RF Bandpass Filter

Introduction

The rf bandpass filter refers to a filter that can pass through the frequency components in a certain frequency range, but attenuate the frequency components in other ranges to a very low level, which is opposite to the concept of band stop filter. An example of an analog band-pass filter is a resistance inductance capacitance circuit (RLC circuit). These filters can also be generated by combining a low pass filter with a high pass filter.

A band-pass filter is a circuit that allows signals to flow between two specific frequencies, although these signals are divided into other frequencies. There are different types of band-pass filters. Some band-pass filters can be designed by external power supply, integrated circuits, transistors and other active components. They are called active band-pass filters. Similarly, some filters use any type of power supply and passive components such as capacitors and inductors, which are called passive band-pass filters.

Principle

An ideal band-pass filter should have a completely flat passband. There is no amplification or attenuation in the passband, and all frequencies outside the passband are completely attenuated. In addition, conversion outside the passband is completed in a very small frequency range.

In fact, there is no ideal band-pass filter. The filter cannot completely attenuate all frequencies outside the expected frequency range, especially there is an attenuated but not isolated range outside the required passband.

Effect

Like notch filter, rf bandpass filter plays an important role in the field of digital power control. For example, in the aspect of resonance suppression control of three-phase LCL inverter, the frequency spectrum near the resonance point can be extracted by band-pass filter for further control strategy. In active power filter, band-pass filter can be used to extract the fundamental frequency of power grid signal for further control.

Type

Bandpass filter can be divided into two types: wideband filter and narrowband filter.

1. Broadband pass filter

Wideband pass filters (WBPF) can be formed by discarding low pass and high pass bands, which are usually different circuits for simple design and operation.

Wideband pass filter is recognized by many practical circuits. By using two parts, a band-pass filter with ± 20 dB/ten octave can be formed, such as a first-order low-pass filter and a high pass filter can be removed. Similarly, a ± 40 dB/decade band-pass filter can be formed by serializing two second-order filters, namely, low pass and high pass filters.

2. Narrowband pass filter

In general, narrowband pass filters (NBPF) use multiple feedbacks. This band-pass filter uses an operational amplifier. The main features of this filter are as follows:

●Another name for this filter is multi feedback filter, because it includes two feedback channels.

●The operational amplifier is used in inverting mode.

Application

The application of rf bandpass filter includes the following aspects:

1. Widely used in wireless transmitters and receivers.

2. It can be used to optimize the S/N ratio (signal-to-noise ratio).

3. The main purpose of the filter in the transmitter is to limit the bandwidth of the output signal to the selected communication frequency band.

4. It is widely used in optical fields, such as lidar, laser, etc.

5. The best application is audio signal processing, where a specific range of sound frequencies is required, just delete the rest.

6. Suitable for sonar, instrument, medical and seismological applications.

7. A communication system for selecting a specific signal from a variety of signals.

Test equipment

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