Special RF interference problems frequently arise in a large installation. These problems cannot be solved entirely by sup-pressing individual components, as described in the earlier chapters. Some of them arise from interaction between com-ponents that are close together. Others are due to common ground impedances, or to coupling between wires that are bundled together. RF interference problems are particularly severe in installations that include both interference sources and sensitive equipment that are receivers of the interference.
Such installations require more complete shielding than is otherwise necessary. Bonding of each component to the struc-ture or supporting frame becomes of major importance in such instances.
The examples treated in this chapter are representative rather than exhaustive. Obviously there are so many types of electronics equipment that it is impossible to cover them all in a book of this size. Although each type of equipment does introduce special problems of its own, the examples given here (together with the background material supplied in the earlier chapters) should be sufficient to allow suppression of practically any RF interference, provided no entirely new principle of RF interference generation is involved.
EQUIPMENT SUSCEPTIBILITY
RF interference susceptibility causes equipment to malfun-tion when any external lead or circuit (except the antenna) is subjected to an RF voltage. The term susceptibility can be expanded to include also that characteristic which causes equipment to malfunction when any power lead is subjected
to an AF voltage. It is necessary, therefore, that the suscepti-bility of electronic equipment be carefully measured according to this restricted definition. Otherwise, the antenna or its lead-in may pick up interference.
The broad objective of susceptibility-limit requirements is to insure that electronic equipment will operate properly when exposed to the highest possible levels of interference in the operating area. Since these levels are not always known, equip-ment must be designed with the least susceptibility that is practical. There is always a point of balance between inter-ference suppression at its source and reduction of suscepti-bility at the receiver.
The first step is to develop test procedures in accordance with susceptibility-control requirements. The second is to de-termine the highest practical degree of susceptibility control.
The latter should therefore be considered in the equipment design. The susceptibility test procedure includes one or more of the following three actions:
1. Subjecting the equipment under test to a strong radiated continuous-wave (CW) field varied over a wide range of frequencies.
2. Subjecting the equipment to a strong radiated broad-band interference field.
3. Applying RF sine-wave signals conductively to all power leads.
Susceptibility Tests
Susceptibility is the characteristic of equipment to develop a change in operation or output indication due to the reception of undesired RF signals, produced by other equipment, via the case, interconnecting cables, antenna, lead-in, or power lines.
The susceptibility test consists generally of applying an intense magnetic field to selected parts of electronic equip-ment. This field is generated by an electrostatically shielded loop probe and powered by a standard signal generator through a 50-ohm cable which is always properly terminated at the generator end. The susceptibility of equipment to this source, in terms of microvolts, may be plotted against frequency for comparison with the susceptibility of other equipment. In this way, the best control requirements can be determined. Be-cause of the intense magnetic field generated, the effect can be duplicated with a receiver and an interfering signal from a transmitter.
The attenuation of electric fields from 0.15 to 1000 mc by metal shielding material greatly exceeds that for magnetic fields. Therefore, the equipment under test must be subjected to magnetic fields of sufficient amplitude. (An electric field is an electromagnetic field in which the magnetic intensity is negligibly small. Likewise, a magnetic field is an electro-magnetic field in which the electric intensity is negligible.)
This susceptibility-determining method makes it possible to simulate the effect of the highest levels of interference within an area from 0.15 to 1000 mc; it is desirable that the established limits should be related to these levels. However, there is no known equipment that can withstand the interfer-ence created by high-powered transmitters. As a result, the criterion for establishing interference-susceptibility limits de-pends on the levels of interference that the best designed equipment in the particular installation can tolerate.
The standard signal source has an impedance of 50 ohms.
It is necessary to calculate the open-circuit voltage at the end of the 50-ohm cable connected to the signal source. Any stand-ard signal generator with a 50-ohm source impedance is espe-cially suitable because the open-circuit voltage is twice the output indication, regardless of the length of the output cable (cable losses neglected). Signal generators having source im-pedances of other than 50 ohms must be modified with appro-priate pads to match the 50-ohm output cable. However, the open-circuit voltage at the end of the output cable will not be twice the output indication but must be calculated. A typical termination network is shown in Fig. 11-1.
Here is the procedure for determining susceptibility: The equipment under test is subjected to magnetic fields from 0.15 to 1000 me generated by a three-inch, single-turn electrostatic-ally shielded loop probe terminated by a 20-foot RG-8/U cable attached to a standard signal generator. The loop probe is secured to the most susceptible area of the equipment being tested. (The area of maximum susceptibility will be almost constant with changes in frequency.) Therefore, the equipment need not be probed with every change in frequency.
In order to determine the most susceptible area of the equip-ment, the following preliminary tests are necessary:
1. The loop probe is secured near the case of the equipment under test, at either the antenna input connector, any large opening, or the power-line entry. The signal gen-erator is then set at maximum output and the spectrum scanned until a maximum leakage frequency is found.
(During scanning, checks are made at equipment oper-ating frequencies. Then the entire device is probed at this maximum leakage frequency and a point of maximum susceptibility is located.
2. The loop probe is placed near the point of maximum susceptibility (as just determined) oriented for maxi-mum coupling, and permanently secured.
Next the frequency range of the generator is again scanned with maximum output. If susceptibility is observed during the scanning, the generator output (open-circuit voltage in micro-volts) is decreased until threshold susceptibility is reached.
This reading, in open-circuit microvolts, is then compared with the specified ;imits for the equipment. If the level is above the specified limits, the unit under test meets the requirements.
A typical test set-up for radiated susceptibility with the signal source (generator) located outside the screen room is shown in Fig. 11-2. Fig. 11-3 shows a typical set-up for con-ducted susceptibility tests. A screen room is desirable because it eliminates both ambient interference signals and difficulties due to generator leakage.
Undesirable response is a variance from the normal opera-tion which does not cause malfuncopera-tioning of the equipment.
Threshold susceptibility results in an undesirable response which is barely recognizable from the normal output.
It is not always possible to indicate specifically what change in normal output constitutes threshold susceptibility or the beginning of a malfunction. In most cases the threshold sus-ceptibility, whether audible or visible, is taken as
approxi-OUTPUT
com-position resistors in parallel.
P2—six-db 50-ohm pad composed of -watt carbon composition resis-tors.
P3—pad to make Z05 equal to 50 ohms; consists of %-watt resistors, including R2.
P2 Ps r ZOUT
R1-20 ohms except at full output, where it varies from 10 to 50 ohms.
R2-250-ohm carbon composition resis-tor or network equal to 250 ohms.
Zin—equal to generator output imped-ance.
Z,,,,t—approximately 50 ohms.
Fig. 11-1. Network termination for signal generator.
LSN shielded, depend'ng on equipment.
If test sample operates with shielded leadin, termination is to a dummy
Ft—screen-room filters for power in-put outside top deck.
LSN—line stabilization network used for each ungrounded power lead.
Nt—type-N fitting (2) installed in an-tenna-termination filter panel.
Nt—type-N fitting installed in screen-room door.
PL—loop probe oriented for maximum output on meter or in phones, and permanently secured at, or very close to, antenna input fitting.
P.—test sample power leads; 24 inches +1 inch. spaced 2 inches apart, and 2 inches above ground plane.
Fig. 11-2. Typical setup for radiated susceptibility tests.
mately a 1-db change in output. The threshold of malfunc-tioning is usually taken as a point where it is difficult to dis-tinguish between the effect of desired and undesired signals.
In some equipment, threshold susceptibility actually cannot be determined because the equipment will malfunction if it is at all susceptible. In general, the performance requirements should be used as a guide for RF-interference susceptibility tests.
A—antenna leadin (see Fig. 11-2).
Ft—screen-room filters for power input located outside top deck.
L.—output cable; 20' of RG-8/U with loop probe at end; probe placed for maximum output at meter or phones.
000R CLOSED
CURING ALL TESTS) SC ROOM
LSN—line stabilization network.
Nt—type-N fitting (see Fig. 11-2).
P.—test sample power leads (see Fig.
11-2).
Fig. 11-3. Typical setup for conducted susceptibility tests.
In addition to the test for radiated fields, it is necessary to check for the effect of conducted signals feeding through power lines. These tests are the same as those described in earlier chapters.
Susceptibility Reduction
Electronic equipment is susceptible to interfering signals which enter through metallic cases, interconnections, cables, power lines, antenna lead-ins, etc. A good example of the setup for investigating receiver susceptibility to transmitter leakage is shown in Fig. 11-4. There are a number of poor designs, and even proper ones, which can cause the equip-ment to be less or more susceptible to interference.
Shielded lead-ins—The susceptibility of receivers decreases considerably when shielded lead-ins are used, such as gener-ally found on equipment operating above 30 mc. Receivers which operate below 30 mc should also be designed to use shielded lead-ins.
Shielded Cables—Shielded cables considerably attenuate the interfering signal and reduce susceptibility; this can be offset by using a cable which offers maximum attenuation. At 270.0 mc, for example, RG-8/U provides attenuation of 26 db less than RG-9/U, and RG-9/U provides 15.8 db more attenuation when encased in aluminum conduit.
Power- and Control-Line Filters—Unshielded leads, power cables, and control lines are very serious means of interference transmission. These wires will conduct both radiated and
N1
A—antenna leadin (see Fig. 11-2). LSN—line stabilization network.
F.—screen-room filters. Ni—type-N fitting.
L.—generator output cable. P.—test sample power leads (see Fig.
Fig. 11-4. Typical setup for testing susceptibility of receivers to transmitter leakage.