As I stated in the alternator article, the frequency of alternator whine is roughly equivalent to the engine RPM. At a steady-state 60 MPH, the average V6 vehicle engine is turning about 1,800 RPM. Four cylinders will be a little higher, and eight cylinders a little lower. So the whine will be in the range of 1,500 Hz, to about 3,000 Hz. Twelve diode alternators will be double this amount. Generally speaking, the output is a series of sinusoidal half waves, and as such the rise time is rather slow. As a result, there is very little harmonic energy generated unless there is a bad diode. Even then there is no appreciable amount of RFI generated.
At the same RPM, a V6 engine will fire three times with each revolution. At 60 MPH, that's a frequency of 5,400 Hz. Because the ignition arc occurs when the field of the ignition coil relaxes, the rise time is very fast. As a result, a whole lot of RFI is generated well into the VHF spectrum. Because of the different gear ratios used, and at any given MPH, the RFI frequency tends to be the same regardless of the number of cylinders. Incidentally, the level of ignition RFI changes with engine load and/or RPM.
Modern engines universally incorporate electromechanical fuel injection. The electromagnetic coil is energized for a very precise period of time by the engine control computer. When the field collapses, a fast rise time pulse is generated. Here too the resulting RFI extends into the VHF spectrum. As a rule, the injector pulse is less bothersome, due in part to a lower current requirement (verses the ignition). However, the new direct-gasoline injection systems use higher current levels as the nominal fuel rail pressure is as high as 150 BAR, therefore they tend to be worse RFI offenders.
Modern diesels aren't much better with respect to impulse noise as they too use electromagnetic injectors or an electromagnetic shuttle system. In fact, the current used to drive the coils tends to be higher than gas engines so more RFI is generated.
DSP systems come in many different forms. They may be audio based, IF based, or external. In a few high-priced radios, the DSP is applied directly to, and at, the incoming signal's frequency. The effect, or lack of it, on the incoming signal is different with each design. However, a few general assumptions can be made with respect to amateur transceivers.
1). We limit the audio bandwidth to about 3 kHz, albeit the front ends may be barn door wide.
2). Fast rise time signals (ignition RFI) tend to be masked better than slow rise time signals (alternator whine), which is, as a rule, part of their algorithm.
3). Bandpass width and depth adjustments change the perceived level and shape of any given type of interference.
4). Signal integration and phase distortion are common problems, especially with inexpensive DSP designs.
As a result of these shortcomings, the perceived interference (the actual audio) we hear out of our speakers can be much different than the actual signal would otherwise sound. This makes definitive identification of the various AFI, RFI, and EMI signals difficult.
There are very few built in noise blanker circuits worth any mentioning. Almost universally, the distortion, IMD products, and crosstalk they generate makes their use in high-level RFI situations antithetical (mutually incompatible). Add in a dose of poor DSP design, and it becomes very difficult to distinguish one RFI source from another.
It is always best to cure an RFI problem at it source. If you don't know the source, you're wasting time and effort. Identifying which is which is difficult at best. There are a few examples of the worst offenders located within my web site article Noise ID (http://www.k0bg.com). These sound files may help you identify which is which. However, they are by no means the only examples.
Nonetheless, here are a few suggestions to make sure which RFI generator you're dealing with.
1). Keep the DSP off, or at least at it's widest bandpass setting and minimum depth (DSP level in some cases).
2). Keep the noise blanker turned off.
3). Use the highest band (frequency) where the RFI can be easily heard.
4). If you remove the antenna and the noise goes away, it's a safe bet it's radiated RFI rather than induced noise through the wiring.
5). If removing the coax from the radio reduces or increases what appears to be alternator whine, chances are it's caused by poor DC wiring and/or grounding.
6). If you hear a rhythmic tick, tick, tick, that is too slow to be ignition, it could very well be a fuel pump.
7). A carrier which continues for a few seconds after turning off the ignition may also be a fuel pump. On Honda vehicles, the carrier shows up every 5 kHz, and can even be heard on 10 meters. It is caused by a mono-stable vibrator which drives the brush-less fuel pump motor. The motor stays on until the oil pressure drops as an aid in purging the system of excess vapor. This fact makes identification somewhat easier.
8). Some fuel pumps use brushed motors even though they are mounted inside the fuel tank. These cause an intermittent hash. There is usually a few seconds of hash when you first turn on the ignition, and then intermittently after the engine is running. Revving the engine changes the stacotic rhythm, but not the level of RFI.
9). If you listen carefully at low RPMs, you'll hear the injection pulse, and then the ignition pulse immediately afterward. If you have a scope, and connect it to the audio output, you can actually see the two pulses. The ignition pulse is always much higher than the injector pulse. The point being, most of the time you can't hear the injectors as their pulse is masked by the ignition pulse.
10). Some lessor known RFI generators are throttle position motors (primarily drive-by-wire systems), IAC (Idle Air Controls), Navi ROM motors, cross-flow ventilation motors (which can run even though the HVAC is off), and servo motors in automatic transmissions.
11). The most common RFI sources (more or less in order), ignition, injectors, cooling fans, HVAC fans, and windshield wiper motors.
If there is anything universal about noise suppression, it is the fact no solution is universal. Here's a few things to keep in mind while you're locating a source:
There are as many "fixes" as there are RFI sources. These include (more or less in order) very robust bonding, proper antenna mounting, proper DC and coax wiring, split beads, shielding, shunt capacitors, and a host of others;
What suppresses some forms of RFI, won't necessarily work with other similar sources;
What suppresses a specific RFI problem on your last vehicle, is no indication it will work on your new one;
Two separate sources can produce the exact same signature. Curing one won't necessarily cure or mask the other. In other words, if your attempt to cure a source (like adding a split bead or two), didn't cure the RFI, don't remove the first cure until you find the second identical signatured one. A good example of this is injector and ignition RFI;
Two, otherwise identical, vehicles can have vastly different levels of RFI, EMI, and AFI. Buying a vehicle because you have heard it is RFI clean is a prescription for owning the worst RFI generator on four wheels. If you're really into mobile operation, take along a decent HF receiver as a precaution;
If a couple of ground straps cured your RFI problem, the only conclusion that can be draw is, you were very lucky;
What ever you do, don't give up! John Pavelich, VE3XKD, has spend about 200 hours, and that much money curing the RFI emitted by his Prius, and he isn't done yet!
On the forefront of automotive technology is a new vehicle breaking system being developed by Siemens, a German electronics company. A short write up of the system appears in the November 13th issue of AutoWeek, on page 10. While a very significant development in many ways, the system uses high frequency signalling to control the electronic actuation motors. It is yet another example of why it pays to be cognisant of cutting edge, digitally controlled devices, which have a potential of causing serious RFI.
FCC Part 15, Subpart B, 15.103 (a) exempts automobiles as unintentional radiators. However, the preamble to that exemption states that manufacturers should endeavor to keep levels below those set by Part 15. Exemption or not, excessive levels of RFI from any vehicle should be brought to the attention of the selling dealership, factory representative, and even the corporate offices of the manufacturer. It is a case of the proverbial squeaking wheel. After all, we don't want automotive digital electronics to become another BPL-like wasteland.
Alan, KØBG
www.k0bg.com
