by Sam Benzacar, Anatech Electronics
As one of the fundamental signal processing components in RF and microwave circuits, the RF filter plays a critical role in determining any system’s ultimate performance. Since the electromagnetic spectrum has become more and more densely populated, RF filter performance has taken on greater significance than ever because interference rejection is of paramount importance, which in many cases desensitizes receivers and data is lost.
This article describes the common filter types and their characteristics such as LC, ceramic, cavity, Surface Acoustic Wave (SAW), and crystal, to provide a simple thumbnail sketch of the most popular filter types along with the advantages and disadvantages of each in various applications.
To the uninitiated, RF filters can seem strikingly simple, since they are passive components that perform a single function: To pass a certain frequency band and attenuate unwanted frequencies, either below or above a pass band. However, the truth lies elsewhere, since there are not only multiple types of filters but also multiple response types, and descriptions of both are the subject of thousands of technical papers and several textbooks.
Specifying a particular filter for a given system invariably requires a trade-off between a wide variety of factors, including power handling, Q factor, insertion loss, operating frequency, size, packaging and mounting, manufacturability, and many other parameters, some of which can add enormous complexity to the design and implementation.
LC or Lumped-Constant Filters
LC filters, also called lumped-element types, can be specified in low-pass, bandpass, band stop, high-pass, diplexer, and duplexer. They are constructed with capacitors and inductors using complex configurations and are available in frequencies ranging from around 10 KHz to slightly above 3 GHz (Figure 1). They range in size from about 0.5 in. at high frequencies to 26 in. at low frequencies, their size being dictated by the size of their capacitors, inductors, and power handling.
LC filters have a variety of advantages, the benefits of which vary depending on the application for which they are being considered. They are a good choice at frequencies between 500 MHz and 1 GHz because their size remains reasonably small compared to cavity filter types. They are the best choice for frequencies between 1 MHz to 1000 MHz where cavity filters would be bulkier.
For example, an LC filter with a center frequency of 500 MHz would typically be one-third the size of a cavity filter (Figure 2). They can be made to provide the widest range of filter topologies, such as Chebyshev, elliptical, Bessel, Butterworth, constant-impedance, and constant group delay.
Depending on the complexity of the requirement, they can be designed with a very sharp transition between the pass band and the rejection. LC type filters are more useful when special characteristics such as very low group delay variation, constant phase linearity and matching networks to match various impedance networks.
LC filters are also very versatile from a mechanical standpoint and can support many types of connectors in various combinations, as well as drop-in, printed circuit board, and surface mount. Many case styles can be accommodated to meet the needs of specific physical environments as well. They have low insertion loss and can handle RF power levels as high as 500 W, and in special cases even more.
However, LC filters are limited in the performance they can achieve, such as extremely narrow bandwidths, extremely low insertion loss due to coupling between the elements, and limited Q factor. At frequencies above 3 GHz, the inductor’s size becomes impractically small. Their power handling is dictated by the size of the inductors and capacitors as well as the design complexity. LC filters might be a good choice for reasonably priced RF filters and will mostly depend on the complexity.
Ceramic filters (Figure 3) use quarter-wavelength resonators as their main tuning elements, are best suited for frequencies between 400 MHz and 7 GHz and can be fabricated in bandpass or band stop configurations with the most prevalent type being the bandpass filter. Their size depends on the dielectric constant of the ceramic resonator, which in most cases ranges between 30 and 90. The lower the dielectric constant, the larger the resonator and the better the temperature coefficient, and vice versa.
Ceramic filters can be made from discrete ceramic resonators or as a monoblock in which the resonators are made from a single piece of ceramic. Their advantages include high Q factor (compared to LC), good insertion loss, low cost, comparatively small size, and the ability to be mass produced cost-effectively in large numbers.
However, ceramic filters are limited to the number of sections, and also limited in their power handling, which is between 5 to 8 W. Temperature stability can pose a problem when using very high dielectric constant. In addition, their construction allows them to be suitable only as Surface Mount Technology (SMT). To achieve full performance, they require a considerable amount of care during assembly to ensure good adhesion to ground. Ceramic filters can be connectorized by installing them inside an enclosure.
Cavity filters can be designed between 20 MHz and 50 GHz and can be specified in band pass or notch filter configurations. They are built as quarter-wavelength resonators typically machined from aluminum and usually in an air dielectric; the result is larger footprints compared with LC filters.
The advantages of cavity filters include high power handling ability, typically 50 to 1000 W depending on the resonator’s size and complexity, low insertion loss, can achieve extremely high performance, and can be manufactured in medium to large quantities. On the downside, they are comparatively large, cannot be used at very low frequencies, and generally can only be supplied in a connectorized or drop-in style package.
A coupled resonator structure is employed when narrow bandwidth is required. Moderate bandwidths are accomplished with a combline structure, and when wider bandwidths require an interdigital configuration. Recent advances in cavity filter design allow them to achieve extremely high performance, with a transition from passband to stopband as low as 500 to 700 kHz in a filter centered at 800 MHz, for example.
Crystal filters consist of crystal resonators, each one made from a single piezoelectric resonance material (Figure 4). They provide a precisely defined fixed center frequency and achieve extremely high Q factors (in the tens of thousands), which allows them to achieve extremely narrow bandwidths of only a few kilohertz. Crystal filters are typically employed in the intermediate frequency (IF) stages of receivers (70 and 140 MHz) and can be specified at frequencies from about 300 KHz to about 225 MHz. They are usually available in band-pass or notch configurations and in Chebyshev topologies.
Crystal filters are primarily used for single-band applications such as a receiver operating at one frequency, which the filter passes while rejecting all others with high precision. They also inherently have high temperature stability. However, crystal filters can handle power levels of only about 5 W. They are difficult to implement, and lead time can sometimes be around 18 weeks or so due to stabilization of the crystal, which sometimes takes between 7 to 10 weeks.
As should be obvious at this point, specifying the most appropriate filter for a given application is not as simple as might be expected, and the ultimate decision requires careful examination of many factors. Manufacturers such as Anatech Electronics can help designers make these decisions and can provide quick solutions to even the most complex problems that can otherwise consume lots of time.