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In this video, I'll show you how to enable the loopback mode feature with the AFE74XX EVM. To learn more about configuring the AFE74XX EVM in loopback mode, visit section 3.7.1 of the Programmers Guide titled "Configuration of the AFE74XX as a Repeater." Download the guide from ti.com/afe74xx.

In loopback mode, the AFE74XX models a repeater application, where the digital output of the ADC is internally routed to the input of the deck, bypassing the need for physical [INAUDIBLE] LAN connections, as well as the need for an FPGA. To learn more about loopback mode, reference section 7.3.3.3 of the datasheet titled "Repeater Mode." When configuring the AFE74XX in repeater mode, both receiver and transmitter paths within the device need to be configured in the same JESD mode, meaning that the LMFS parameters should match in order to ensure correct JESD coding and decoding. After this is done, a configuration file can be loaded to the AFE74XX GUI to enable loopback mode.

For example, looking at functional operating modes in the datasheet, it can be seen that, in mode 1, RX and TX have different LMFS JESD parameters, and therefore are not eligible for loopback mode. However, mode 4's RX and TX LMFS JESD parameters are the same, which means mode 4 is eligible for loopback mode operation. Let's get started.

First, bring up the AFE74XX using a mode that is loopback compatible. Remember that the RX LMFS and TX LMFS JESD parameters must match in order for the mode to be eligible for loopback mode. In this example, I've programmed the AFE74XX EVM in mode 4. Once the AFE74XX EVM is up and running, and you verified that the DAC and the ADC are configured, load the following configuration file detailed in the Programmers Guide. Do this by navigating to the low level tab, pressing the Open File icon, and loading the file RXTXLoopback.

The signal generator connected to ADCA is set to 1,800 megahertz. The RX NCO is set to 1,700 megahertz. This means that the 1,800 megahertz input signal is down converted to 100 megahertz before it reaches the DAC. The DAC then up converts the signal using the TX NCO, which is set to 1,700 megahertz. Therefore, the final RF frequency should be 1,800 megahertz at the output of DAC A.

Turn on the signal generator connected to ADCA and verify the tone is looped back to DAC A and outputted at 1,800 megahertz. To change the location of the output, either the RX or TX NCO can be adjusted. To visibly see the AFE's total operational bandwidth in loopback mode, enable the Max Hold function on the spectrum analyzer and sweep the input frequency. In the case of mode 4, we can visibly see that the repeater bandwidth is 750 megahertz.

Loopback mode is a great tool for debugging to verify that the ADC and the DAC are up and running. If I'm not able to get a capture on my FPGA, I can use loopback mode to verify that my ADC is capturing correctly. This may indicate that the problem is related to the AFE74XX FPGA interface. To learn more about the AFE74XX and how it can speed up your design process, visit ti.com/afe74xx and make sure to check out the other videos in this series. Thanks for watching.