The Radio420X field-programmable gate array (FPGA) mezzanine card (FMC) is a powerful multi-mode software-defined radio (SDR) RF transceiver module designed around a state-of-the-art Lime Microsystems LMS6002D RF transceiver. It was designed with the following specifications to provide a high degree of flexibility:
- With its low-pin-count (LPC) connector, you can stack two Radio420Xs on the same FMC high-pin-count (HPC) connector
- Many fixed filters
- Clock matrix to use many different source clocks
- RF amplifier
Depending on the application, not all the capabilities of the Radio420X may be required. To optimize the cost and power consumption of the Radio420X, a component redesign may be useful.
Figure 1, taken from the Radio420X User Guide, shows its main components.
Figure 1: Clock distribution circuit block diagram
Nutaq can redesign the Radio420X for developers with specific needs. Except for the LMS, all the other parts can be exchanged for something else based on the application. Basically, the Radio420X can use a wide range of clock sources and frequencies. A specific application, for example, may only require a fixed clock source.
The Radio420X, in its original configuration, uses up to 7W. The LMS itself has a power consumption of around 2W. The remaining 5W can be lowered by selecting different parts. If we go back to the default design specifications, here is what can be done to lower the cost and power consumption:
- The LPC connector: The two complex programmable logic devices (CPLDs) are there exclusively for voltage translation and I/O expander functions because the card’s design uses only the LPC part of the FMC connector. You can use the HPC part of the FMC connector to control the LMS (and other devices) and remove one or both CPLDs.
- Many fixed filters: The LMS has a high-band and a low-band output and the Radio420X has many fixed filters that provide flexibility to the user. Depending on the application, only one band of the LMS could be used and the filters can be used only as needed. All the RF switches used to select filters can be removed as well.
- Clock matrix: Depending on the application, a fixed clock source could be put onboard with a clock distributor instead of a phase lock loop (PLL) for multiple frequency choices
- RF amplifier: In the RF path (not shown), there is an amplifier that can be changed if the gain in the LMS is sufficient for the user
The Radio420X can be used to evaluate the LMS6002D when operating in different modes:
- All the LMS registers can be monitored via SPI from the baseboard.
- The LMS 12-bit analog-digital/digital-analog converters (ADC/DACs) can be bypassed to enter/output an analog signal directly in/from the RF path of the LMS.
To simplify development, the Radio420X includes many U.FL connectors that can be used to analyze the signal in the RF path. You can also use a signal generator to insert a signal into the LMS RF path in order to validate the RF output stage. The same goes for the RX part – one can monitor the RX variable-gain amplifier (VGA2) output to validate the RF path or insert a signal into the RX ADC to validate conversion to the baseboard.
One of the most time-consuming tasks of working with the LMS direct-conversion chip is to implement the IQ imbalance and LO leakage auto-calibration process. Starting with the next release of Nutaq’s ADP software (Release 6.5), the source code for the MicroBlaze soft processor will be open source. The routines used for the auto-calibration will be available to Radio420X users to reuse in their own designs.