Radio Interference Filter

Options for interference mitigation.

Stage 1: ADC Sample Filter

The first line of defence is the sample-based interference filter that is programmed into the FPGA and works directly on the time series of raw voltages. This filter examines the time series of voltages (individual frames) that come directly from the ADC and filters out data frames with too many spikes (outliers).

One disadvantage of using this particular filter is that it can result in an increase in the sampling periods of the sensor. This is because it can take longer to gather enough frames for a complete integration in case of strong interference. In areas with high radio frequency interference (RFI) levels, the integration time can be several seconds.

Stage 2: Polarization Discrimination Filter

Radio interference caused by human activities (known as RFI or EMI) is typically polarized in either a circular or linear manner. Conversely, the thermal radio emission from the soil is unpolarized when the sensor is pointed straight down (i.e., the incidence angle is zero). This difference can be used to distinguish between the two sources of signals.

As a general guideline, we have discovered that discrimination filtering can greatly enhance sensitivity and signal quality, provided the radio interference is within moderate limits. However, once RFI becomes the primary signal source, this method is unlikely to salvage the data effectively.

It is important to note that any electronic device that contains a clock or crystal can generate radio interference. For instance, some LCDs have been found to be strong emitters of RFI. Nevertheless, it is difficult to determine beforehand which devices may cause issues. For example, unshielded flight controllers and their processors may pose potential problems, but DJI controllers have been found to work well with the sensor.

Preventing the generation of RFI in electronics by shielding them is challenging. Just enclosing the offending devices in a metal container is usually not enough. Additional line filters are required since the power supply and data lines connected to and from the device can act as antennas and continue to emit RFI.