Choosing the Right RF Filter for Industrial Applications

Why RF Filters Matter in Industrial Systems

Electromagnetic energy can come from sources like switching power supplies, motors, digital electronics, and wireless transmitters, among others in industrial settings. Electromagnetic sources can generate noise that causes interference with circuits or communication channels.

The use of the correct filter will enable the suppression of unwanted frequencies while ensuring the passage of the desired frequency. The RF filter should not only consider the nominal frequency but rather the electrical and mechanical characteristics of the filter.

• Operating and unwanted frequency bands
• Required attenuation or rejection
• Passband insertion loss
• Characteristic impedance
• Signal or power level
• Environmental conditions
• Installation of physical nature
• Electromagnetic compatibility (EMC) considerations

One key factor in the evaluation of a filter is insertion loss since unwanted frequencies can be rejected while introducing loss within the desired signal. RF passive components are measured by parameters including insertion loss and return loss.

Types of RF Filters

The first step in choosing an RF filter is understanding what the circuit needs to pass or reject.

The cutoff frequency separates the passband from the attenuation region, while the filter’s attenuation characteristics determine how strongly unwanted frequencies are suppressed.

Key Factors When Choosing an RF Filter

1. Identify the Required Frequency Range

Define the frequency which is needed to be transmitted/received by the system. Define the frequencies which need to be attenuated.

For instance, an RF receiver operating in a particular band of frequencies may need a band-pass filter to remove any out-of-band signals. A low-pass filter may be used in another system to reduce high-frequency noise.

Clear definition of the frequency helps avoid too much limitation of the filter.

2. Look for the Insertion Loss

The insertion loss shows the losses of signal when the filter is applied in the RF path. The lower insertion loss is preferable in the wanted frequency bands since the excess losses may decrease the available power of the signal.

However, the required value depends on the system itself, not on the filter parameters.

3. Pay Attention to the Impedance Matching

Most RF systems use standard values of the characteristic impedance which equals to 50 Ohms or 75 Ohms depending on the application. All components, cables and measuring instruments should be properly matched to minimize reflections.

The reflections and power losses may occur due to poor impedance matching; hence, the filter should have the appropriate impedance.

4. Determine the Required Attenuation

There is a need to reject interference sufficiently at these frequencies. Identification of these frequencies and amount of attenuation required is also essential. In case of EMC, it would be useful to conduct a measurement of the actual interference spectrum for selecting the right type of filter.

As per Rohde & Schwarz, measurement could assist in assessing the conducted emissions and improving the EMI filter performance.

5. Power and Environmental Considerations

The industrial filters may have to work in an environment with temperature variations, vibration, humidity, dust or high electrical loads. The component must be suitable to these conditions.

In high power cases, engineers must also take into account current, voltage, thermal and mechanical ratings apart from frequency.

RF Filter Selection Criteria

Prior to making a purchase or designing the RF filter, consider the following:

• The desired passband of the RF filter.
• The frequencies to be attenuated.
• The insertion loss.
• The required stopband rejection.
• The system impedance.
• Signal or power levels.
• The temperature and other environmental considerations.
• Connector and mounting considerations.
• The performance of the RF filter in terms of measurements.
• EMC or industry considerations.

Testing and Verification

Performance specifications for the filters should always be validated in practice in the actual system wherever possible. The filter which functions fine in the lab environment may behave differently when installed owing to parasitics such as cables, connections, PCB layout, grounding, etc.

For characterisation of RF passive components, S-parameter measurement is widely employed. For measuring various parameters such as S11 and S21, the vector network analyzer can be used.

In case of an EMC test, both conducted and radiated emissions tests will determine whether any interfering signals are suppressed. Pre-compliance testing during the design phase will identify issues before the compliance testing.

Pro Tip: Always consider the performance of RF filters not only within its operating frequency range but also insertion loss, attenuation, impedance, power-handling capability, and other characteristics at your frequency range of interest.

How Diamond Microwave Chambers Ltd Can Support RF Testing

Diamond Microwave Chambers Ltd is a company that is working in the RF & microwave testing industry using various technology like anechoic chambers, RF absorbers and antenna measurement systems used in defense, aerospace, space, satellite and semiconductor industries.

RF testing under controlled conditions can be of much use to engineers, especially when it is necessary to make repeatable measurements under minimum external interference of electromagnetic radiation and reflections. RF & EMC Chambers are designed to create controlled conditions for measurement.

Ideally, the RF filter will satisfy all needs of the system. Frequency response, attenuation, insertion loss, impedance, power handling, environmental and installation requirements should be taken into account at once. Thanks to proper RF measurement and choosing appropriate filters, industrial manufacturers will learn more about signal integrity and EMC.

Industrial companies developing and testing RF & microwave equipment can participate in a larger RF testing and measuring program, where Diamond Microwave Chambers Ltd can play its part.