Understanding the difference between modeled performance and test data is often the most challenging part of an RF filter or microwave filter build. This difference can be broken down into two parts – modeling inaccuracies and manufacturing tolerances. The possible sources of modeling inaccuracies are vast and won’t be addressed here. The various impacts of manufacturing tolerance, how to predict them, and tune around them, are issues we can work to correct. Two existing filter designs will be used as examples – A 3-port LC
, with passbands from 100-500MHz and 700MHz-6GHz, and a
distributed element broadband bandpass filter, with passband from 2.5-6GHz.
The capacitors used in the LC diplexer design are +/-5% tolerance over nominal. Each capacitor forms a pole in the pass band, so the variance in frequency of each of the poles will affect the electrical response of the filter. The response of the diplexer with nominal capacitance values is shown in Figure 1. A yield analysis of varying capacitor values over the +/- 5% tolerance is shown in Figure 2. It shows a yield of 65%, meaning 35% of manufactured parts would need to be tuned to meet the electrical spec.