Getting back to basics: Filter terminology
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Filters are indispensable in radio communications systems. Receivers and transmitters use them in RF stages, intermediate frequency stages and audio frequency stages. Much of the basic filter terminology applies to all types of filters used in receivers, transmitters, multicouplers, combiners and antenna systems.
Bandpass
Bandpass filters pass a given band of frequencies and reject frequencies outside the passband.
When selecting a bandpass filter, first determine how much rejection or attenuation must be provided by the filter at frequencies outside of its bandwidth. The bandwidth extends to frequencies above and below the filter’s center frequency.
Frequencies above and below the center frequency, at which the filter response falls to a level 3dB below the center frequency, define the limits of the bandwidth. Figure 1 illustrates how to calculate bandwidth. Simply subtract the lower frequency (F1) where the response drops by 3dB from the higher frequency (F2) where the response drops by 3dB. The difference in these two frequencies equals the filter’s bandwidth.
Another important filter characteristic is “Q.” The filter’s center frequency divided by its bandwidth defines its Q. (See Figure 1.) The higher the Q, the more narrow the bandwidth at a given center frequency. In land mobile radio work, cavity resonators achieve high Q. Crystal filters offer high Q, but they have higher insertion loss. And crystal filters can’t be subjected to high transmitter power.
Selectivity makes a big difference in selecting the proper filter for a particular application. Usually, the selectivity is described as “so many decibels down” at some frequency removed from the filter’s center or pass frequency. For example, the selectivity of a cavity resonator at 155MHz might be described as “15dB down” at a frequency 1MHz above or below the center frequency.
A filter’s response isn’t always symmetrical. In fact, it seldom is. Therefore, the actual response might be “12dB down at 1MHz above the center frequency” and “15dB down at 1MHz below the center frequency.”
Shape factor expresses filter selectivity in another way, as a ratio of the filter’s response at 60dB of attenuation and 6dB of attenuation. Generally the shape factor is defined as shown in Figure 2. The lower the shape factor, the steeper the skirts of the filter’s response curve. For example, a shape factor of unity would indicate that the response curve of a filter has a rectangular or square shape — not available in the real world.
Figure 3 shows additional filter characteristics of ripple and return. Ripple occurs within the filter’s passband. Expressed in decibels, it quantifies the difference in the level of the maximum (crest) to minimum (trough) response. Don’t confuse “filter return” with transmission line “return loss.” Return is an unexpected and unwanted peak in the response curve in the frequencies outside of the passband, known as the stopband.
Insertion loss is another important characteristic to consider in selecting a filter. It is the loss presented to the signal as it travels in the desired signal path. Generally, it is desirable to keep insertion loss to a minimum. In the real world, however, a certain amount of insertion loss must be accepted to achieve the desired or required degree of selectivity. Fortunately, insertion loss doesn’t always harm the system performance. In many cases, several decibels of insertion loss might be tolerated before system performance begins to degrade. An example is a transmitter site with high ambient noise — more the rule than the exception these days.
One important thing to remember about insertion loss is that the filter’s power input rating must be based on its insertion loss. Insertion loss causes the filter to dissipate power. The higher the insertion loss, the more power dissipates within the filter.
For example, when 100W is applied to a filter with 3dB insertion loss at the operating frequency, the filter must be rated to dissipate 50W of power. Power dissipates as heat, and the filter must be able to withstand the dissipation.
Inherent high insertion loss is cited as a reason not to use crystal filters. But where crystal filters are needed, it is rare that the insertion loss would cause serious degradation to the receiver system performance. Crystal filters normally are considered in the first place when site noise is usually high — so high that the insertion loss is of little consequence.
Band-reject
Band-reject filters reject a frequency or a band of frequencies while passing all others. The response of a simple band-reject filter looks like an inverted passband filter response. Photo 1 shows a CRT display of the response of a band-reject filter built into a broadband preamplifier. This band-reject filter attenuates signals from stations in the FM broadcast band.
Bandpass/band-reject
Bandpass/band-reject filters are sometimes simply called pass-reject filters. You may see the name abbreviated as “BpBr” (or something similar). The pass-reject filter is commonly used in duplexer configurations. It can be designed to reject one frequency and pass another frequency with little separation between the two. Repeaters often have closely spaced transmitter and receiver frequencies and a need for a pass-reject duplexer.
In today’s crowded spectrum, few of us can get by without having to purchase and install filters of one type or another. On the bright side, most applications engineers at companies that manufacture and sell such filters are quite knowledgeable and helpful. Discuss your needs with them, and they will usually come up with a solution for your interference problem. The more you know about filters, the better you can communicate with the applications engineer.
Until next time — stay tuned!
Contributing editor Kinley, MRT’s technical consultant and a certified electronics technician, is regional communications manager, South Carolina Forestry Commission, Spartanburg, SC. He is the author of Standard Radio Communications Manual, with Instrumentation and Testing Techniques, which is available for direct purchase. Write to 204 Tanglewylde Drive, Spartanburg, SC 29301. His email address is [email protected].
