Reducing Electromagnetic Spectrum Fratricide
On August 8, 2019, the Under Secretary of the Navy issued a memorandum regarding “Reducing Electromagnetic Spectrum Fratricide”. The memorandum provided clarification on three SECNAV instructions regarding policy and responsibilities for research, development and acquisition of all spectrum-dependent systems (SDS). It stated that all SDS must assess the risk of electromagnetic spectrum (EMS) fratricide and defined EMS fratricide as “inadvertent self, friendly, or neutral electromagnetic interference (EMI) caused by SDS”. This memorandum emphasizes the importance (and now requirement) for thorough electromagnetic compatibility (EMC) design and analysis throughout all stages of research, development and acquisition for the Navy.
The Navy memorandum underscores the importance of undergoing thorough EMS risk assessments as early as possible. At EMA, this is a message that we also emphasize to our customers. It is much more cost effective to address potential interference problems early in the design stage of a new program or a planned upgrade to an existing platform. When interference problems are discovered in the final stages of testing, potential solutions are almost always expensive and add additional weight and power requirements to the platform or they limit the performance of the RF systems. Further, EMI discovered shortly before deployment of a new SDS generally results in delays in providing capabilities to the warfighter, which is never a welcome message.
When one considers the potential for interference between RF systems, there are many factors and considerations. Often for our DoD customers, we work on cosite interference problems where the RF systems are located on the same platform. However, interference can and does occur between RF systems on different platforms, or between RF systems on platforms and infrastructure. So, it is often not sufficient to only consider the RF systems on an isolated platform. The integrator must analyze the environment in which the platform will operate, which includes nearby transmitters and receivers that could potentially be threats or victims.
A common misunderstanding with regards to military RF systems and potential for EMI is with regards to MIL-STD-461 requirements. We frequently encounter individuals who believe that if an RF system passes the various conducted and radiated tests specified in MIL-STD-461 that their RF system can be installed on a platform, and no interference will occur. There could be nothing further from the truth. The MIL-STD is specifying test thresholds that are considered “reasonable” and as noted in the appendix A.5.6 of MIL-STD-461G, the standard cannot address every possible scenario. Some key sentences from the appendix A.5.6, “The limit for transmitters in the transmit mode is placed primarily at levels which are considered to be reasonably obtainable for most types of equipment. Suppression levels that are required to eliminate all potential electromagnetic compatibility situations are often much more severe and could result in significant design penalties. The limit for receivers and transmitters in standby is placed at a level that provides reasonable assurance of compatibility with other equipment.”. So, the standard is clearly stating that the limits specified do not ensure EMC for all cases. Even after unit level tests are performed and passed, system level analysis and testing are required.
What is often eye-opening for individuals new to the field of EMI is the out-of-band performance of receivers and other components such as filters and amplifiers. The out-of-band performance of transmitters is generally a concept that is widely understood. The transmitter emits the fundamental frequency, harmonics of the fundamental, some spurious emissions and broadband noise. Before a transmitter is characterized, an engineer might not be able to easily predict specific amplitudes for the harmonics and spurious emissions. However, it is widely understood that these harmonics exist and that they are potential sources of EMI. However, receivers can also have out-of-band responses. These occur as mixer products and spurious responses. These are frequencies at which an external signal can cause interference to the receiver. In the figure below, the measured broadband susceptibility of a receiver is plotted. The in-band performance is shown near 160 MHz (-133 dBm). However, notice all of the other responses of the receiver at other frequencies. One can imagine a nearby transmitter that is capable of emitting a signal of sufficient amplitude to cause interference to the receiver at one of these out-of-band responses. Manufacturers of RF receivers almost never provide details on this out-of-band performance.
Integrators must consider filters and other devices that also have out-of-band performance. Specification sheets for a filter will typically state the in-band insertion loss and out-of-band rejection. A plot of the in-band performance of the filter is typically shown with very nice out-of-band rejection. However, such plots tend to not show any information about what is happening far away from the in-band performance. The out-of-band rejection of the filter almost never stays below the value stated on the specification sheet for all frequencies. What is happening at two, three, four times the in-band frequency? Has the out-of-band rejection changed dramatically? The short answer is that, yes, it is very likely that the out-of-band rejection varies considerably at higher frequencies. In the figure below, the S21 plot for a bandpass filter designed to pass signals near 70 MHz is shown. The out-of-band rejection of the filter is generally very good up until 400 MHz. Then we see that the rejection decreases dramatically. For a cosite interference analysis, we must know this out-of-band performance of the filter.
For an EMI analysis, integrators must consider the out-of-band performance of all components and the antenna-to-antenna coupling. Every piece of equipment between the output port on the transmitter and the input port on the receiver must be characterized in a broadband sense in addition to understanding the broadband performance of the transmitter and receiver.
EMA has several decades of experience working with our customers to solve challenging EMI problems. In particular, our team of scientists at EMA work with commercial and military customers to solve a wide variety of EMI problems related to interference between RF systems installed on the same or different platforms. We have been able to save our customers millions of dollars and help keep projects on schedule by identifying EMI problems during the design phase before equipment was procured and integrated into the platform.
EMA was recently awarded an SBIR contract from NAVAIR to develop an automated RF measurement system (ARMS) for characterizing the broadband performance of RF systems. This measurement capability will allow for extremely accurate EMI analyses by using high-fidelity measured data for transmitter and receiver performance instead of the low-fidelity performance data found on specification sheets.
EMA offers our customers a wide variety of consulting services that help ensure that platforms and products meet certification requirements and that their designs work correctly the first time. Consulting and measurement services are provided for the following technical areas: lightning direct and indirect effects, HIRF, P-Static, EMP, spacecraft charging, RF cosite interference, installed antenna performance and radar signature. If we can be of assistance to your team, please Contact Us.
