Performance indicators of RF filters

RF filter is one of the essential key components in RF systems, applied in the RF, IF, and baseband parts of receivers. This article mainly introduces its performance indicators.

The electrical performance technical indicators of RF filters include:

1) Order (series): For high pass and low-pass filters, order is the sum of the number of capacitors and inductors in the filter. For bandpass filters, the order is the total number of parallel resonators; For a band stop filter, the order is the total number of series resonators and parallel resonators.

2) Absolute bandwidth/relative bandwidth: This indicator is usually used for bandpass filters to characterize the frequency range of signals that can pass through the filter and reflect the frequency selection of the filter. Relative bandwidth is the percentage of absolute bandwidth to the center frequency.

3) Cut off frequency: Cut off frequency is usually used for high pass and low-pass filters. The cutoff of a low-pass filter represents the highest frequency range that the filter can pass through; For high pass filters, the cutoff frequency represents the lowest frequency range that the filter can pass through.

4) Standing wave: S11 measured by the vector network, representing the degree of matching between the filter port impedance and the required impedance of the system. Indicates how much of the input signal fails to enter the filter and is reflected back to the input end.

5) Loss: Loss refers to the energy lost by a signal after passing through a filter, which is the energy consumed by the filter.

6) Passband flatness: the absolute value of the difference between the maximum and minimum losses within the passband range of a filter. Characterize the difference in energy consumption of filters for signals of different frequencies.

7) Out of band suppression: The amount of attenuation outside the passband frequency range of a filter. Characterize the filter's ability to select unwanted frequency signals.

8) Ripple: The difference between the peaks and valleys of the S21 curve within the passband of a filter.

9) Phase linearity: The phase difference between a filter's passband frequency range and a transmission line with a delay equal to the center frequency. Characterize the dispersion characteristics of the filter.

10) Absolute group delay: The time it takes for a signal within the passband of a filter to be transmitted from the input port to the output port.

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11) Group delay fluctuation: the difference between the maximum and minimum absolute group delay values within the passband of the filter. Characterize the dispersion characteristics of the filter.

12) Power capacity: The maximum power that can be input into the passband signal of the filter.

13) Phase consistency: The difference in phase of the transmission signal between different filters of the same batch with the same indicator. Characterize the differences (consistency) between batch filters.

14) Amplitude consistency: The difference in transmission signal loss between different filters of the same batch for the same indicator. Characterize the differences (consistency) between batch filters.