The tests in RTCA/DO-160 Section 21 are performed to ensure that the Equipment Under Test (EUT) does not emit Radio Frequency interferences that exceed the specified limits. Conducted RF emissions exposed to interconnecting cables and power leads are measured. Radiated RF emissions from the EUT, power leads and interconnecting cables are also measured.
Measurements must be made with an instrument using a peak detector, and with IF bandwidths, frequency step size, and dwell time as specified in RTCA/DO-160 Section 21, Table 1, for the frequency range being scanned.
A Line Impedance Stabilization Network (LISN) must be placed in series with each power lead and ungrounded power return lead, with a 10 uF capacitor connected between the power input of the LISN and the ground plane.
Interconnecting cables should be at least 3.3 meters in length, and power leads will be no more than 1 meter in length for these tests unless otherwise specified.
Ambient emission levels must be at least 6 dB below the applicable limit and must be measured and recorded if any signals are found to be within 3 dB of the applicable limit.
Clamp-on current probes are used to measure conducted RF current on interconnecting cables and power leads. The current probe is to be positioned 5 centimeters away from the EUT and measurements are recorded over the frequency range of 150 kHz to 152 MHz.
A linearly polarized antenna is used to measure Radiated RF fields over the frequency range of 100 MHz to 6 GHz. As seen in RF radiated susceptibility testing in Section 20, there are two RF radiated emissions test methods allowed in RTCA/DO-160 Section 21: The Reverberation Chamber Method and the Anechoic Chamber method.
The Reverberation Chamber method which requires a Field Uniformity Validation from Section 20. After the EUT is installed in the chamber, EUT Loading is measured, and this data is used as a correction factor for the radiated emissions measurement. A minimum of 200 sweeps of the analyzer or measurement receiver is required over one rotation of the tuner, for each measured frequency range.
The Anechoic Chamber method requires a chamber lined with RF absorber, and the minimum performance of the absorber is specified. The measurement antenna distance is 1 meter, and multiple antenna positions are required when the beamwidth of the antenna does not totally cover the system. If the EUT has apertures, connectors, seams, or other points of penetration in the EUT enclosure, all of these must be directly exposed to the test antenna, requiring multiple EUT positions during testing.
There are 6 Equipment Categories (B, L, M, H, P, and Q) that indicate the location of the equipment and the separation between the equipment and aircraft antennas. In general, the closer the equipment is to an aircraft antenna, and the more it approaches a “direct view” of an aircraft antenna, the tighter the emissions limits.
|Changes between DO-160 D, E, F, and G
|From C to D
||From D to E
||From E to F
||From F to G
|Change Category Z requirements to reflect the need for additional scrutiny in the VHF COM NAV and GPS frequencies.
||All of the figures were cleaned up and made easier to read. The Radiated Emissions limits for Categories M and H were modified slightly so that the notches that occur in various frequency ranges now have sloped bottoms to match the rest of the limit. The frequency range of the first notch has also been changed for a more precise match with the communications/navigation frequency range that the notch protects.
||The conducted emissions frequency range has been broadened to 150 kHz - 152 MHz, and the Radiated Emissions test frequency range has been narrowed to 100 MHz - 6 GHz. A new procedure for radiated emissions measurements in a reverberation chamber was added. In the anechoic chamber method, clarification has been added regarding the requirement to expose all apertures and openings of the EUT. A new limit category has been added, Category P, to provide added protection for certain receivers by use of even deeper notches than previously used.
|Added optional in-line low noise preamp to diagrams in Fig. 21-11, Fig. 21-12, and Fig. 21-13 per description in Table 1, Note 1. Corrected values at 152 MHZ notch points, 37.5dB in Fig. 21-8, 27.5 dB in Fig. 21-9, and 27.5 dB in Fig. 21-10. Added acronym definitions for GPS and GNSS at first occurrence of the acronym in Section 21. Added verbiage in Section 21 User Guide discussing the use of equivalent dipole directivity in formula found in Section 21.6.3.i. Revised Figure 21.6 and 21.11 to show bends in the cable bundle as it exits the EUT. Revised frequency ranges in Table 1: 100 kHz bandwidth upper frequency limits = 0.4 - 0.96 GHz and 1 MHz bandwidth lower frequency limits = 0.96 – 6.0 GHz. Revise Note 1 to Table 1 to eliminate the option to use an uncorrected 10 kHz bandwidth in the measurement of category M & H emission notches above 960 MHz. Substituted a new Note 1 which recommends the use of a low noise preamplifier if necessary, to achieve sufficiently low measurement noise floor in the notches. Added Category Q. Revised Paragraph 21.2 to include Category Q definition. Inserted two new Figures for Category Q, Fig. 21-4 and Fig. 21-5. Renumbered remaining figures as required. Added Section 21 User Guide to Section 21.