Introduction
This article discusses RS101 and RS103, including the updates contained in MIL-STD-461 revision “G”, the current revision. These tests quantify the ability of a device to tolerate undesired signals present as a field at the device and associated cables. The field has the potential of coupling the interference into the device circuitry and cause performance degradation or device damage.
These test methods have been a part of the MIL-STD-461 test program from the onset using RS01 (Magnetic Field), RS02 (Magnetic Induction Fields), RS03 (14 kHz-10 GHz electric field) and RS04 (14 kHz-30 MHz Parallel Plate Electric Field) numbering. MIL-STD-461 assigned applicability and test levels for various applications. In addition, reference is made to MIL-HDBK-235 to determine the anticipated environment for establishing test levels for any particular application.
RS01 magnetic field testing was accomplished much like today over the 30 Hz to 30 kHz frequency range with a radiating loop placed 5 cm from the test article. A 1-ohm resistor was placed in the wiring to the loop to measure voltage of 1-volt per 1-amp of loop current. The radiating loop design produced 50 μT per ampere of current flowing through the loop. Assuming a low frequency acceptance level of 160 dBpT plus an additional 20 to 30 dB for a specified test level, the current would need to be approximately 20 amperes ((10^(180/20)*1E-12)/5E-5).
RS02 cable magnetic induction field testing wrapped 2-turns of wire per meter (1-meter of cable under test) of cable wound tightly around the cables. A current of 20-amperes at the power frequency was flowed through the wound wiring while monitoring the EUT for susceptibility. Additionally, spikes of 200V were applied to the wound wiring (spikes of 10μS and 150nS could be specified) while monitoring for susceptibility. Case RS02 testing locates the wound wiring around the EUT chassis and the same induction field susceptibility tests are accomplished.
RS03 E-field testing was much like today, however modulation of the interference signals was not specified. A statement in the standard that consideration be given to the operational electromagnetic environment as part of the test planning. This consideration could have provided for AM, FM or pulse modulation for various frequency bands used at that time. Test levels varied from 1 V/m to 200 V/m as defined in MIL-STD-461.
RS04, the parallel plate method was available as an alternative approach for RS03 in the 14 kHz to 30 MHz frequency range. This configuration produced a vertically polarized field between the upper and lower plates with a RF amplifier driving the applied voltage. This method helped reduce some of the cost and size elements associated with field radiation in this frequency range while providing a viable test approach especially for higher test levels.
In 1993 MIL-STD-461D defining requirements and applicability was released along with MIL-STD-462D describing the test method. This revision brought forth several changes:
- A table for the frequency step size based on specific test frequency ranges.
- Specifying a one second dwell time for susceptibility testing. RS101 provided an exception that allowed scanning three times faster than stated in the scan rate table.
- RS02 induction field testing and RS04 parallel plate method were eliminated.
- The magnetic radiating loop wiring was changed to produce 9.5E7 pT per ampere of applied current. An associated loop sensor was defined to verify the radiating loop performance.
- Use of an anechoic chamber was specified with the absorber performance defined.
- The numbering system changed to RS101 and RS103 for magnetic field and electric field testing respectively.
- The RS101 test frequency range changed to 30 Hz – 100 kHz.
- The RS103 test frequency range changed to 10 kHz – 40 GHz (18 GHz unless the procurement specification required 40 GHz).
- RS103 testing not applicable to the tuned frequency of an antenna connected receiver unless specified by procuring agency.
In 1999 revision ‘E” incorporated a few changes:
- The frequency step dwell time was increased to a minimum of 3-seconds and stated that the EUT response time could increase the dwell time if necessary.
- The frequency step size table was changed in many of the test frequency ranges.
- RS101 alterative procedure using a Helmholtz was introduced.
- RS103 testing below 2 MHz was deleted from the requirements.
- RS103 alternative procedure using a reverberation chamber mode tuned method was allowed.
- The RS103 test level for the tuned frequency of antenna connected receivers was change to 20 dB above the RE102 limit for the associated application.
Revision “F” was released on 2007 bringing forth a few updates:
- The frequency step size table was changed in the upper test frequency ranges.
- RS101 scanning at three times the standard rate was changed to using the standard scan rate.
- RS103 testing at the tuned frequency of antenna connected receivers was not applicable except for surface ships and submarines. Test levels for the receiver tuned frequency was not provided, so the standard level was assumed which often created a susceptibility in the form of desensitization.
Revision “G” released in 2015 is the current standard and will be used as the basis for the following discussion detailing the test procedures for RS101 and RS103.
RS101 Radiated Susceptibility, Magnetic Field
As with most MIL-STD-461G tests, we start with a calibration check to verify the integrity of the instrumentation. If we had a way to monitor the radiated field during test this step could be eliminated but the field monitoring loop is not present during test plus the EUT will distort the field preventing accurate measurement.
Figure 1 shows calibration configuration where the field monitoring loop is accessible for field measurement and a current probe is located to monitor the current flowing in the radiating loop. Recall that the radiating loop winding should produce 9.5E7 pT per ampere flowing in the coil so the calibration process verifies this relationship. Once the relationship is verified then the current is monitored to establish the field during test.
Let’s step through the process for the calibration check. The radiating loop provides a mounting guide for the field monitoring loop to position the two items at the calibration location. The standard calls for the check to be accomplished at 1 kHz with a flux density of 110 dBpT, so we translate to linear units (10^(110/20) = 316E3 pT. We divide the 316E3 by 9.5E7 to obtain 3.33 mA of current to produce the field. Note that we could have converted the 9.5E7 to 159..55 dBpT and subtract from 110 dBpT to arrive at the same answer. We measure the field sensor and current probe voltage and after addition of the factors, we arrive at a relationship of 39.5 dB difference between the current and the field. This relationship holds throughout the test frequency range – i.e. the field in dBpT is equal to the current in dBμA plus 39.5.
Figure 1: RS101 Calibration Check Configuration
Once the calibration check proves successful, the field monitoring loop is removed to allow the radiating loop to be placed 5 cm from the EUT as shown in Figure 2. Scan the test frequency range (3-second dwell at each frequency step) at each test point. A test point is each 30 cm x 30 cm section of the EUT and each interface connector. It is a common practice to eliminate test points that do not have circuits in the area such as a blank panel in a rack of equipment. The testing is accomplished at a test level 10 dB above the acceptance limit up to a maximum of 183 dBpT (15-amps).
If susceptibility is observed, a threshold of susceptibility is to be measured. The threshold is determined by reducing the interfering signal amplitude until the EUT does not show susceptibility and then lower an additional 6 dB to compensate for hysteresis effects then increase the signal amplitude until susceptibility is observed. This applied amplitude in dBpT is the threshold. Until revision G was released the number of threshold measurements was not specified. Now the standard indicates that a minimum number of threshold measurement to identify the start and end frequencies of susceptibility and the worst-case measurement. I believe that more points should be used to help map the profile of the susceptibility such a 2 to 3 frequencies per octave of the frequency range where susceptibility was observed. Note that threshold measurements should be provided for each different susceptibility indications if applicable.
RS101 alternative procedure employs a Helmholtz coil to produce the magnetic field. The Helmholtz coil consists of a pair of near identical coils wound to support current flow in the same direction. This current flow supports the creation of a uniform magnetic field between the two coils. The distance between the two coils is equal to the coil radius. Note that if the coil is wound incorrectly, the fields from the two coils will tend to cancel.
The pre-test calibration of the Helmholtz coil is accomplished much like the standard method using a 13.3 cm loop (the RE101 loop antenna) in the coil center to measure the field while measuring the applied current to establish the current to field relationship. During test, the relationship is used to determine the applied current throughout the frequency range.
The EUT is placed in the coil center and operation is established. The frequency ranges is scanned using the standard dwell time established for the EUT and frequency step size. The EUT is oriented in the next orthogonal plane and the frequency scan is repeated and then the final orthogonal plane. The Helmholtz method provides the distinct advantage in complete testing in the plane with only one frequency scan. If your product line tends to be a common size, construction and use of the Helmholtz coil is very useful. Please note that the standard provides more details on the size of the Helmholtz coil and the maximum EUT size.
Figure 2: RS101 Test Configuration
RS103 Radiated Susceptibility, Electric Field
RS103 covers the frequency range of 2 MHz to 18 GHz with an option to extend the upper frequency to 40 GHz if specified in the procurement. The tuned frequency of antenna connected receivers is not applicable for Army and Air Force requirements. Receivers with permanently connected antennas may exhibit reduced performance during in-band testing but must recover after the in-band exposure is removed. Horizontal and vertical antenna polarization testing is applicable above 30 MHz.
The standard RS103 testing in MIL-STD-461G uses active sensor monitoring to measure the field during test. Configure the EUT in the test setup boundary with the radiating antenna 1-meter or more from the boundary as shown in Figure 3. Note that for larger test items, multiple antenna positions may be necessary to provide complete coverage within the radiating antenna 3 dB beamwidth usually necessary to attain the test level.
Placing the E-field sensor at the front of the test setup boundary centered along the 2-meters of cable and elevated at least 30 cm above the ground plane is normal for the radiating rod and biconical antennas. The beamwidth of these antennas cover the entire test boundary except for cases where the EUT is very large such as multiple racks. In the 200 MHz to 1 GHz frequency range the antenna positioning should expose the EUT plus 35 cm of cable at the EUT end of the cable. Above 1 GHz the position includes the EUT plus 7 cm of cable. With the various antenna positions the E-field sensor is placed to be in the antenna coverage and note that above 1 GHz the elevation of 30 cm may be reduced to place the sensor in the area being illuminated by the radiating field. Avoid placing the sensor at EUT corners of edges that may provide a shadow effect that could create a sensor measurement error.
Once the configuration and EUT operation is established the radiating field is increased until the test level is attained. Modulation (1 kHz square wave pulse modulation) is then applied and maintained for the greater of 3-seconds or the EUT cycle time. The frequency is stepped per the frequency step table from section 4 of the standard. The process continues in this manner until all required antenna positions and polarizations have been completed. If susceptibility is noted, threshold measurements are applicable.
Figure 3: RS103 Test Configuration
RS103 may be accomplished with the reverberation chamber method as an alternative. Details of the mode tuned method are provided in the standard with details on the calibration. The method has some distinct advantages with the calibrated volume but limitations on locating the point where susceptibility enters the EUT. IEC 61000-4-21 is a good reference for reverberation chamber testing and may be used for chamber calibration. Most reverberation chambers commercially available are supported by the manufacturer for the calibration process. Unless the EUT is directly involved in the calibration process, the testing may subject the EUT to over-testing because the loading effects of the EUT are simulated by an absorber volume.
Summary
The RS testing is included with most qualification test programs especially RS103. The testing can be very time consuming but can be automated with the feedback from the sensor. Monitoring the EUT performance continually is an integral part of the process and note that the simulation and monitoring equipment can easily be susceptible so isolation in the test configuration design needs to be considered.
