Co-site Electromagnetic Interference Troubleshooting and Validation of Suppressing Performance by Extrapolation Method
Ma Xie
Southwest Communication Institute, Postbox 810, Chengdu, CHINA E-mail: [email protected]
Abstract
Co-site interference is becoming an increasingly significant problem in electronic systems. Ever increasing demand for more sophisticated, higher performance communication systems on one hand and the need for high density integrations in congested electromagnetic environments on the other have introduced numerous challenges to system designers. In this paper, the procedure of troubleshooting interference from vehicle to co-site radio system is detailed, and an approach for suppressing the interference is proposed. With the equivalent experiment and extrapolation method, the efficacy of suppressing is easily and effectively predicted. Also the signal coverage test and error analysis of prediction are reported.
1. Introduction
The modern military operations and emergency communication for rescue have created the need for more co- operation and higher efficiency. Thus, the lightweight wheeled vehicle integrated kinds of electronic systems forms the high-mobility unit with multi-function. Since last two decades, the high mobility multi-purpose wheeled vehicle has been widely used in both military and civil rescue. There has been much effort put into the design of radio systems and antennas in order to increase the wireless signal coverage during the past years. But meanwhile, the co-site Electro- Magnetic Interference (EMI) caused by the close proximity of antennas mounted on a space limited vehicle, has become a significant problem. Issues associated with co-site interference are multifold. Techniques to mitigate such effects are thus critical for ensuring reliable communications.
2. Analysis and Suppression
Normally, a typical vehicular communication system consists of switches, routers, radios and computers. In this case, the system has three radios covering the High Frequency (HF), Very High Frequency (VHF) and Ultra High Frequency (UHF). During the system performance test, the VHF radio keeps loud and clear communication when the vehicle engine is off. Once the engine starts, the voice quality drops badly, even unable to distinguish from background noise. The covering range of VHF radio in the whole working frequency band decreases 7.6 km when the vehicle engine is on.
2.1 Interference Diagnosis
The interference mode should be figured out first. The conducted interference may affect the VHF radio through power cable or signal cables or grounding, and the radiated interference may directly go through gaps into device or go into Radio Frequency (RF) unit by antenna coupling. The following steps are carried out for interference diagnosis.
1) Uninstall the VHF radio and its attaching antenna from the vehicle, disconnect all the input/output cables, place the antenna at the same position as before on the top of the vehicle without any electrical contact, and use the battery units to supply the power for VHF radio separately. Repeat the system performance test, the result shows that the covering range of VHF radio decreases when vehicle engine is on and restores to normal when engine is off. This step indicates the interference is not in conducted way nor caused by grounding.
2) Make the vehicle in a simulation of driving and full-featured state in the Semi-Anechoic Chamber (SAC), test and record the electrical field strength of the vehicle chassis running. Then according to the procedure of RS103 in MIL- STD-461, Test the electrical field radiated susceptibility of the VHF radio with the field strength as recorded before in the SAC. In this test, the VHF radio is placed in testing room and the antenna is placed in an open area to transmit signal, the RF cable connects them goes through RF trap out of the test chamber with small copper scraps. The test result shows that the VHF radio is capable to withstand the radiated electrical field at the level vehicle radiated and still has the normal covering range. This step rules out the possibility of the radiated interference goes into VHF radio or its attached cables directly.
3) From results of the two steps above, an assumption of the interference coupled into VHF radio via the antenna can be proposed. To validate the assumption, a simple but efficient approach is used in the EMI testing chamber. First,
978-1-4673-5225-3/14/$31.00 ©2014 IEEE connect the VHF radio under test and the antenna on the vehicle and a VHF radio accompanies in test with a 3-port RF coupler and RF cables, as Fig.1 shows. For adjusting the amplitude level, an external adjustable attenuator needs to be activated in the RF circuit.
Fig.1 Configuration of verify test with a 3-port RF coupler
During the test, keep the vehicle engine off, establish the communication connection between the two VHF radios, and test the voice quality until it scores 5 of 5 (full mark). Then decrease the amplitude of the signal, tune up the attenuator value as much as possible, set the voice connection into a critical situation but the voice is still loud and clear, the voice quality should score 3 of 5. Start the vehicle engine, the voice quality degrades immediately. The interference goes into the VHF radio under test in this configuration can be confirmed. After that, uninstall the 3-port coupler and the antenna, connect the two VHF radios directly with attenuator and RF cables only, as Fig.2 shows. In this test, the communication is still in critical status as before, but the VHF radio communication is not affected by start/stop the vehicle engine anymore. Thus, the assumption of the interference coupled into VHF radio via the antenna can be proved.
Fig.2 Configuration of verify test with RF cable connecting directly
2.2 Equivalent Measurement
For the convenience of EMI troubleshooting and performance revalidating in the laboratory environment, an equivalent measurement method is designed as Fig.3. The method only requires the two VHF radio systems in communication in a close distance, and transmitting wireless signal at a micro-power level.
Fig.3 Configuration of equivalent measurement at short range
The VHF radio accompanies in test emits the power to antenna in a fixed level, and the VHF radio under test in the vehicle is set to receive. Because the VHF radio works in the hopping frequency normally, a wide frequency band is not good for performance fast validation. Thus, three frequency bands are separately tested.
First, keep the vehicle engine off, tune up the attenuator value as much as possible, set the voice connection into a critical situation, and record the value of the attenuator. Then, start the vehicle engine, the communication is seriously affected. Adjust the attenuator in the smallest step to increase the amplitude of the signal transmitted from VHF radio accompanies in test, until the voice communication restores the performance as previous, record the new value of the attenuator (Attn.). The difference value of the attenuator (D-value) of each frequency band can be calculated as Table I shows. The D-value indicates the severity of the effects from vehicle to the co-site VHF radio.
TABLE I ATTENUATOR VALUES BEFORE EMI SUPPRESSION
Frequency Band Attn. (engine OFF) Attn.(engine ON) D-value 30.000 MHz~49.125 MHz 30.0 dB 27.0 dB 3.0 dB 49.125 MHz~68.275 MHz 34.0 dB 30.0 dB 4.0 dB 68.300 MHz~87.975 MHz 29.6 dB 25.6 dB 4.0 dB
2.3 Interference Suppression
To suppress the EMI from vehicle, the principle of cause and where it comes from should be diagnosed first. Once the vehicle engine starts, plenty of electrical devices and components in the engine cabin start to work in the meantime. An effective way to locate the regions of fields radiated by device or cable is performing near-field electromagnetic scan with electric-field probe and spectrum analyzer. Set the scanning frequency range covers VHF band, and then scan the entire engine cabin with probe when the vehicle engine is on. In the carpet scanning process, the noise increases conspicuously when the probe gets close to Electrical Unit Assembly (EUA) and its connected cables, and the input cables can be measured in higher amplitude. The EUA consists of many relays and converters for various electronic appliance of the vehicle. Then it can be confirmed that EUA is the source of the interference, and the power cable for EUA is a high efficiency antenna for the interference signal.
The most effective and complete way to solve EMI problem is control the source. In this case, the EUA is installed in a plastic case which cannot be replaced by conducted material under the hood of engine cabin to prevent potential safety hazard. In the meantime, utilize filters to filter noise and/or ferrite core to absorb interference for each cable from/to EUA is infeasible. Considering the cost-effectiveness and the limited space in engine cabin, the only acceptable measure is to install a high performance filter and/or ferrite core on the power cable which has significant antenna effect to radiate the interference signal, but the background noise is not reduced substantially. The EMI suppression from source has low feasibility in engineering.
Due to the risk of performance reduced if filtering the signal for victim and suppressing the EMI from source is not available, decrease the EMI radiation from cable to antenna through space. The power cable for EUA comprises two stages, one is from vehicle batteries to the front panel of terminals; the other one is from the front panel of terminals to EUA. The method to decrease EMI is to shield the radiated interference from the two-stage cable with a type of tinned copper braid shield widely used in EMC engineering. The cables in engine cabin are mostly connected in screws but not plugs and sockets, so the 360 degree bonding and grounding is not possible in this situation. A simple but effective approach is to grounding the two-end of the shield in a pigtail, and grounding the shield out layer in every 10 cm. After these processes, re-test the background noise VHF antenna received when the engine is on. The result shows a spectrum nearly the same as the background when the engine is off. Thus, the EMI suppression to the paths is successful.
3. Validation
The field coverage test is costly, so check the efficacy of EMI suppression by the equivalent measurement. The measurement uses the same configuration as Fig.3 and the same steps in chapter 2.2. The result is shown in Table II.
TABLE II ATTENUATOR VALUES AFTER EMI SUPPRESSION
Frequency Band Attn.(engine OFF) Attn.(engine ON) D-value 30.000 MHz~49.125 MHz 28.1 dB 28.0 dB 0.2 dB 49.125 MHz~68.275 MHz 30.2 dB 30.0 dB 0.2 dB 68.300 MHz~87.975 MHz 25.7 dB 25.6 dB 0.1 dB
Comparing to the attenuator values before EMI suppression, the D-value has been decreased significant. In the three testing frequency bands, D-value is not more than 0.2 dB, which means the VHF communication in critical situation has been affected minimum by the vehicle engine.
3.1 Prediction and Field Re-test
Due to the different background noise in different frequency bands, the VHF communication covering range in each band should be predicted separately. From the changes of D-values, the decreased range after EMI suppression can be calculated by extrapolation. The result is shown in Table III. Take the frequency band 30.000 MHz to 49.125 MHz for an example, before the EMI suppressing, the range decreased because of vehicle engine is 7.6 km and the measured D- value is 3.0 dB (1.995:1). After the EMI suppressing, the new D-value is 0.2 dB. By the extrapolation method, the reference of the range decreasing is 0 dB (1:1), then decreased range can be predicted as 0.359 km.
TABLE III COVERING RANGE PREDICTED BY EXTRAPOLATION METHOD Before EMI Suppression After EMI Suppression Frequency Band D-value Range Decreased D-value Decreased Range in Prediction 30.000 MHz~49.125 MHz 3.0 dB (1.995:1) 7.6 km 0.2 dB (1.047:1) 0.359 km 49.125 MHz~68.275 MHz 4.0 dB (2.512:1) 7.6 km 0.2 dB (1.047:1) 0.236 km 68.300 MHz~87.975 MHz 4.0 dB (2.512:1) 7.6 km 0.1 dB (1.023:1) 0.116 km
In the field test, establish the VHF communication first to get the largest communication covering ranges of the VHF radio in each frequency bands when the vehicle engine is off. Start the engine, the voice communication is overwhelmed. Then drive the vehicle to decrease the distance between two VHF radios, stops when the voice communication restores to normal. The test result shows that the covering ranges in each of frequency bands decrease 0.29 km, 0.12 km and 0.02 km respectively. The result indicates that the interference affects the VHF communication covering range from vehicle is successfully suppressed.
3.2 Error Analysis
The EMI suppression is effective but the prediction data is not equal to field test result perfect. The error of the prediction is manifold: the original coverage test is in the whole frequency band but not in separate bands which are used for validation after EMI suppression, the original test result is recorded in tenths which has less precision, the D-value may be affected by the tolerance of equivalent measurement and the subjective uncertainty of communication critical situation determine, and also the different test condition like terrains between original test and the field re-test may lead the error in covering range calculating. In conclusion, the error between prediction and measurement happens for reasons and can be controlled better in future, but in this case the error less than 0.1 km is acceptable in engineering.
4. Conclusion
The EMI between equipment and vehicle chassis increases significant due to the high density integrations. Using the method of system-level EMC diagnose to locate the cause and to find a suitable measure to suppress the interference is fast and effective. By utilizing the equivalent experiment and extrapolation method, the efficacy of suppressing can be easily and quickly accomplished without the complete costly signal coverage performance test, and the predict result is helpful for validation of EMI suppressing performance in engineering.
5. References
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