The propagation delay through the digital path is shown by the API write of the new gain value, denoted by TRFdc_API, and the adcXY_pl_event assertion, denoted by TDelta_Latency. TDelta_Latency is chosen by the user application to match the Digital and Analog latencies. After the digital gain update has been applied, the threshold can be automatically deasserted, if the Auto-Clear function is enabled.

It should be noted that the application of the QMC gain using the RFDC driver API might incur some driver overhead. In applications where this latency is undesirable, the PL Digital Gain Compensation method can be used by issuing an adcXY_pl_event after the event source has been set by a call to the XRFdc_SetQMCSettings API function.