Considerations for selecting microwave filters

2022-06-10
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Considerations for selecting microwave filter

microwave filter is simple to build, but complex to understand. They perform a basic function in the system: blocking some signals from passing through other signals. However, this function can be realized in many different ways, and there are many different side effects, such as system amplitude and phase response distortion. Therefore, it is helpful to understand the difference of mechanism analysis between them before selecting filters

filters have various configurations: low-pass, high pass, band-pass and band stop or band suppression filters. As the name implies, the low-pass filter minimizes the attenuation of the signal below the cut-off point and suppresses the signal above the cut-off point. High pass filter is the opposite. The bandpass filter has the minimum attenuation in the passband around the central frequency and has high suppression of signals above and below the passband. On the contrary, the band stop filter blocks the signal within the narrow band around the center frequency and allows all other signals to pass through. In addition, a pair of band-pass filters with different frequency ranges can be combined to form a diplexer, or three band-pass filters can be combined to form a triplexer, while low-pass and high-pass filters can be combined to form a duplexer

ideally, the attenuation of the filter to the signal to be passed by the design should be 0dB, and the attenuation of the signal to be suppressed by the design should be infinite. In practical applications, dielectric substrate materials, conductors, passive components and connectors will cause losses and non ideal filter behavior. Therefore, many factors need to be considered to select which filter for a given application

filter response types include Butterworth, Chebyshev, Bessel and elliptical filters. Each filter has different response curves and is suitable for specific applications. For example, in order to minimize the amplitude change in the passband, Butterworth filter sacrifices the steep transition from passband to stopband. Chebyshev filter has a steep transition from passband to stopband. It is a filter with high quality factor (high Q), but there is a certain compromise in amplitude flatness and passband insertion loss. Bessel filter has good amplitude and transient response, but the stopband attenuation index is sacrificed. Therefore, Bessel filter is recognized to have linear phase characteristics and flat group delay in the whole passband. The change of elliptic filter from passband to stopband is also very steep, at the cost of large passband amplitude fluctuation and passband group delay

the performance levels of different types of filters can be compared through a common set of indicators, including insertion loss, suppression, VSWR and power processing capacity. Insertion loss is related to the signal in the passband of the filter and refers to the difference between the amplitude of the output signal and the output signal (unit: dB). As mentioned earlier, the ideal passband insertion loss should be 0dB, but the actual data is high, usually more than 1dB, depending on the signal frequency and filter type

The stopband of

filter refers to a frequency range within which the signal needs to be attenuated by a certain value. This attenuation value may be 20dB or higher, depending on how the filter manufacturer characterizes their filter. For a specified application, the degree of suppression should at least reduce the amplitude of useless signals to a small enough level, such as lower than the sensitivity of the receiver front end in the same system. In some filter types, the stopband rejection value may be 80dB or higher

The cut-off frequency of the

filter clearly separates the passband and stopband. The cut-off frequency is defined as the frequency at which the insertion loss (or suppression) is equal to 3dB or half power point. Low pass or high pass filters have only one cut-off frequency. The band-pass or band stop filter has two cut-off frequencies, which are located above and below the passband in the experimental machine products developed for displaying experimental data and results in the band-pass filter, and on both sides of the stopband or notch in the band-stop filter. In addition, for bandpass filters, the center frequency is generally the geometric average of the low-end cut-off frequency and the high-end cut-off frequency. For example, if the low-end cutoff frequency of a band-pass filter is 2400mhz and the high-end cutoff frequency is 2500mhz, its center frequency will be 2450MHz and the 3dB bandwidth will be 100MHz

the voltage standing wave ratio (VSWR) of the filter is an index to measure the matching degree between the filter and the characteristic impedance of its system. The VSWR of one port of the filter is the impedance seen from the other port when the other port accurately matches the characteristic impedance of the system (generally 50 Ω in high-frequency system). Therefore, the index of a filter usually includes the typical value and maximum value of input VSWR and output VSWR at the same time, which represents the matching degree between the filter and the source and load impedance to which it is connected. VSWR is expressed in proportion to 1, such as 1.50:1, but it can also be expressed as the reflection loss of the filter (in DB). If a filter shows impedance matching in both passband and stopband, the filter is considered as an absorption filter. When impedance matching is only achieved in passband, the filter is considered as a reflection filter. The latter filter has a higher VSWR in the stopband, such as 20.0:1 or higher. The power processing capacity of the filter is usually related to the physical size, working frequency range, filter technology, substrate material type, packaging type and heat dissipation limit of the material. The type of pulse or the type of continuous wave (CW) signal used is also a function of the maximum power of the signal

many companies provide downloadable white papers or application notes on the concise reference materials written on the filter specifications, including anatech electronics's "how to regulate RF and microwave filters", which provides a fruitful overview of different filter types; Similarly, the company's "lumped element (LC) filter", this paper reviews these popular RF filters; The application note "filter: introduction, term definition, Q & a" of mini circuits introduces the meaning of filter technical indicators and provides some application examples based on the company's compact filter

there are many types of filters, including fixed and adjustable types at low and high frequencies, such as lumped element filters based on discrete inductors (L) and capacitors (c), crystal filters, ceramic filters, cavity filters, surface acoustic wave (SAW) filters, bulk acoustic wave (BAW) filters, thin film bulk acoustic resonance (FBAR) filters, micro electromechanical system (MEMS) filters, and even active semiconductor adjustable filters. Lumped elements or LC filters are usually used in applications with frequencies below about 3GHz. The size of these filters depends on the operating frequency and the size of LC elements

the spiral filter is composed of a series of magnetically coupled cavities. It also belongs to LC filter, which is also limited to the frequency of about 3GHz in passband format only. Although it has a steeper response than the traditional LC filter, the input power is limited to about 5W

ceramic filter is made on a very thin ceramic substrate and uses discrete or integrated components to form a resonant circuit. According to the dielectric constant of ceramic substrate. Ceramic filters can be made especially small. The higher the dielectric constant of the material, the smaller the filter can be made. The use of mass production method can make the cost very low and the size small. The working frequency of ceramic filter is limited to about 6GHz and the power level is limited to about 5W, but it is very suitable for making band-pass and band stop filters requiring small size. The cavity filter can handle power levels up to about 500W and has outstanding insertion loss performance. The operating frequency of the cavity filter can be as high as about 30 GHz. Compared with LC and ceramic filters, they are larger and more expensive, because they are generally processed from aluminum blocks

saw, BAW and FBAR filters are manufactured by semiconductor manufacturing technology and use photoetching process to produce fine patterns, while MEMS filters use these processes to form three-dimensional structure. All these filters can be as small as 2x2mm, although they are limited in power processing capacity. Saw, BAW and FBAR filters are usually used in cellular communication, and the maximum operating frequency is about 3GHz. The working frequency of MEMS filter may be above 18GHz. (end)

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