One of my current projects is the modification of an existing Nutaq FPGA mezzanine card (FMC) module to create a new version that can be used on different platforms.

Before I discuss it, here’s some information about the Vadj  power in the FMC world. It’s the job of every FMC carrier board to power the FMC modules attached to it. The provided power comes in four versions: 12V, 3.3V, 3.3AUX and Vadj. The FMC power Vadj  (called “V-adjust”) can be chosen by the carrier based on the value required by the FMC. The FMC standard allows for the following values: 1.2V, 1.8V, 2.5V or 3.3V. Some carrier boards like Nutaq’s Perseus601x can provide all four Vadj  values to an FMC, while others (like a lot of evaluation modules, or EVMs), have fixed values for the Vadj power. Xilinx EVMs have fixed values for this power; the ML605 Vadj  is fixed at 2.5V and the VC707 and VC709 Vadj  are fixed at 1.8V.

As developers start experimenting with newer FPGAs from the Virtex-7 family for their projects, they logically end up using an evaluation carrier board based on that family, such as the VC707 and the VC709 from Xilinx. Since the Vadj provided by these boards is fixed at 1.8V, FMC modules that only accept Vadj  values of 2.5V, for example, are not compatible. This compatibility issue is true for Nutaq’s Radio420X FMC module as well, but, as customers asked us about the possibility to use the Radio420 with Virtex-7-based boards, we took action and created a version of the card that supports a Vadj of 1.8V.

Functional differences between the 2.5V and 1.8V version

The main and only functional difference between the 2.5V and the 1.8V version of the Radio420 is the clock distribution circuit. Since it could not be powered at 1.8V, the clock switch (present in the original 2.5V version) was removed. This results in the Radio420 1.8V FMC not being able to send clock signals to a carrier’s FMC_CLK0 and FMC_CLK1 signals.

Figures 1 and 2 show the clock distribution circuits of each version of the Radio420.

Figure 1: Clock distribution circuit block diagram of Radio420X 2.5V


Figure 2:  Clock distribution circuit block diagram of Radio420X 1.8V

Preliminary performance results

The preliminary performance results of the new Radio420 1.8V are very encouraging. The number of units tested is still limited and we have not yet performed a complete characterization, but we have not seen so far any significant performance changes in the modified 1.8V version and that the two versions cannot be differentiated based on measurements.

You can find the specifications of our Radio420 FMC 2.5V here.