Step-by-Step: Measuring Filter Parameters With A Network Analyzer

Welcome to our series of Step-by-Step posts - a no-frills series that gives you a general, illustrated step-by-step guide to get your job done. This, our first post in the series, guides you on how to make filter measurements using a network analyzer.

A filter is one of the essential components in RF/microwave circuits and systems. In a nutshell it is used to reject unwanted frequencies and allow desired frequencies to pass through. An ideal frequency response of a filter is like a brick wall whereby it either accepts or rejects certain frequencies. However, in the real world, the unwanted frequencies are not completely cut off and the desired frequencies may not be completely passed through. To quantify these phenomena, the most commonly measured filter characteristics are bandwidth and insertion loss.

Here is how you do it:

  1. Set the appropriate frequency range on the Network Analyzer. This range should be wider than the expected filter bandwidth.
  2. Perform a full 2-port calibration on the Network Analyzer together with all the cables and adapters that are required to connect the filter. The filter must be disconnected during calibration.
  3. Connect the filter to both ports of the Network Analyzer.
  4. Select either S21 or S12 measurement modes.
  5. Select the Log Magnitude format.
  6. Trigger/sweep the measurement.
  7. Execute autoscale.
  8. Execute the automatic bandwidth search capabilities of the Network Analyzer if available. You should get all the necessary results such as bandwidth, insertion loss, Q-factor and center frequency listed out automatically. Otherwise, please proceed with the following steps.
  9. Search for the maximum magnitude using marker peak search function (Marker 1).
  10. Turn on delta for Marker 1 and move it to the left to a level that is 3 dB (or 6 dB depending on the way the filter bandwidth is specified) below Marker 1. The delta marker reading should give give as close to -3dB as possible. Convert this delta marker position to become Marker 2.
  11. Turn on delta for Marker 2 and move it to the right, passing the peak, until you reach a position that indicates as close to 0 dB as possible. This means the delta marker should be on the same magnitude level as Marker 2. The frequency readout for this delta marker is the bandwidth of the filter.
  12. Convert the delta Marker 2 position into Marker 3 and calculate the midpoint frequency between Marker 2 and Marker 3. Place Marker 4 on that position. That midpoint frequency is the center frequency of the filter.
  13. Read the magnitude (dB) on Marker 4. That is the insertion loss of the filter at the center frequency.
  14. Obtain the Q-factor of the filter by using the following ratio: center frequency (Marker 4) / bandwidth.

If you are looking for the right equipment needed to make filter measurements, check out the range of Network Analyzers that MAT offers that can help you get the job done.