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Pasternack BlogRF Frequency filters, or just filters, are components/devices that modify a signal’s amplitude and phase angle based on frequency. Filters are either passive or use active tuning electronics. Filters are designed with a predetermined frequency dependent transmission response and are predominately used to attenuate portions of the spectrum where undesirable signal components, interference, or noise may be present. Passive filters are composed of passive structures and components, such as resistors, capacitors, inductors, resonators, attenuators, etc. Active filters may include tuning circuitry, amplifiers, and other active/passive circuits. There are also RF digital filters, which implement signal conditioning functions to digitized signals using digital hardware, such as application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or digital signal processors (DSPs).
For non-digital filters, which have other parameters/considerations, RF filters are typically specified using the following parameters:
• Insertion Loss
• Attenuation/Rejection
• Percent Bandwidth
• Passband Frequency (Bandwidth)
• Stopband Frequency
• Passband and Stopband Ripple
• Group Delay
• Peak Power and/or Continuous/Average Power
Typical RF Filter Types
Bandstop (notch): attenuates frequencies in a specified band
Bandpass: attenuates frequencies outside of a specified band
Lowpass: attenuates frequencies above a specified band
Highpass: attenuates frequencies below a specified band
Testing RF Filters
RF filters may be tested in a variety of ways, using LCR bridges, digital multimeter, spectrum analyzers/tracking generators, or with vector network analyzers (VNAs). For filters made of discrete components, an LCR bridge may be used to ensure that the discrete components (inductors, capacitors, and resistors) used match the designed values of the filter as closely as possible. This may be a critical step, as any deviation from the designed values of inductance, capacitance, or resistance of a filter may significantly impact the frequency performance of the filter. In this way, a digital multimeter may also be used to test the resistance and capacitance of certain discrete filter components.
If a tracking generator feeds a signal to a filter under test and a spectrum analyzer is connected on the output of the filter, then a filter’s frequency response can be measured relatively accurately if the tracking generator, spectrum analyzer, and interconnect cabling is properly calibrated. The transmission response of a filter over frequency can reveal the insertion loss, attenuation/rejection, percent bandwidth, passband frequency, and passband/stopband ripple. However, this setup isn’t capable of measuring the phase or reflection portion of the filter’s response, only the transmission.
For this, a VNA can be used to do a full 2, or more, port vector network analysis. A VNA can yield a much more accurate filter response measurement, but only for small signal levels. In this way, group delay and other phase related response aspects of a filter’s behavior can be measured.
Peak Power and continuous/average power handling measurements are more likely to be performed with a signal generator, amplifier, directional coupler, power meter, and termination. There are VNAs with much higher dynamic range and power handling that may be used to measure some filters at their peak power and/or continuous/average power levels.
In some cases, usually with lower frequency filters, an oscilloscope and signal generator may be used to measure the time domain performance of a filter. From this, frequency domain information can be calculated using a Fourier Transform.
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