Ⅰ. What is an RF Filter?

In a radiofrequency/microwave system, it's common to have to distinguish the useful frequency signals from the worthless ones in the signal spectrum. A filter is the piece of equipment that completes this purpose.

The filter is an important radio frequency component of a wireless communication system.

Image frequency interference is filtered out, noise is reduced, and frequency division multiplexing is used.

Oscillation, amplification, frequency multiplication, and mixing circuits with excellent performance

In the passband range, a radio frequency/microwave filter refers to a filter in the radio frequency and microwave frequency bands. One of the most significant components in consumer electronics is the radio frequency filter, commonly known as a "radio frequency interference filter." The radio frequency filter is a filter circuit made up of capacitors, inductors, and resistors that are used to filter the signal frequency in communication channels. The filter allows signals that meet a specific frequency to pass while suppressing unwanted frequency signals. It is widely used in radio frequency signal processing systems for base stations and terminal equipment to solve the problem of signal interference between different frequency bands and communication systems. The filter is situated on the backside of the power amplifier in the radio frequency transmission path, and on the front side of the low noise amplifier in the radio frequency receiving path, from the perspective of the layout of the radio frequency signal processing system.

Network and coupling structure for effective broadband impedance matching

Ⅱ. The classification of RF Filter

The working attenuation LA is usually used to describe the amplitude characteristics of the filter.

According to attenuation characteristics

Low-frequency signals can be sent from the input port to the output port with a minor amount of attenuation thanks to the low-pass filter. When the signal frequency exceeds the characteristic cut-off frequency, the signal attenuation increases significantly, resulting in a reduction in signal amplitude at the output port.

A high-pass filter has the exact opposite features. When the signal frequency exceeds a particular cutoff frequency, the signal is attenuated and sent from the input port to the output port.

Specific lower and upper frequencies separate band pass and band stop filters into frequency bands. In comparison to other frequency bands, the signal attenuator in this band has low (band pass) or high (band stop) attenuation.

According to structure

Tuned electronic equipment is used in active filters. The wave device is designed to have a specified frequency-dependent transmission response, which is mostly used to suppress unwanted signal components, interference, or noise in the spectrum.

Resistors, capacitors, inductors, resonators, attenuators, and other passive structures and components make up passive filters.

Digital hardware, such as application-specific integrated circuits (ASIC), field programmable gate arrays (FPGA), or digital signal processors, are used to execute signal conditioning functions on digitized signals in radio frequency digital filters (DSP).

Low-pass and band-pass filters are the most often used filters.

Low-pass filters are commonly utilized in the mixer's image suppression and the frequency source's harmonic suppression.

Receiver front-end signal selection, spurious suppression after transmitter power amplifier, and frequency source spurious suppression all require bandpass filters.

Ⅲ. The technical index of RF Filter

Order (number of series)

The order of high-pass and low-pass filters is equal to the total of their capacitors and inductors. The order of band-pass filters is the total number of parallel resonators, whereas the order of band-stop filters is the total number of series and parallel resonators.

Absolute bandwidth / relative bandwidth

This indicator is typically used for band-pass filters to characterize the frequency range of the signal that can pass through the filter and to represent the filter's frequency selection. The relative bandwidth to the center frequency is expressed as a proportion of the absolute bandwidth.

Cut-off frequency

High-pass and low-pass filters are commonly utilized. The cutoff frequency characterizes the highest frequency range that a low-pass filter can pass, while the cutoff frequency characterizes the lowest frequency range that a high-pass filter can pass.

Standing wave

That is, the S11 value determined by the vector network represents the degree of matching between the filter port impedance and the system's needed impedance. The percentage of the input signal that does not pass through the filter and is reflected back to the input.

Out-of-band suppression

Outside of the filter passband frequency range, there is "attenuation." Describe how well the filter can select undesired frequency signals.

Ripple

The difference between the S21 curve's peak and trough in the filter's passband.

loss

Indicates how much energy is lost when the signal passes through the filter, or how much energy the filter consumes.

Passband flatness

Within the filter's passband, the absolute value of the difference between the maximum and smallest losses. The filter is used to characterize the difference in energy consumption of different frequency signals.

Phase linearity

The phase difference between a transmission line with the same time delay as the center frequency and the phase in the filter's passband frequency range. Determine the filter's dispersion properties.

Absolute group delay

Within the passband of the filter, the time it takes for the signal to travel from the input port to the output port.

Power Capacity

The maximum power of the filter's passband signal can be input.

Phase consistency

The phase difference between different filters in the same index and batch when transmitting a signal. Determine the distinction (consistency) between batch filters.

 Amplitude consistency

The difference in signal transmission loss between different filters in the same index and batch. Determine the distinction (consistency) between batch filters.

Operating temperature

The filter design's operating temperature range.

Ⅳ. The application of RF Filter

The radio frequency filter is a unique technology that allows frequency band filtering to be accomplished. It can pass select frequency components in the signal while drastically attenuating or suppressing others, allowing for filtering, coexistence, duplexing, aggregation, and other radio frequency signal functions.

It can be classified into two broad categories from the standpoint of application: "civilian" and "military."

Filters are widely employed in mobile phones, tablet computers, smart homes, cars, and biological sectors for civilian usage, and the market is enormous;

In terms of military applications, filters are useful in areas such as GPS navigation and electronic countermeasures. It's fair to say that it's a crucial component of national information security.