We use filters to select or reject certain signals. A very important task of filters is the reduction of input noise, because the noise bandwidth is rather close to a 3-dB
Figure 4.3 This is how or why power divider phase character!sties affect antenna array performance. Any phase differences of the divider will be direclly added lo the phasing network values.
bandwidth in most RF equipment. The main characteristics of filters are their frequency response {both attenuation and group delay) and impedance matching (4j. Often manufacturers specify the passband attenuation and stopband attenuation separately. Many filters are reflective in nature, which means that there is large mismatch for frequencies within the stopband \S\. The losses in the passband should normally be a lot below 1 dB, but at very high millimeter-wave frequencies this may turn out to be difficult. Some simple filters do not give much more than 20 to 30 dB of attenuation in their stopband, but most are capable of 50 to 60 dB or even more.
Commercially available filters are found in all four main categories: lowpass, highpass, bandpass, and band reject [3]. Normally, the frequency response is fixed. We can also purchase tunable components, which arc based on pure mechanical adjustments. Sometimes the adjusting mechanism is driven by a stepper motor. Faster and durable devices rely on electronics, which can be found as varactor diode designs and as yttrium iron garnet (YIG) blocks. Tuning typically compromises other performance figures. Mechanically tuned filters show generally better attenuation responses but suffer from slow speed and wear in use. YIG and varactor filters can be very fast—several gigahertz per second—but this is achieved only when a couple of decibels of additional passband attenuation and not so steep slopes can be tolerated. The systems designer can obtain additional degrees of freedom by suitably combining low and high pass designs in order to get tailored pass bands. One such result is illustrated in Figure 4.4. Physical constructions include coaxial ("tubular") filters, microstrip and stripline designs (also using air as the dielectric), and waveguides. Narrowband filters are made in the surface acoustic wave (SAW) scheme or as piezoelectric crystals. Although filters arc normally very low-loss devices also in the stopband, they still have an upper limit of signal levels that thev can handle.
Initially, we have to find a filter having the correct frequency range. This is not particularly complicated if we arc dealing with a predefined system {e.g., SSR radar). Then we have to check the interface (i.e., coaxial or waveguide) and the passband loss. This might be surprisingly high in a tunable device even at moderate
Figure 4.4 Suitably selected commercial lowpass and highpass Tillers in cascade can provide a semicustom passband for our system. This shows the response when a highpass filter with a cutoff at 530 MHz is used in series with a lowpass filter with a 630-MHz cutoff. Actually, both filters are sold as 600-MHz devices.
RF frequencies. Many filters have considerable attenuation ripple in the passband (see Figure 4.5), which in FM systems may cause unwanted FM-to-AM conversion. The real struggle in filter selection is often in getting the wanted low passband attenuation in conjunction with sufficient rejection capabilities quite close to the passband. Steep filters are often sought. Modern systems present a further difficulty through the group delay requirement. If a filter has steep slopes in attenuation, we may find severe fluctuations in the delay curve close to the passband edges as indicated in Figure 4.6, and some suggested design procedures unfortunately yield to less satisfactory results (6). So-called constant-delay filters are special designs intended to overcome this problem |7|. Unfortunately, they are not broadly available as ready-made units for arbitrary frequencies.
Figure 4.5 Excessive attenuation ripple In the lilter passband (around A) may cause unwanted FM-to-AM conversion in frequency-modulated systems.
Figure 4.6 If a fitter has a reasonably steep amplitude response as desired in many systems, its phase characteristics might be far from linear. Here we show the group delay performance of a seven-stage stripline filter.
One of the cases in which custom designs are easily justified is a tailored system-specific filter. This partly comes from the fact that we—and the whole system—can benefit from a suitably narrow bandwidth—just tuned to our needs whereby the noise input will be lowest. Military radio systems require specific filters also due to enhanced jamming resistance [8], Of course, exact filter characteristics are kept as classified information due to their importance in electronic countermea-surcs (ECM) and antijamming (AJ) tasks. Examples of design equations and related data can be found in |2]. Recent trials in the author's team with selected commercial simulation software packages have indicated that a fully functional RF filter still requires at least one physical manufacturing iteration cycle for optimum performance [9|. For example, the time needed from the announcement of system specifications to produce the first-in-scries stripline bandpass filter (prototype illustrated in Figure 4.7) for an L-band radar was about 3 months.
Figure 4.7 A high performance stripline bandpass filler (or L-band radar. Completing this all-milled design from the announcement of system specifications took about 3 months.
Libro: Circuits and Components for System Evaluations and Design
Autor: Pekka Eskelinen
Nombre: Josmar Eduardo Depablos Rodriguez
Asignatura: Circuitos de Alta Frecuencia
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