Basic knowledge of filters: types/functions/principles

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Basic knowledge of filters: types/functions/principles [Copy link]

The filter is one of the essential key components in the RF system. It is mainly used for frequency selection - allowing the required frequency signal to pass through and reflecting the unwanted interference frequency signal.

The classic application example of the filter is the receiver or transmitter front end, as shown in Figure 1 and Figure 2:

As can be seen from Figure 1, filters are widely used in the RF, IF and baseband parts of the receiver. Although with the development of digital technology, digital filters have replaced the analog filters in the baseband part and even the IF part, the filters in the RF part are still irreplaceable. Therefore, the filter is one of the essential key components in the RF system.

There are many ways to classify filters. For example:
according to the frequency selection characteristics, it can be divided into: low-pass, high-pass, band-pass, band-stop filters, etc.;

According to the implementation method, it can be divided into: LC filter, surface acoustic wave/bulk acoustic wave filter, spiral filter, dielectric filter, cavity filter, high temperature superconducting filter, planar structure filter.
According to different frequency response functions, it can be divided into: Chebyshev, generalized Chebyshev, Butterworth, Gaussian, Bessel function, elliptic function, etc.
For different filter classifications, the different characteristics of the filter are mainly described from different filter characteristic requirements.
The many different characteristics of the filter described by this many classification methods of filters reflect that the demand for filters in actual engineering applications needs to be considered comprehensively, that is, when designing for user needs, user needs need to be considered comprehensively. When
selecting a filter, the first thing to determine is whether to use a low-pass, high-pass, band-pass or band-stop filter.
The following first introduces the frequency response characteristics and functions of high-pass, low-pass, band-pass and band-stop classified by frequency selection characteristics.

Butterworth Chebychev Bandpass Filter

Butterworth Chebychev High Pass Filter

The most commonly used filters are low-pass and band-pass. Low-pass filters are widely used in image suppression in the mixer part and harmonic suppression in the frequency source part. Band-pass filters are widely used in signal selection at the front end of the receiver, spurious suppression after the transmitter power amplifier, and spurious suppression of the frequency source.

Filters are widely used in microwave radio frequency systems. As a functional component, they must have corresponding electrical performance indicators to describe the system's performance requirements for the component.

Different applications have different requirements for certain electrical performance characteristics of the filter.

The technical indicators that describe the electrical performance of the filter are:

Order (number of series)
absolute bandwidth/relative bandwidth
cut-off frequency
standing wave
out-of-band suppression
ripple
loss
passband flatness phase
linearity
absolute group delay
group delay fluctuation
power handling
phase consistency amplitude
consistency
operating temperature range

The following is an explanation of these electrical performance indicators of the filter.

Order (level): For high-pass and low-pass filters, the order is the total number of capacitors and inductors in the filter. For bandpass filters, the order is the total number of parallel resonators; for band-stop filters, the order is the total number of series resonators and parallel resonators.
Absolute bandwidth/relative bandwidth: This indicator is usually used for bandpass filters to characterize the signal frequency range that can pass through the filter and reflect the frequency selection of the filter. Relative bandwidth is the percentage of absolute bandwidth to center frequency.

Fifth-order high-pass filter

Cut-off frequency: Cut-off frequency is usually used for high-pass and low-pass filters. For low-pass filters, the cut-off frequency represents the highest frequency range that the filter can pass; for high-pass filters, the cut-off frequency represents the lowest frequency range that the filter can pass.
Standing wave: that is, S11 measured by the vector network, which indicates the degree of matching between the filter port impedance and the impedance required by the system. It indicates how much of the input signal fails to enter the filter and is reflected back to the input end.

Ninth-order low-pass filter simulation curve

Loss: Loss refers to the energy lost after the signal passes through the filter, that is, the energy consumed by the filter.
Passband flatness: The absolute value of the difference between the maximum loss and the minimum loss within the filter passband range. Characterizes the difference in the energy consumption of the filter for signals of different frequencies.
Out-of-band suppression: The "attenuation" outside the filter passband frequency range. Characterizes the filter's ability to select unwanted frequency signals.
Ripple: The difference between the peaks and troughs of the S21 curve within the filter passband.
Phase linearity: The phase difference between the phase within the filter passband frequency range and a transmission line with the same delay as the center frequency. Characterizes the dispersion characteristics of the filter.
Absolute group delay: The time taken for the signal to be transmitted from the input port to the output port within the filter passband range.
Group delay fluctuation: The difference between the maximum and minimum absolute group delay within the filter passband range. Characterizes the dispersion characteristics of the filter.
Power handling: The maximum power of the passband signal that can be input into the filter.
Phase consistency: The difference in the phase of the transmission signal between different filters of the same batch with the same index. Characterizes the difference (consistency) between batch filters.
Amplitude consistency: The difference in the transmission signal loss between different filters of the same batch with the same index. Characterizes the differences (consistency) between batches of filters.
Phase linearity: The phase difference between the phase within the filter passband frequency range and a transmission line with the same delay as the center frequency. Characterizes the dispersion characteristics of the filter.
Absolute group delay: The time taken for the signal to be transmitted from the input port to the output port within the filter passband range.
Group delay fluctuation: The difference between the maximum and minimum absolute group delay within the filter passband range. Characterizes the dispersion characteristics of the filter.
Power capacity: The maximum power of the passband signal that can be input into the filter.
Phase consistency: The difference in the phase of the transmission signal between different filters of the same batch with the same indicator. Characterizes the differences (consistency) between batches of filters.
Amplitude consistency: The difference in the transmission signal loss between different filters of the same batch with the same indicator. Characterizes the differences (consistency) between batches of filters.

LC Filter

Surface acoustic wave/bulk acoustic wave filter
Surface acoustic wave filters use the method of converting electrical energy into surface acoustic waves and using the acoustic wave resonance effect to achieve filtering. The characteristics of this surface acoustic wave filter are very small size, high Q value relative to LC, and suitable for mass production using semiconductor technology. The volume of a filter of about 800MHz is about the size of a 0805 capacitor. Its disadvantages are small power capacity, unsuitable for small batch customized products, long R&D cycle, and high R&D cost.
Surface acoustic wave filters are usually used in terminal consumer electronic products.

Spiral filter

Spiral filter:
Spiral filter is a semi-lumped parameter filter, which uses the self-resonance of spiral inductors placed in the cavity to realize the resonator, and realizes coupling through the spatial magnetic field of adjacent resonators.
Its advantages are: smaller volume than the cavity, higher Q value and power capacity than LC. Its disadvantages are: it is difficult to achieve broadband, and the high-frequency inductance is not easy to realize.
Spiral filters are usually used in occasions with 20% relative bandwidth below 500MHz, 100W average power, and certain requirements for insertion loss.

Dielectric filter

Ripple: The difference between the peak and trough of the S21 curve in the filter passband.

Dielectric filter

Dielectric filter is a semi-lumped filter implemented by a dielectric-filled quarter-wavelength short-circuit or half-open-circuit. Its advantage is that the Q value is higher than LC, and a filter with a higher frequency than the LC filter can be realized. Its disadvantage is that the parasitics are close and the resonator needs to be customized.

Comb Cavity Filter

The biggest feature of the interdigital filter is that it can achieve broadband. If redundant resonant rods are used, considering that the machine is linear, its relative bandwidth can usually be as wide as 60%. At the same time, in the K band, the broadband comb filter can basically not be machined and the debugging screws cannot be placed, so the interdigital structure is usually used under this condition. Compared with the comb structure, the parasitic passband of the interdigital structure is closer, and its parasitic passband is usually around 1.8F0. Under the same volume, the power capacity of the interdigital filter is larger than that of the comb filter.

The filter is an indispensable key component of the wireless communication system.

There are many types of filters, and various filters have different performance characteristics. Therefore, when selecting filters, it is usually necessary to comprehensively consider the customer's actual use environment and customer performance requirements to make a correct, effective and reliable choice.

When the customer has a vague concept of the filter index, it is usually necessary to ask the customer about the volume, loss, frequency to be suppressed out of the band, suppression degree, power capacity, etc. According to these simple index requirements, the type of filter can be basically determined.