RFSoMs (Radio Frequency converter enabled Systems on Module) are convenient, take care of all the RFSoC’s complex power requirements and memory interfaces, are typically compact, and save months, if not quarters in development time. But are they any good? Will you get the same RF performance as you might in a carefully designed board-down approach ?

YES — if you source well designed modules.

In your preliminary SOM selection process, whether you’re considering Intel/Altera or AMD/Xilinx products, you should look for specific features. You can spot many by looking at product pictures or the bullet points on a glossy product flyer, but some information must be obtained by asking the vendor for performance data.

  1. Is there a clean separation between analog and digital domains? This includes the placement of the low noise LDOs that power the sensitive converters. Ideally, the LDOs are placed along the edge of the FPGA that features the converters, so that the analog domain for power supply and RF signals share a common, well-isolated GND island. Are the LDO converters powered by polyphase (aka, easy to filter) pre-regulators that delineate the first transition from the noisy digital domain to a less noisy intermediate voltage? Both of these features are important, as any design compromise in the power supply will lead to a diminished SNR in the analog domain. A noise characterization report from the vendor will shed light on how well the SoM is designed and if you are getting all of the RFSoC’ s analog performance potential. While digital designs are somewhat forgiving, in an RFSoM “form follows function” is an essential rule. It should be easy to spot if that rule was followed or ignored.
  2. Did the designer ensure sufficient spacing and shielding (in layout and connector signal assignment), so that neighboring channels are not causing crosstalk, either from DAC to ADC or within ADC or DAC channels? This is more difficult to spot by simply looking at pictures of the SoM, but a quality RFSoM vendor can provide that data upon request.
  3. Does the solution offer a high-quality (low jitter) clocking system either on SoM or as a shared carrier design? The quality of the sampling clock has a substantial impact in the overall performance of the RF converters, and any design tradeoff will lead to signal quality impairments. While some RFSoM designs incorporate the clocking subsystem, which is definitely convenient, others place it on the carrier, where it may easily be adapted to specific needs, either allowing cost reduction, adaptations to specific needs, or further performance enhancements.
  4. Does the SoM have adequate DC/DC converter power reserves on all its rails? Power reserves translate to better reliability, as the power stages will operate at lower temperature, which translates into longer MTBF. Yes, the MTBF is dominated by the complex parts of the design (FPGA and RAM), but a stressed converter is never a good idea, even from an EMI point of view. Look for designs that deploy polyphase power converters for minimal voltage ripple and fewer hot spots on the SoM, as such architectures will outlast lesser designs.
  5. With power consumption comes heat, even if the converter efficiency is high. Does the SoM under consideration offer a meaningful heat management solution? Not just a generic, afterthought heatsink with a fan, but a controlled thermal interface that can be used in passively cooled (dock the RFSoM to your enclosure, or radiator in space, or other fan-less applications) or actively cooled environments with a heatsink and fan assembly? To ensure that proper cooling in harsh environments is possible, the SoM must be designed with that in mind from the ground up. Typically, this means all the main power stages are on top of the module, where they can be interfaced to a heat spreader plate.
  6. Does your application require the SoM to be rugged, or is it just for a benign rack-mount or bench-top application? A well-designed heat spreader plate also offers opportunities for ruggedizing the SoM. Since shear forces may act on heavy components such as the FPGA, converter inductors or the connectors, a dedicated heat spreader plate can also offload those forces from the solder joints and pads of the PCB via extra compression and additional fastening points. Only the strategic placement of heavy components on the RFSoM will allow for this, and if done well, the payoff is a SoM that can withstand a launch or any environment with sudden acceleration.
  7. System integration capabilities are very important as well. Does the SoM offer well-documented power-up behavior? Does it allow for external circuits to intervene in the sequence? Or does it offer control signals to ensure that secondary power supplies on a carrier do not cause cross currents through the FPGA I/Os that interface to the system? Is there a facility that allows for telemetry data extraction by an external supervisory circuit.
  8. Since the evaluation carrier or kit is not really part of the SoM, it is close to the bottom of the list. The carrier or kit is a valuable element, at least during the initial steps towards your own design. A well-designed kit may offer assemblies that can be reused in quick turnaround projects or at least offer known good starting points. That is particularly important if the clocking solution is not part of the SoM. And with that kit, look for example designs or applications you will receive. They will save you several days, already offsetting whatever the kit may cost.
  9. At last, features of the RFSoM. Yes, it’s last because if the quality points above are not met, it does not matter if the list of features matches your needs, as they will not be fulfilled to your satisfaction. Plus, how many substantial feature differentiations can you really have in an RFSoM? The only feature I can think of that cannot be augmented in a carrier design is the RAM. How much bandwidth do you need? Is ECC required? Does it need PS memory only or also PL, one or two interfaces on the PL? My suggestion: go big! If a scale up to volume requires further cost reductions, most vendors will agree to do an assembly variant with less memory — but don’t limit your options by skimping on memory.

I would not be surprised if you think these points are self-evident, but now that your analytical mind is primed by this little check list, please go look at the global offering of RFSoC modules again. You’ll be amazed by how many products you’ll find that fall short in one or more points. Then, to recover from that disappointment, come back see our stuff. 😉