How to choose the right RF filter?

If the basic elements of the filter type and minimum technical specifications are not taken into account, the product may not pass the "test", and as a result the product has to be restarted, resulting in costly production delays. On the other hand, knowing how to accurately determine the filter parameters will help to produce the products that meet the customer's production standards and functions. In fact, this knowledge helps to control production costs while improving the chances of success in the market.

Starting from the foundation

In today's wireless world, the fierce expansion of bandwidth competition forces people to pay more attention to the performance of the filter. If the filter parameters are determined to be inaccurate, it will eventually lead to frequency conflicts, which in turn will cause the design team to deal with crosstalk, dropped calls, data loss, and network connectivity interruptions.

Part of the reason for the incomplete or inaccurate filter definition is that the electronics market is currently keen on digital electronics. According to some statistics, 80% to 90% of new electronic design engineers are in software and digital. The knowledge gap is here because, regardless of whether the transmitted information is in digital form, the carrier signal always obeys the laws of electromagnetic physics when the information is transmitted by radio or microwave.

Fortunately, quickly revisiting some important foundations of filter performance parameters can help engineers correctly identify filters that meet specific applications. If you choose the right one at the beginning, you can save time and money, and you can be sure of the price when you order these essential components.

1. Understand the basic response curve

The basic response curves of the filter include: bandpass, lowpass, highpass, bandstop, duplexer, as shown in Figures 1A-1F. Each specific shape determines which frequencies pass and which cannot.

Undoubtedly, the most common of these groups is the bandpass filter. All engineers know that a bandpass filter allows signals between two specific frequencies to pass, suppressing signals at other frequencies. For example, surface acoustic wave filters (SAW), crystal filters, ceramics, and cavity filters. For reference, the cavity bandpass filter manufactured by Anatech Electronics has a frequency range of 15 MHz to 20 GHz and a bandwidth in the range of 1% to 100%. The table below gives all the technical parameters of the lumped component bandpass filter from Anatech Electronics. All manufacturers have adopted a method of defining the passband with 0.5 dB, 1 dB or 3 dB attenuation points on either side of the filter center frequency.

2, including all necessary technical parameters

This often happens, and engineers give a short request for a "100 MHz bandpass filter," which is clearly too small. It is difficult for filter suppliers to sign orders based on such information.

Give all the necessary information starting with giving all the frequency parameters in detail, such as:

Center Frequency (Fo): The midpoint between two 3 dB points, usually defined as a bandpass filter (or bandstop filter), typically expressed as the arithmetic mean of two 3 dB points.

Cutoff Frequency (Fc): The transition point from the passband to the stopband of the lowpass filter or highpass filter, which is typically 3 dB points.

Suppression frequency: A particular frequency or group of frequencies at which a signal attenuates certain specific values or sets of values. Sometimes the frequency region outside the ideal passband is defined as the suppression frequency or frequency group, and the attenuation that passes through is called suppression.

The filter type determines the specific frequency. For bandpass and bandstop filters, the specific frequency is the center frequency. For low pass and high pass filters, the specific frequency is the cutoff frequency.

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

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

Isolation: In the duplexer, the ability to suppress the transmission (Tx) frequency when considering the reception (Rx) channel, and the ability to suppress the reception (Rx) frequency when considering the transmission (Tx) frequency, is called Rx/Tx isolation. The higher the isolation, the stronger the filter's ability to isolate the Rx signal from the Tx signal, and vice versa. The result is a cleaner transmission and reception signal.

Insertion Loss (IL): A value that represents the power loss in the device, IL = 10Log (Pl/Pin), independent of frequency, where Pl is the load power and Pin is the power input from the generator.

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

Group Delay (GD): The group delay represents the magnitude of the phase linearity of the device. Since the phase delay occurs at the output of the filter, it is important to know if this phase shift is linear with changes in frequency. If the phase shift varies nonlinearly with frequency, the output waveform will be distorted. Group delay is defined as the derivative of the phase shift as a function of frequency. Since the derivative of the linear function is constant, the group delay caused by the linear phase shift is constant.

Shape Factor (SF): The shape factor of a filter is typically the ratio of the stopband bandwidth (BW) to the 3 dB bandwidth. It is a measure of the steepness of the edge of the filter. For example, if the 40 dB bandwidth is 40 MHz and the 3 dB bandwidth is 10 MHz, the form factor is 40/10=4.

Impedance: Filter source impedance (input) and termination impedance (output) in ohms. In general, the input impedance and output impedance are the same.

Relative attenuation: The difference between the measured attenuation at the minimum attenuation point and the attenuation of the ideal suppression point. Typically, relative attenuation is expressed in dBc.

Ripple (Ar): Indicates the flatness of the filter passband, usually expressed in decibels. The size of the filter ripple affects the return loss. The larger the ripple, the more severe the return loss and vice versa.

Suppression: Ibid.

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

3. Don't pursue unrealistic filter characteristics

Engineers sometimes ask for the following: "I need a passband of 1,490 to 1,510 MHz, and a suppression of 70 dB at 1,511 MHz." This requirement cannot be achieved. In fact, the inhibition is gradual, not a sharp drop of 90°, and the more practical parameter is about 10% off center frequency.

Another situation is that the filter is required to, for example, "suppress all components above the frequency of 1,960 MHz." At this point, the engineer must be aware that it is not possible to attenuate the rejection frequency until all frequencies between infinitely high frequencies. Some boundaries must be set. A more realistic approach might be to attenuate the specific rejection frequency near the passband by two to three times.

4, strive to achieve a reasonable VSWR

The voltage standing wave ratio (VSWR) is often used to indicate the efficiency of the filter. It is a ratio between 1 and infinity and is used to indicate the amount of reflected energy. 1 means that all energy passes without loss. All values greater than 1 indicate that some of the energy is reflected, which is wasted.

However, in practical electronic circuits, a 1:1 VSWR is almost impossible to achieve. Usually, the ratio 1:5 is more practical. If the required value is less than this value, the benefit-to-cost ratio is reduced.

5, consider power handling capabilities

The power handling capability is the rated average power in watts above which the filter performance may be degraded or disabled. It should also be noted that the size of the filter is somewhat determined by its power handling capabilities. In general, the greater the power, the larger the board area occupied by the filter. Manufacturers, such as Anatech, have been working on new algorithms to meet these challenging benefits, and planning ahead with algorithms can save costs.

6. Separation factors in simultaneous and two-way communication

Isolation is a particularly important aspect of duplexers. When viewed from the receive channel, isolation represents the ability of the filter to reject the transmission frequency, and vice versa. The greater the isolation, the more open the two are, the cleaner the transmitted and received signals are.

7, pay attention to make a choice

The higher the performance, the higher the cost. This is why it needs to be accurately defined, because accurate definitions can reduce unnecessary extremes and thus avoid unnecessary expenses.

In addition, other factors need to be weighed against each other. For example, the closer the suppression frequency is to the center frequency, the more complex the filter, which sometimes results in greater insertion loss.

In addition, the higher the performance of the filter, the larger the board area. For example, a very steep transition from passband to rejection requires more cavity and number of segments, making the filter more complex. But if board cost is important, performance must sometimes be cut.

8. Find a manufacturer that can balance the various requirements

While filter vendors are independent of the inherent characteristics of filter performance, when choosing a filter vendor, you need to pay attention to this as much as you pay attention to the component itself. An excellent and stable manufacturer specializing in the production of filters, which can often produce specific components to compensate for product design flaws.