Electromagnetic compatibility vs. mobile radio installations
With the proliferation of digital electronic control devices or electronic control units (ECU) within vehicles, the problem of electromagnetic compatibility (EMC) has really come to the forefront. How can all of these devices peacefully coexist within the confined space of vehicles? Now, throw in the mobile radio, with its relatively high RF power output and sensitive receivers, with an ever-increasing number of channels and range of operating frequencies, and the problem really gets complicated!
The problem is really a two-way (no pun intended) problem. That is, the radio transmitter can cause mild-to-severe degradation of the on-board electronics that control such things as ignition, braking and transmissions. Conversely, the ECUs might cause severe degradation to the mobile receiver. I remember a time when the communications technician only had to worry about a thing called ignition noise. And this only occurred at VHF low-band frequencies-right? Wrong. Contrary to some writings, the ignition noise problem could be severe at VHF high-band frequencies. Now that we have a multitude of other noise sources to concern us, the old ignition-noise problem pales in comparison.
Electronic ignition systems I remember my first encounter with the problem of the mobile radio transmitter interfering with the ECU that controls the ignition system of a pickup truck. The complaint was: “The engine stalls when I key the transmitter.” It was one of those unusually cold January days in upstate South Carolina when I had to go out and check this problem. Like everything else that slows down in cold weather, my brain seemed to be no exception.
Anytime I have to deal with intra-vehicular interference, one of my first objectives is to determine whether the interference is conducted or radiated. It is important to make this determination because this factor determines how the problem is going to be attacked and, one hopes, solved. Radiated interference means that the interference is entering or leaving the radio antenna. Conducted interference means that the interference is entering or leaving via cables, usually the power leads.
My first test was to confirm the complaint, just to satisfy myself that the problem did exist. So, I cranked the truck and keyed the mobile radio transmitter. To my chagrin the truck engine immediately shut down. This radio had been installed a couple of years ago by another technician and apparently had worked well for some time before this problem appeared.
Next, I removed the antenna connector from the radio and connected a dummy load instead. Again, I cranked the truck and keyed the transmitter. This time nothing happened-the truck ran on without skipping a beat. So, this interference was of the radiated type. The next step was to check the antenna. The forward-reflected power ratio looked good. The antenna was installed on the truck fender, next to the hood, so I raised the hood of the truck to explore underneath. I discovered that the antenna was sitting adjacent to the ECU that controls the ignition.
After this discovery, I suspected that the problem was the proximity of the antenna to the ignition ECU. Relocation of the antenna might solve the problem, but why had the problem just started when the installation had been working fine for some time? I quizzed the customer about the timing of the problem. Had anything been changed recently? Exactly when did the problem start? At first the customer expressed no knowledge of any changes around the time the problem started. But when asked specifically about the ECU he suddenly remembered that the ECU had been changed because of an ignition problem. Sure enough, that had been about the time the problem started with the radio.
The replacement ECU was apparently different from the original manufacturer’s part. At this point, it appeared that the problem might best be solved by relocating the antenna to a point further away from the ECU. However, before spending more time in that miserable cold, I decided to try a cheap trick on a hunch. Using a piece of regular kitchen aluminum foil, I carefully wrapped all the wires entering the ECU. Then, to test the results, I cranked the truck and keyed the transmitter. To my delight, the truck ran on.
Transmission ECU Recently, a problem was discovered in an International truck that is used to transport a crawler tractor used for forest fire suppression. The complaint was that something on the truck was interfering with normal radio reception on our state two-way radio receiver. The transceiver is a 32-channel VHF high-band with frequencies ranging from 151MHz to 159MHz. The interference was more severe on some channels than on others.
The first step in trying to resolve the problem was to determine the source of the interfering signal. The first possibility to come to mind was the ECU that controlled the engine. In order to search out the source of the interference, a Tektronix 495P spectrum analyzer was used with a sniffer probe attached, and a preamplifier was connected between the spectrum analyzer input and the sniffer probe as shown in Figure 1 on page 8. The sniffer probe is made by winding several turns of enameled wire into a self-supporting coil and attaching it to an SO239 connector as shown in Figure 2 on page 8. The preamplifier was a Radio Shack model used for CATV-not exactly a low-noise amplifier-but considering the noise figure of a spectrum analyzer, the amplifier was of sufficient quality for such work. With about 20dB of gain, the amplifier would allow us to see signals on the spectrum analyzer that normally would not be seen.
Using this test arrangement, a couple of mechanics, our communications technician and I set out to find the source of interference. First, the ECU that controls the engine was checked and found to be clean. At that point we did not realize that there was another ECU on board. Eventually, during the searching process, the second ECU was discovered behind the seat. That ECU controlled the automatic transmission for the vehicle.
In the interim, we had discovered that the truck did not have to be running to cause interference. With the radio receiver set to receive at 159.270MHz, just turning on the ignition switch would cause an intermittent carrier to be heard in the radio receiver. Of course, the CTCSS squelch system had to be bypassed by removing the microphone from the hook or pressing the monitor button. While holding the sniffer probe near the automatic transmission ECU, and turning on the switch, many spurious signals could be seen on the spectrum analyzer. We had found the source of the interference.
The severity of the problem was determined by using a test setup as shown in Figure 3 above.The test procedure is:
1)Set the signal generator to the receiver frequency, and with the truck switch turned off, set the signal generator to 3kHz deviation at 1kHz and adjust the generator level to produce 12dB sinad on the sinad meter. Record the signal generatorlevel in dBm.
2) Start the engine. Increase the signal generator level to return the sinad meter reading to 12dB. Record the new signal generator level.
3) The difference between the signal generator level in steps 1 and 2 is equal to the amount of degradation caused by the interference.
In this particular case, the interference was causing 19dB to 20dB of degradation. This level of interference could not be tolerated by our radio system, so some remedy had to be found. Because this automatic transmission was an Allison World Transmission model, we decided to contact Allison for assistance in solving the problem. After several communications back and forth, it was decided to try a couple of different ECUs in the truck to see if any improvement resulted. This was done by Allison at one of their local service facilities. We were requested to run our test to see what, if any, improvement resulted from the different ECUs being installed.
The two ECUs were set at different operating frequencies. One was at 7.996MHz and the other was at 8.004MHz. The Allison service representative installed first one, then the other, ECU as we tested for results using the previously described test procedure. Tests were run at 159.270MHz, 154.130MHz and 151.400MHz. The interference at 159.270MHz was always the most severe, even with the newer ECUs. However, the amount of degradation dropped from 19dB-20dB to about 6dB. Although this was still undesirable, it represented progress. Maybe 2dB to 3dB would be tolerable, but not 6dB.
The service representatives at the Allison service center are currently working with the manufacturer of the ECU to reduce the interference to a tolerable level. The work is still ongoing, and the ultimate solution is yet to be found. I will pass along information on the final solution when it is reached. A newer International truck, with a similar transmission and an ECU made by the same manufacturer, was tested and found to produce only minimal interference.
Problems such as these require the manufacturer’s cooperation to arrive at an effective solution. Without such cooperation, the probability of a successful outcome would be low. Even with full and unswerving cooperation, the solution is not easy.
Because of more and more EMC problems, I have advised our fleet manager to put something in the specifications to include the effects of electromagnetic interference (EMI) to our radio system (and from our system). Because more electronic devices are being added to vehicles, the problem is sure to get even worse in the future. The EMC problem is best handled at design time, rather than trying to find solutions after units are manufactured and sold to unsuspecting users. When buying such equipment, be aware of such problems and if possible, test one of your system radios in an actual vehicle under typical operating conditions before purchasing the vehicle.
A complaint from one of our county rangers was: “My windshield wipers start up when I key the transmitter on one of the repeater channels.” In order to get to the electronics that controls the wipers, I removed a portion of the dash panel. As I was preparing to set that portion of the dash panel aside, I noticed that the mini-UHF coax connector on the radio was hanging by the center pin. The shield of the plug was not even making contact with the radio connector. Yet, the radio had been working well enough that the user had no complaint about the radio operation.
Because the shield was not connected, I immediately thought this might be the problem causing the wipers to start up when the radio transmitter was keyed. To check, I simply connected the coax connector firmly to the radio and tested for the result. Bingo-that was it. Such a simple solution for such a seemingly complex problem. I had seen this type of connector get loose before and cause other symptoms such as a loud buzz in the broadcast radio in the vehicle. This is the type of thing that should be checked first.
Until next time-stay tuned!
