When workers who are not familiar with RF technology try to add RF filters to the circuit, the first confusion is that they do not understand how other places will be affected, and it is urgent to understand RF knowledge. Failure to take into account the basic situation in terms of the type of filter and the minimum technical requirements can lead to a variety of problems with the test results, which not only requires redesign, wastes materials, and leads to delays in delivery.

      In the field of wireless communication, in order to expand bandwidth and increase traffic, designers pay more attention to the performance of filters. If the parameters of the filter are not accurate, it will eventually lead to frequency conflict and mutual influence. In this way, designers need to deal with group delay, phase consistency, out-of-band suppression, crosstalk and other problems, if not handled properly, it will eventually lead to data loss, offline, network interruption problems.

     When the information is transmitted wirelessly through the base station, the carrier signal must obey the laws of electromagnetism. The cognitive gap for technicians who only have knowledge of analog and digital circuits is that the definition of filters is incomplete or inaccurate. If you can quickly improve the basic knowledge of some important performance and parameters of the filter, you can help technicians find out how to meet the specific application, so that you can ensure that there are fewer detourts at the beginning and save time, the following points are for your reference.

一、Select realistic filter characteristics

       For example, the technician proposed that the bandwidth is 1575~2050MHz, and the suppression at 2051MHz is greater than 85dB; All components above 2060MHz are suppressed." While the inhibition is actually a gradual change, rather than a sharp drop in a straight 90 degree line, the more practical parameter is a deviation from the center frequency of about 10%. Therefore, this requirement is not achievable. In addition, the technician must be aware that it is not possible to attenuate this suppressed frequency until all frequencies in between are infinitely high. Certain boundaries must be set. A more realistic approach might be to attenuate specific rejection frequencies near the passband by a factor of two or three.

二、Understand the basic response curve

     The basic response curve of the filter includes: low-pass, high-pass, band-pass, band-stop, duplex, and each specific shape determines which frequencies can pass through. What doesn't pass. The most common is the bandpass filter, the bandpass filter is to allow the signal between two specific frequencies to pass, the signal of other frequencies is limited, such as the sound meter filter, ceramic filter, cavity filter, the frequency range of this kind of filter is basically 15MHz~20GHz, The attenuation points of 0.5dB, 1dB or 3dB on both sides of the center frequency of the filter are used to define the pass band.

三、Provide the necessary technical parameters whenever possible

     Often people only give a short requirement that requires "1950 to 2050 MHz bandwidth", obviously, this information is too little, and the filter supplier may find many different models of filters. So provide as many important parameters as possible,Such as：

     1、Center frequency (fo) : Usually defined as the midpoint between two 3dB points of a bandpass filter (or band-stop filter), generally expressed as the arithmetic average of two 3dB points.。

  2、Attenuations：The signal attenuates some specific value or set of values to a specific frequency or group of frequencies, defining the frequency region outside the ideal passband as the suppressed frequency or group of frequencies, so excessive attenuation is called suppression. The closer the rejection frequency is to the center frequency, the more complex the filter, sometimes resulting in greater insertion loss.

     3、Insertion loss (IL) : Indicates the value of the power loss in the device, IL= 10Log(PI/Pin) is frequency independent, where PI is the load power and Pin is the power input from the generator.

     4、Return loss (RL) : A measure of a filter's performance, indicating the degree to which the input and output impedances of the filter are close to ideal impedance values. The backfiltration loss is defined as RL= 10Log(Pr/Pin) independent of frequency, where Pr is the power of the reflected back generator.

     5、Group delay (GD) : The group delay represents the magnitude of the phase linearity of the device. Since the phase delay occurs at the output end of the filter, it is important to know whether this phase shift is linear with frequency; if the phase shift is nonlinear with frequency, the output waveform will be distorted. The group delay is defined as the derivative of the phase shift with frequency.

     6、Impedance: is the filter source impedance (input) and terminal impedance (output), in ohms. In general, the input impedance and the output impedance are the same.

     7、Ripple（Ar）：Represents the passband flatness of the filter. Generally expressed in decibels. The size of the filter ripple affects the return loss, the larger the ripple, the more serious the return loss, and vice versa.

     8、Operating temperature: refers to the filter design operating temperature range.

    Notice：The type of filter determines the specific frequency, for band-pass and band-stop filters, the specific frequency is the center frequency; For low-pass filters and high-pass filters, the specific frequency is the cutoff frequency. For completeness, the technician should also define, stopband: the band of frequencies between which the filter does not transmit a specific frequency value; Isolation: In a duplex/multiplexer, the ability to suppress the transmit (Tx) frequency is considered when receiving (Rx) channels, and the ability to suppress the receive (Rx) frequency when considering the transmit (Tx) frequency. This is called Rx/Tx isolation. The higher the degree of separation, the better the ability of the filter to separate the Rx signal from the Tx signal. And vice versa. The result is cleaner signals for both transmission and reception.

四、Strive to achieve reasonable V.S.W.R

　  The voltage standing wave ratio (V.S.W.R) is commonly used to express the efficiency of the filter, and its value is a ratio between 1 and infinity, which is used to express the size of the reflected energy. 1 indicates that all energy passes through without loss. Any value greater than 1 indicates that some energy is reflected, that is, wasted.

However， in actual electronic circuits, 1:1 VSWR is almost impossible to achieve. In general, a ratio of 1:5 is more practical. If the required value is less than that, the benefit-cost ratio will be reduced.

五、Power handling capacity

  Power handling capacity is the rated average power in watts, beyond which the filter performance will degrade or fail. It should also be noted that the size of the filter is determined to some extent by the size of its power processing capacity, and the larger the area of the circuit board occupied by the filter, the greater the power.

六、Isolation factor in two-way communication

  Isolation is an important requirement for diplexers, and when viewed from the receiving channel, isolation indicates the filter's ability to suppress the transmission frequency, and vice versa. The greater the isolation, the more separated the two, and the cleaner the transmitted signal and the received signal.

七、Aipple in BW

  Aipple in BW refers to the range of ups and downs of the signal in the passband of the filter, usually expressed in dB. It reflects the linear distortion of the amplifier, and generally requires in-band fluctuations within +/-0.5dB. When the tolerance range is exceeded, it is equivalent to strong interference, affecting the reception.。

八、Find a balance between demands

The components themselves require a certain relationship with an excellent manufacturer specializing in the production of filters, excellent manufacturers often produce specific parts to make up for product design defects, the higher the performance, the higher the cost, which needs to be accurately defined. Accurate definitions can reduce unwanted extremes and thus avoid unnecessary costs. In addition, the higher the filter performance usually makes its area larger. For example, a very steep transition from passband to rejection requires more cavities and segments, making the filter more complex.