How to Test RF Filter Quality

How to Test RF Filter Quality

In the realm of Radio frequency (RF) filters, there are generally four filter functions that are desirable: band-pass filter, band-stop filter, low-pass filter, and high-pass filters.

Band-pass filter is one that will transmit a selected group or banding of frequencies with no great loss, but will attenuate all frequencies either higher or lower than the desired band. Low-pass filtering, allows only the frequencies below a certain cut off point to pass through, with little to no loss. High-pass filtering, allows only the frequencies above a certain cut off point to be transmitted with little to no loss – but there is great attenuation. And last but not least, the band-stop filter gets rid of any frequencies that you do not want.

Often times what is called the ‘Q’ or quality factor concept can help to tell you a lot about performance in Radio Frequency or other circuit. Values for the quality factor are often times used to define the performance.

One aspect of quality factor is that it decides the qualitative behavior of simple damped oscillators. With this comes three things to consider:

  • Low Quality Factor (Q<1/2) – A system with a low quality factor, that which is below 1/2, is said to be overdamped. This kind of system doesn’t oscillate and the system will exponentially decay. As the quality factor decreases, the system slows.
  • High Quality Factor (Q>1/2) – A system with a high quality factor, that which is above 1/2, is said to be underdamped. This type of system combines oscillation at a certain frequency with the decay of the amplitude of the signal. Underdamped systems may oscillate a few times before dying out. As the quality factor increases, the amount of dampening that occurs goes down.
  • Intermediate Quality Factor (Q=1/2) – A system with intermediate quality factor equal to 1/2 is said to be critically damped. This type of system does not oscillate, or overshoot a steady-state output. This system responds very quickly to a unit step input. These specifications allow for some overshoot for faster initial response or require a slower initial response for safety against overshoot.

When dealing with RF, there are other reasons why quality factor is important.

  1. Bandwidth – It is reduced with increasing q factor, as losses decrease the energy that is being is stored better in the circuit and the tuned circuit ends up being sharp.
  2. Oscillator Phase Noise – The q is able to minimize the phase noise, just by being increased.

With RF filters, testing for quality is important for optimum functionality. Q Factor is a good tool to use with this as helps with you with different facets of RF and other circuit performance.

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