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How to tune RF filters

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May 13, 2015
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How do you tune an RF filter circuit. what is the test setup like and what are the basic procedures

How do you TUNE RF circuit that are Lumped Element filters, filter bands, waveguide filters, Millimeter Wave Filters in the Low Pass, Band Pass, Band Stop, High Pass that are in the ranges from low MHz to the High GHz frequency ranges?

How do you measure Filters Parameters like S-Parameters, Phase Linearity, Filters Ripple, Pulse Ringing?

Filter tuning is an ART and you must learn it.
Any filter should be pretuned before final tuning. An operator should use a S21 test setup, best calibrated, with a swept-frequency input, and a detector feeding a display to see filter frequency response.
A typical procedure is to tune the resonators and couplings so that you can see some frequency response. Then try to adjust gradually all resonators one after another to get the wanted filter response. Tune couplings also one after another until you get the response into the template. Retune all tuners as needed.

If your filter must be tuned to S11, S21, S22 ad S12 specifications, get a Vector Network Analyzer and test your filter after tuning it for the best S21 response. The VNA works slower, so tuning the filter during VNA test is not productive.
For Phase linearity use the VNA, for Ripple use the S21 setup. Ringing needs another setup with matched source and load.
Take some experiments, there is no way else to learn but by trying it.
As was mentioned above, tuning an RF filter could be harder even than designing the filter itself.
The same as exist polynomials for filter design, there are math algorithms for filter tuning. Most of the algorithms have as a target, tuning the filter with minimum number of iterations.

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If your filter must be tuned to S11, S21, S22 ad S12 specifications

What is a test setup like for S11,S21,S22,S12 and the test equipment used for S11,S21,S22,S12?

What does S11,S21,S22,S12 mean?

What parameters and measurements can I do when measuring a filter?

Each stage s-parm's as a function of frequency, time, temperature and voltage , then verified individually with test coupons ( small PCB's) with controlled impedances and pre-determined matching and taps to 50Ohm output SMA's for example to a Spectrum Analyzer with synchronous RF sweep generator or better a VNA. THe coupon test fixture must be calibrated adequate S parms when the circuit is bypassed and compared with circuit installed. Then interactive effects from mismatched stages must be tested.

With a good VNA and test fixture, one can calibrated a fixture to >40dB RL and 0.1dB insertion loss up to the desired GHz range. THe Anritsu VNA I Worked with using a customer made leadless chip carrier fixture for a customer chip had a flat response and accuracy of 0.1dB and a resolution of 0.01 dB up to 6GHz where Tx loss must be low and every 0.1dB for a Sat Phone means a compromise of range or harsh weather.

I setup automated tests for 3IP, s-parms, filter ripple, BW, skirt rejection, Group Delay ripple, etc etc. for the Iridium Satphones for Motorola & C-MAC

Dishal's method works well for bandpass. In filters with tuning screws, you can short out resonators by turning the screws all the way in, and then backing them out one at a time while watching S11.

in printed filters, which are a pain to tune if they have a poor passband due to the solder pads falling off after a few component replacement can solder a tiny jumper wire from the resonators to ground, and cut the wires, one at a time, while tuning the chip capacitors while watching S11

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