Selection Techniques of RF and Microwave Filters

Welcome to the IT technology community forum and interact with 2 million technical staff in today's wireless field. The fierce competition to expand bandwidth forces people to pay more attention to the performance of filters. If the filter parameters are inaccurate, the system will eventually cause frequency conflicts. In turn, the design group will have to deal with crosstalk, disconnection, data loss, and network connection interruption.

Welcome to the IT Community Forum and interact with 2 million technical staff> in today's wireless industry, fierce competition to expand bandwidth forces people to pay more attention to the performance of filters. If the filter parameters are inaccurate, the system will eventually cause frequency conflicts. In turn, the design group will have to deal with crosstalk, disconnection, data loss, and network connection interruption.

Welcome to the IT Community Forum and interact with 2 million technicians>

In today's wireless field, the fierce competition to expand bandwidth forces people to pay more attention to the performance of filters. If the filter parameters are inaccurate, the system will eventually cause frequency conflicts. In turn, the design group will have to deal with crosstalk, disconnection, data loss, and network connection interruption.

The incomplete or inaccurate definitions of filters are partly due to the current popularity of digital electronics in the electronic market. According to some statistics, 80% ~ 90% of new electronic design engineers are software and digital. The knowledge gap lies in this, because no matter whether the transmitted information is in digital form, when the information is transmitted through radio or microwave, the carrier signal always follows the laws of electromagnetism.

Fortunately, quick review of some important foundations of filter performance parameters can help engineers find filters that meet specific application requirements. If you choose the right option at the beginning, you can save time and money and make sure the price is good when you order these essential components.

1. Understand basic response curves

The basic response curves of filters include Band-pass, low-pass, high-pass, band-resistance, and dual-worker, as shown in 1A-1F. Each specific shape determines which frequencies can pass and which cannot pass.

Undoubtedly, the most common in this group is the band-pass filter. All engineers know that a band-pass filter allows signals between two specific frequencies to pass through and suppress signals at other frequencies. Such as SAW, crystal filter, ceramic and cavity filter. As a reference, Anatech Electronics manufactures a cavity band-pass filter with a frequency coverage of 15 MHz ~ 20 GHz, bandwidth in 1% ~ The value range is 100%. The following table lists all the technical parameters for the band-pass filter of the total component of Anatech Electronics. All manufacturers have adopted the method of defining the pass band by using the 0.5 dB, 1 dB, or 3 dB attenuation points on both sides of the filter center frequency.

2. including all necessary technical parameters

This often happens when engineers give a short requirement for a 100 MHz band-pass filter, which clearly contains too little information. It is difficult for filter suppliers to sign orders based on such information.

Give all necessary information starting from giving all the frequency parameters in detail, for example:

Center frequency (Fo): it is usually defined as the midpoint between two three dB points of a band-pass filter (or band-blocking filter). It is generally expressed by the arithmetic mean of two three dB points.

Cut-off frequency (Fc): the conversion point starting from the channel to the impedance of a low-pass filter or a high-pass filter. The conversion point is generally 3 dB points.

Suppression frequency: the frequency or frequency group at which the signal degrades a set of specific values or values. It is sometimes defined as the suppression frequency or frequency group, and the attenuation is called suppression.

The filter type determines the specific frequency. For Band-on and band-blocking filters, the specific frequency is the center frequency. For low-pass and high-pass filters, the specific frequency is the cutoff frequency.

For completeness, engineers should also define the following features, such:

Impedance band: The frequency band between the specific frequency values that the filter does not transmit.

Isolation: In the dual-worker, the ability to suppress the transmission (Tx) frequency when considering the receiving (Rx) channel, and the ability to suppress the receiving (Rx) frequency when considering the transmission (Tx) frequency, it is called Rx/Tx isolation. The higher the isolation, the more powerful the filter can isolate the Rx signal from the Tx signal, and vice versa. The result is that both the transmission and receiving signals are clean.

Insertion Loss (IL): A value of the power loss in the device. IL = 10Log (Pl/Pin) is irrelevant to the frequency. Pl indicates the load power, pin is the power input from the generator.

Return Loss (RL): a measure of the filter performance, indicating that the input and output impedance of the filter are close to the ideal impedance value. The return loss is defined as RL = 10Log (Pr/Pin), which is independent of the frequency. Pr is the power of the reflected return generator.

Group delay (GD): group delay indicates the linear phase of the device. Because the phase delay occurs at the output end of the filter, it is important to know whether the phase shifting changes with frequency linearly. If the phase shift changes along with the frequency nonlinear, the output waveform will be distorted. Group latency is defined as the derivative of the phase shift with frequency variation. Because the derivative of a linear function is a constant, the group delay caused by linear phase shifting is a constant.

Shape factor (SF): The shape factor of the filter is usually the ratio of the barrier bandwidth (BW) to the bandwidth of 3 dB. It is a measure of the steep degree of the filter edge. For example, if the bandwidth of 40 dB is 40 MHz and the bandwidth of 3 dB is 10 MHz, the shape factor is 40/10 = 4.

Impedance: Source impedance (input) and end impedance (output) of the filter in ohm ). Generally, the input and output impedance are the same.

Relative attenuation: the difference between the attenuation at the minimum attenuation point and the ideal attenuation point. In general, the relative attenuation is expressed in the unit of dBc.

Ripple (Ar): the flat size of the pass band of the filter, which is generally expressed in decibels. The ripple size of the filter affects the ripple loss. The larger the ripple, the more serious the ripple loss, and vice versa.

Suppression: Same as above.

Operating temperature: the operating temperature range of the filter design.

3. Do not pursue unrealistic filter features

Engineers sometimes make the following request: "I need a bandwidth of 1,490 ~ 1,510 MHz, and 70 dB at 1,511 MHz ." This requirement cannot be implemented. In fact, the suppression is gradually changing, not a sharp decrease of 90 °. The more practical parameter is the deviation center frequency of about 10%.

Another scenario is to require a filter, for example, to "suppress all components above the 1,960 MHz frequency ." At this time, engineers must realize that it is impossible to decay the suppression frequency until all the frequencies between the infinitely high frequencies. Some boundaries must be set. The more practical method is perhaps to reduce the specific suppression frequency near the band by two to three times.

4. Strive to achieve reasonable VSWR

The voltage standing wave ratio (VSWR) is used to indicate the efficiency of the filter. It is a ratio between 1 and infinity to indicate the size of the reflected energy. 1 indicates that no energy passes through. All values greater than 1 indicate that some of the energy is reflected, which is a waste.

However, in actual electronic circuits, a VSWR of is almost impossible. Generally, the ratio of is more practical. If the required value is smaller than this value, the cost-effectiveness ratio is reduced.

5. Considering power processing capability

The power processing capacity is the rated average power per watt. If this value is exceeded, the filter performance will be reduced or invalid. In addition, it should be noted that the size of the filter depends on its power processing capability to some extent. Generally, the larger the power, the larger the area of the circuit board occupied by the filter. Manufacturers, such as Anatech, have been committed to using new algorithms to meet these challenging interests, and pre-planning algorithms can save costs.

6. Isolation factors in simultaneous and bidirectional communication

Isolation is a particularly important aspect of the dual-worker. From the perspective of the receiving channel, isolation indicates the ability of the filter to suppress the transmission frequency, and vice versa. The larger the isolation, the more open the two, the cleaner the transmission signal and the receiving signal.

7. Make a trade-off

The higher the performance, the higher the cost. This is why accurate definition is needed, because accurate definition can reduce unnecessary extreme situations and avoid unnecessary expenses.

In addition, other factors also need to be weighed. For example, the closer the suppression frequency is to the center frequency, the more complicated the filter, which sometimes causes a larger insertion loss.

In addition, the higher the performance of the filter, the larger the area occupied by the filter. For example, the steep transition from the pass-through band to the suppression requires more cavities and segments to make the filter more complex. However, if the cost of the circuit board is very important, sometimes the performance must be reduced.

8. Find manufacturers that can balance requirements

Although the filter vendor has nothing to do with the inherent characteristics of the filter performance, when selecting the filter vendor, you still need to pay attention to this as required by the attention component itself. An excellent and stable manufacturer specializing in the production of filters can often generate specific components to compensate for product design defects.