Types of spurious emissions:
In modern equipment, there are three main types of spurious emissions:
1. Instability and Parasitic Oscillations
-Description: If a stage in a transmitter is unstable and is able to vibrate then it may generate RF at either a frequency close to the operating frequency or at a very different frequency.
-Detection: One good sign that this is occurring is if the antenna is transmitting signals even without an output from the radio equipment. Another sign is if the output power suddenly increases erratically when the input power is increased just slightly. In a Class C stage this behaviour can be seen under normal conditions.
-Prevention: The best defence against this transmitter defect is a good design from a reputable company, and it is also important to pay good attention to the neutralization* of the valves or transistors. Inadequate neutralization may be a sign of faulty equipment components.
*Neutralization is a term used in electronics for negative feedback which is used to make the system more stable.
2. Harmonics
-Description: Every oscillator generates harmonics, which are multiples of a given frequency in addition to the signals at their fundamental (wanted) frequencies. For example, the third harmonic of 100 Hz is 300 Hz. Harmonics can interfere with other users of the radio spectrum since the generated frequencies are usually well out of their allocated band. All systems produce some level of harmonics, therefore FCC regulations were enforced to specify limits for harmonics (along with other spurious radiation) to reduce instances of interference with other transmitters. If operating a 100 W output transmitter, harmonic signals can total no more than 10 milliwatts. A transmitter that complies with the rules still generates some harmonic energy, but that energy is so small that it is not likely to cause problems.
-Detection: Harmonics can be tested for using a RF spectrum analyzer or with an absorption wave-meter, depending on budget. If a harmonic is found at the same frequency of the signal wanted at the receiver, this spurious emission can prevent the reception of the desired signal. For new equipment, it is easy to be sure that it does not generate excessive harmonics because the FCC requires all transmitter manufacturers to prove that their equipment complies with its regulations before they are approved for retail. This means that commercially manufactured equipment usually produce clean signals, whereas home-made and poorly designed equipment suffer significantly from harmonic generation.
-Prevention: Purchase only equipment that are FCC approved, or research the product before buying. Ensure that there are properly tuned circuits, which reduce or eliminate spurious signals between stages in transmitters. The tuned circuits allow signals at the desired frequency to pass, but they attenuate (reduce) harmonics. In addition to an efficient design of the amplifier stages, the transmitter's output should be filtered with a low-pass filter to reduce the level of the harmonics.
3. RF leakage
Description: This condition occurs when frequencies are being unintentionally transmitted (and sometimes received) from a device due to defecting RF shielding.
Detection: If a transmitter is suspected of being responsible for a interference problem then it should be run into a dummy load, which is a resistor in a screened box which will allow the transmitter to generate radio signals without sending them to the antenna. If the transmitter does not cause interference during this test then it is safe to assume that a signal has to be radiated from the antenna to cause a problem. If the transmitter does cause interference during this test then a path exists by which RF power is leaking out of the equipment, which can be due to flawed shielding. This is a rare but insiduous problem and it is sometimes vital to be tested for if equipment is causing inteference.
Prevention: All equipment using RF electronics should be inside a screened metal box, and all connections in or out of the metal box should be filtered to avoid the radiation or reception of radio signals. A common and effective method of doing so for wires carrying DC supplies, 50 Hz AC connections, audio and control signals is to use a feedthrough capacitor. This is a capacitor which is mounted in a hole in the shield, one terminal of the capacitor is its metal body which touches the shielding of the box while the other two terminal of the capacitor are the on either side of the shield. The feed through capacitor can be thought of as a metal rod which has a dielectric sheath which in turn has a metal coating.
Also, be sure to reinstall and tighten all the screws before operating equipment again, if the covers needed to be removed for any reason. In addition to protecting the electronic components from physical damage, the covers also provide shielding to the circuit, stopping any spurious emissions or unwanted RF signals from being radiated.
You are most likely to see RF leakage on homemade equipment or equipment which has been modified. It is also possible to observe RF leaking out of household microwave appliances.
Extra Information:
Apart from the three main forms of spurious emissions previously explained, there are a few additional factors that can contribute to the generation of excessive spurious emissions:
Improperly adjusted controls:
It is possible for improperly adjusted equipment settings to cause undesired emissions. For example, a Single Side Band transmitter with the microphone gain set too high can cause interference to nearby frequencies, a condition known as splatter. Talking too loudly into the microphone, or having the microphone gain set too high can also cause the transmitter to over-modulate its signal, creating spurious emissions that could interfere with other stations.
Unwanted mixing products
Frequency mixing is a method in which one frequency (known as the intermediate frequency) is mixed with another frequency to create a resulting signal that is then amplified and transmitted. If the intermediate frequency enters the power amplifier and is not adequately suppressed then it could be also be radiated, having the potential to interfere with nearby radio frequencies.
Spectral Splatter
In radio electronics or acoustics, spectral splatter (also called switch noise) refers to spurious emissions that result from an abrupt change in the transmitted signal, usually when transmission is started or stopped. For example, a device transmitting a sine wave produces a single peak in the frequency spectrum; however, if the device abruptly starts or stops transmitting this sine wave, it will emit noise at frequencies other than the frequency of the sine wave. This noise is known as spectral splatter, and in some cases a filter can be used to remove unwanted emissions. The need to minimize spectral splatter arises because signals are usually required to be contained in a particular frequency band, defined by a spectral mask, but spectral splatter can cause emissions that violate this mask.
Unintentional Radiators:
An unintentional radiator is any device which creates radio frequency energy within itself, but does not radiate a significant amount of this energy past its housing unless it is damaged or malfunctioning. A computer is an excellent example, as all spurious emissions should normally be contained within the case. The device's housing should be checked for leaks and ensured that all compononts are securely fitted. If the problem still continues (and causes significant inteference to other devices), the faulty equipment should be replaced altogether.
