|Home | Audio Magazine | Stereo Review magazine | Good Sound | Troubleshooting|
(source: Electronics World, Aug. 1964)
Noise suppression in mobile installations has become an urgent problem-what with FM, CB, and v.h.f . receivers.
By John Frye
“YOU know, Mac," Barney said to his employer, "automobile noise suppression jobs are getting more numerous and tougher every day. I'm not talking about broadcast-band car radios. I'm talking about the FM receivers, the CB receivers, and the commercial v.h.f. receivers so common in cars today. Believe you me: there's a whale of a difference between ‘de-noising' an ordinary car radio and keeping the noise out of one of these high-frequency jobs."
"I reckon we both know why," Mac answered. "Ignition noise usually peaks somewhere between 25 and 100 megacycles. On top of that, a broadcast receiver is ordinarily tuned to a nearby station radiating hundreds or thousands of watts from an antenna several hundred feet high. The CB receiver is trying to pull in a signal from a transmitter running a measly three watts into an antenna mounted, at best, on a rooftop.
"Incidentally, I dislike that term 'noise suppression.' It sounds as though all you need do is prevent popping sounds issuing from the speaker. Squelch or a.n.l. circuits will do that, especially if you're content to receive only strong stations. In fact, in an FM receiver with good limiters and a properly operating FM detector, you won't hear quite heavy ignition interference; yet that interference will be seriously degrading your reception by loading up the a.g.c circuit and reducing the receiver sensitivity. A good way to check on the presence of such interference is to use the diode probe of a signal tracer to pick off the signal at the grid of the first limiter.
"It's very important to keep basic concepts clearly in mind and to proceed in a methodical fashion in clearing interference. Remember any electrical spark is a miniature broadband interference transmitter. The higher the voltage producing the spark, the stronger is the interfering signal.
Since ignition voltage is several hundred times greater than other voltages present in the car, that's why ignition interference should be suppressed first.
"Wires leading to sparking electrodes serve both as radiating antennas and linear tuned circuits peaking the interference. Resistance in these wires lowers the ‘Q' of the tuned circuits they represent and damps out r.f. oscillations in them.
The r.f. oscillations can be short-circuited to ground through a bypass capacitor of sufficiently low impedance at the interfering frequency where this does not interfere with the ignition spark. Lumped resistance inserted in the wire or bypass capacitors will be most effective when placed as near as possible to the interference-generating spark, for this effectively shortens the radiating portion of the lead."
"You mentioned interfering with the ignition spark. Some guys insist any kind of resistance noise suppression degrades engine performance. What do you think ?"
"What I think doesn't matter, but tests conducted by automobile manufacturers have proved that where the ignition system is in good shape, the addition of approved resistor--type spark plugs, ignition cable, or suppressors does not affect engine performance. Of course, if the system performance is already marginal because of a weak coil, leaky capacitor, worn-out plugs, or some similar condition, the addition of suppression resistance may cause the engine to miss. In that case the answer is to correct the ignition defect, not take off the suppressors."
"Am I right in thinking the fundamental steps in preventing ignition interference are: (1) attenuate the pulses of interference greatly through the use of resistor suppression and bypassing; (2) bottle up the remaining interference inside the engine compartment by judicious bonding, shielding, and bypassing of all routes by which it can leak or be conducted outside; and (3 ) make sure the only way any signal--including ignition interference-can reach the receiver is by way of the antenna itself ?"
"You've got the big picture! The first thing to do when starting on a job is determine how many of these measures have already been taken. Is the engine equipped with resistor--type spark plugs? Champion precedes the plug number with an ‘Y.' Autolite and A.C. resistor plugs have an 'R' in the plug number. Is resistance ignition cable used? If so, what kind? General Motors. resistance cable is plainly marked ‘GA1 Radio TVRS.' It has a resistance of about 4000 ohms per foot. Other kinds will be marked ‘HTLR' (3000 to 7000 ohms per foot) and ‘HTHR' (6000 to 12,000 ohms per foot). This cable will not stand rough handling and should never be cut to install a screw-on suppressor or to attach a new end terminal. When resistance value exceeds 18,000 ohms per foot, the cable should be replaced."
"How about combining resistor plugs, resistance cable, and lumped-resistance suppressors? If one is good, three ought to be dandy."
"That's like taking a whole bottle of aspirin to cure a headache! Actually, resistor plugs are often used in conjunction with resistance cable or suppressors in two-way radio installations. The Law of Diminishing Returns prevents the improvement of adding external resistance to resistance plugs from being breathtaking, but it is usually noticeable. Increasing resistance, though, makes it tougher for the ignition system to produce the proper spark.
"A good ignition system in top shape may put out as much as 30,000 volts across an open circuit. In operation, this voltage is lowered to the amount actually needed to jump a spark across the plug electrodes. This required voltage goes up under heavy acceleration, and it also goes up if the plug is dirty or too widely gapped. With a single type of suppression installed, there is enough reserve to take care of ordinary variations, but adding resistance to the high-tension system cuts down on this reserve. The kind of driving also determines how much suppression an ignition system can stand. Stop-start, long-idle driving will sometimes foul plugs in an overly suppressed installation. If you're worried about the car's performance, it's a good idea to have the car checked with an electronic ignition analyzer, such as the Heath job or the Champion Plug-Scope,' after the suppression is in place."
"To avoid over-suppressing, a guy has to be on guard against hidden built-in suppressors," Barney offered. "Some older cars and trucks have 15,000-ohm suppressors built into the distributor rotor arm or concealed in the cap.
"Right, and in concentrating on the high-tension circuits, you must not overlook the importance of proper bypassing of the coil primary. Ignition pulses from the secondary can ‘kick back' through the primary and spread out over the whole low-voltage system. A 0.1-uf. coaxial bypass capacitor should have its center lead inserted in the lead coining from the BAT connection of the coil as close to that connection as possible. An ordinary capacitor here will not work at v.h,f. and u.h.f. frequencies. It has too much inductance. Only a coaxial capacitor presents sufficiently low impedance to the high-frequency r.f. to do an adequate job of bypassing. Both the capacitor case and the coil case should have an excellent. paint-free electrical connection to the firewall or block."
"How about bonding ?"
"That's what confines the residual ignition interference to the motor compartment. Heavy duty Belden 8662 bonding braid should be used to bond all four corners of the motor, the steering column, the firewall, the exhaust pipe, and the tail pipe to the frame. Lighter Belden 8668 can be used to bond metallic tubes and rods to the firewall at the point where they pass through this wall.
Tooth-type washers that will bite through the paint should be used to secure the ends of the bonding braid. In some cases, it may be found necessary to bond both sides of the hood and the trunk lid." "How can we be sure any signal reaching the receiver comes only by way of the antenna. I don't quite dig that."
"It's mostly a matter of proper installation. The case of the receiver must make a good connection to the car frame Or body. The antenna lead shield must have good ground connections at "both the receiver end and the antenna end.
A body- or fender-mounted antenna can be mechanically solid and yet have a few ohms of resistance between the grounding lug and the body because of paint or undercoating. In the case of a bumper-mounted antenna, the bumper itself may have to be bonded to the frame. A shielded lead should bring power directly from the battery to the receiver--ideally by way of a feedthrough coaxial capacitor through the firewall. It's a good idea to install a 0.5-uf capacitor between the hot battery connection and ground at the receiver if one is not already present. When the receiver is not picking up any signal through the battery lead or through a poorly grounded case or antenna lead, disconnecting the antenna will reduce the ignition noise to zero." "That about takes care of the three main steps I mentioned. What if you still have some ignition noise ?" "You probably will be able to hear a little, especially after you have cleaned up voltage-regulator, generator, instrument, and other kinds of masking noise and when you are not tuned to a station.
But it will disappear when even a weak station is tuned in. If you want to get the ultimate in noise suppression and are willing to pay for it, you must go to complete shielding of the ignition system as the aircraft and military people do. That means completely shielding the coil, the distributor, all high-tension wiring, and the spark plugs themselves. The shielding must be as complete and free of r.f. leaks as that used in keeping a transmitter from causing TVI."
"Can't you buy this sort of shielding and install it yourself ?"
"Yes, the Hallett Mfg. Co. in Los Angeles manufactures two lines of shielding. The ‘Signal-Saver' line provides complete customized shielding for practically any motor. In ordering, you give the make, model, and year of the car.
plus the displacement of the engine; and the shielding sent you is tailored to fit your particular motor.
"The same company manufactures the ‘Eliminoise' universal noise suppression kit which is marketed through the E. F. .Johnson Company. With this kit, which sells for considerably less than the customized line, you have to do more fitting and trying; but step-by-step instructions are sent to help you."
"You've been reeling off facts and figures pretty recklessly," Barney observed shrewdly. "I somehow get the feeling you didn't learn all this on your own."
"Never said I did," Mac defended himself. "I learned long ago to supplement my own experience and observation with expert help whenever possible; so I've written spark-plug companies, capacitor manufacturers, automobile makers, ignition-component manufacturers, and shielding manufacturers for any facts they had turned up in their research. All were most helpful. Champion Spark Plug puts out a free booklet entitled ‘Giving Two-way Radio Its Voice' that is full of helpful tips. Both Cornell-Dubilier and Sprague have special catalogue sheets describing their capacitors specifically designed for noise suppression. Delco-Radio has good material on the subject, and Hallett Mfg. Co. has promotional material and installation manuals that show you just how their shielding is installed. Well, we better get to work, but what say we continue the discussion by talking about stopping noise from non-ignition sources in the near future ?"
"Fine with me," Barney said; "but before we leave ignition noise, I've got a question: do you think transistorized ignition systems are going to give more noise trouble than conventional systems?"
"While I could get no auto manufacturer to stick his neck out on this subject, I got the impression they are anticipating no special problems with the transistorized systems they expect to use."
Prev. | Next