Automatic Gain Control Systems - 2.6 English

Automatic gain control (AGC) is commonly used in RF-ADC applications where the dynamic range of the input can vary considerably. It provides a way of using the input range of the RF-ADC and maximizing Signal-to-Noise ratio (SNR), while at the same time offering the flexibility to respond to varying signal amplitudes.

AGC systems consist of the following components:

• Variable Gain Amplifier (VGA)
• RF-ADC
• Signal amplitude monitoring
• AGC Algorithm/Decision logic
• Digital gain compensation

The AMD Zynq™ UltraScale+™ RFSoC RF-ADC channels allow the implementation of custom AGC solutions by integrating the signal amplitude monitoring and compensation features into the RF-ADC tiles. These features can be used with an external VGA and AGC logic embedded in the FPGA PL. Zynq UltraScale+ RFSoC Gen 3/DFE devices integrate DSA in each RF-ADC tile. This is shown in the following figure.

This figure shows an example AGC application. The signal amplitude monitoring is implemented within each RF-ADC channel using the threshold feature. This feature provides two thresholds that can be programmed per RF-ADC channel. When a threshold level is violated, this is indicated directly in the PL, thereby bypassing any latency within the datapath.

A sample high level operation of the AGC is as follows:

1. At system initialization, the threshold levels and modes, including enabling the threshold clearing function from the PL, are set by the RFdc driver API.
2. If a threshold level is violated:
   1. The real-time over threshold output flags assert.
   2. The PL-based AGC algorithm makes a decision and computes the new VGA gain and compensation gain
   3. Gain values are programmed into the VGA and digital compensation logic
   4. adcXY_pl_event is asserted by the AGC logic.
   5. Thresholds are cleared.