RF filters play a crucial role in manipulating frequencies, either by allowing specific bands to pass through or by reducing their intensity. These filters can be created using a combination of LC, RC, LCR, LR, or distributed components like microstrips. Additionally, they can be categorized as either active or passive. Active filters incorporate amplifiers along with passive components, while passive filters solely utilize lumped or distributed components. In certain passband and stopband applications, ceramic and crystal filters are commonly employed. These filters greatly enhance a filter's shape factor, enabling steep skirts, and offer a wide range of bandwidth options, from ultra narrowband to wideband. Another commonly used type is the surface acoustic wave (SAW) passive filter, which boasts excellent shape factors across a wide range, from narrowband to extremely wideband, and finds everyday use in RF applications.
The primary function of most filters is to allow frequencies within their designated passbands to pass through with minimal attenuation, while effectively reflecting undesired signals in the stopband back towards the source without absorbing them. However, these reflections can present challenges in wireless system designs, as we will explore shortly.
In addition to shaping signals and rejecting spurious frequencies, filters must also maintain consistent input and output impedance throughout their passbands, matching the impedance of the system, typically around 50 or 75 Ω.
Different types of filters, such as LC, crystal, SAW, ceramic, and distributed filters, find common usage in specific frequency bands. This preference may arise due to considerations such as size, cost, or performance. However, we will not delve into waveguide filters due to their high cost, weight, and bulkiness in this discussion.
LC Filter
An RF LC filter is a type of passive filter that employs inductors (L) and capacitors (C) to selectively pass or attenuate certain frequencies. It consists of a combination of these reactive components arranged in a specific configuration to achieve the desired frequency response. Low pass filter, High pass filter, Bandpass filter, T-type filter and Pi-type filter structures using inductors(L) and capacitors(C) are shown below. LC filters are useful in the range from 1 kHz to around 1.8 GHz.
LC filters are also available in monolithic LC type ceramic type. Monolithic LC bandpass filter IC offers cost-effectiveness, moderate insertion losses (1 to 3 dB), and high return losses (> 10 dB). These filters can handle medium RF input powers (0.5 to 1 W) and boast a compact physical size. They are readily accessible up to 5.8 GHz and possess the capability to attenuate both the 2nd and 3rd harmonics of a 2.4-GHz frequency signal. These LC integrated circuit type filters have gained prominence as the predominant method for RF bandpass filtering in numerous low-power, high-frequency consumer applications within microwave transmitters and receivers. They are constructed as multilayer planar structures, incorporating distributed L and C elements deposited on a high dielectric ceramic substrate.
LC Ceramic filters are available in both bandpass and lowpass configurations. However, unlike discrete LC filters discussed earlier, ceramic filters are limited to specific popular frequencies and bandwidths. It's worth noting that certain ceramic bandpass filters may exhibit a slightly asymmetric high-side frequency response.
Crystal Filter
Crystal filters utilize the piezoelectric properties of crystals, typically quartz, to create a highly selective filter. The crystal resonator vibrates at a specific frequency when an electric field is applied, allowing precise filtering of desired frequencies. Crystal filters are known for their narrow bandwidth and excellent selectivity.
Crystal filters are available in various forms, including single monolithic packages, discrete crystal ladder and lattice topologies, as well as active filter types. Generally, they tend to be more expensive compared to ceramic filters. Crystal filters are primarily utilized at intermediate frequencies (IF) below 250 MHz and in audio frequency applications.
One limitation of crystal filters is the presence of undesired spurious stopband modes that can affect their ability to attenuate specific frequencies effectively. Additionally, crystal filters typically have a limited input power capacity. However, they excel in selectivity characteristics and exhibit an extremely high quality factor (Q). Depending on the specific topology, crystal filters can provide a wide to very narrowband (0.001%) filtering capability, allowing for precise frequency control.
SAW Filter
Surface Acoustic Wave (SAW) filters use acoustic waves that propagate along the surface of a piezoelectric substrate, typically made of quartz. These filters are widely used in RF applications due to their compact size, high selectivity, and low insertion loss. SAW filters can provide a range of bandwidths and exhibit excellent shape factors. The following shows a basic structure of a SAW filter.
Surface acoustic wave (SAW) filters utilize a piezoelectric crystal substrate with gold electrodes deposited on it. These filters offer an alternative to LC filters in specific wideband applications spanning from 20 MHz to 2.4 GHz. SAW filters exhibit an exceptional passband with an almost perfect brickwall response. Some SAW filters even provide respectable stopband rejection capabilities up to 6 GHz. They are available at a moderate cost, deliver minimal ripple, and maintain constant group delays.
However, SAW filters do have certain limitations. They have a limited selection of resonant frequencies and bandwidths readily available off-the-shelf. Depending on various factors, SAW filters can experience insertion losses ranging from 3 dB to an extremely high 25 dB. In some cases, input/output impedance matching components may be required. SAW filters can withstand a maximum RF input level of +15 dBm or lower. Furthermore, they may exhibit poor spurious or harmonic suppression characteristics.
Ceramic Dielectric Resonator Filters
Ceramic dielectric resonator filters use ceramic materials with high dielectric constants to create resonant structures that filter specific frequencies. They offer compact size, good temperature stability, and high Q-factor, making them suitable for RF circuits. These filters are commonly used in applications where size and stability are critical factors.
However, it is important to note that ceramic dielectric resonator filters can become physically large and costly, especially when employed in combination with lower frequencies and/or multiple poles. Additionally, they may exhibit poor stopband performance at or beyond the 2nd or 3rd harmonic frequencies, making them less suitable for applications requiring effective suppression of harmonic frequencies, such as power amplifier stages. The performance of ceramic dielectric resonator filters is also sensitive to the impedance presented at the ports, requiring close adherence to a 50 Ω impedance to avoid passband rippling and minor frequency deviations.
These filters can be custom-made to suit specific applications, even in small production runs, offering flexibility and tailored performance.
Distributed Filter
Distributed filters, also known as transmission line filters, are constructed using transmission lines such as microstrips or striplines. These filters utilize the distributed properties of the transmission lines to achieve filtering characteristics. Distributed filters are often employed in RF circuits due to their wide bandwidth, low insertion loss, and compact size. They are suitable for applications where high-frequency performance is required.
However, it's important to consider certain aspects when utilizing distributed filters. Depending on the frequency and design, they may occupy a significant amount of board space due to their larger physical size compared to other filter types. Distributed filters can also experience significant reentrance modes, which can limit their harmonic stopband attenuation. To achieve optimum performance, high-quality (and often expensive) PCB substrates are required.
Another variant of distributed filters is the hybrid type, which incorporates both distributed and lumped elements. Among these, the combline topology shown below is particularly prevalent.
