Inconsistent Impedance Matching Network of a SAW Filter - Page 1
Forum section: electronics / rf-microwave

author: wonderfulnx
author_rank: Newbie
timestamp: September 12, 2023, 04:32:09 am
content: Hi everyone, I've been trying to measure and use some EPCOS SAW Filters recently and found something strange with their impedance matching network. This strange point exists in many of their models and here I'm using the model "B39321B3741H110" as the example. I've attached the datasheet and the Impedance matching application note they provided below.This SAW filter requires input and output matching at the same time. According to the datasheet, the impedance on both input and output ports (based on an ideal matching) is a 490 Ohm resistor in parallel with a 2.8pF capacitor (490 Ohm || 2.8 pF). In order to match a 50-ohm source/load, a simple LC matching is used as shown (center frequency 315Mhz).See attachment 1.ideal matching.pngThis matching network is consistent with the schematic given in application note 18 on page 11, where a 72nH inductor is used (not 74nH, probably considering the parasitic inductance of the PCB itself), as shown:See attachment 2. Matching in simulation.pngThe 0.2pF capacitance is said to be considered only in simulation so I guess no need for them in actual PCB. But here comes the strange thing, the datasheet of this component clearly shows a matching network as this:See attachment 3. Datasheet Given.pngwhere the 0.2pF capacitor (close to the SAW) is gone and the series inductance continues to reduce to 68nH. And there is another capacitor in parallel that is close to the 50-ohm source. So why is there a 1.8pF capacitor here? Its purpose is not to substitute the 0.2pF one since they aren't in the same location. And it does not help the impedance matching here I think...... So which one of the matching should I focus on if I want to use a source different from 50 Ohm?Also, How much is the typical value of parasitic inductance on a PCB, should the series inductance continue to reduce like this?BTW, the suggested layout for this matching network is this:See attachment Layout.png

author: RoV
author_rank: Regular Contributor
timestamp: September 13, 2023, 01:47:54 pm
content: A short piece of transmission line in series behaves approximately as an inductor, so the 74 vs 68 nH are easily explained. The small shunt capacitor like the 1.8 pF on the 50 ohm side may be necessary to improve the matching, again in the presence of other parasitics.I think you should rely on the impedance given by the manufacturer and compute your matching net from it. If you have a CAD like MWO you can model your parasitic elements (lines, component pads, etc) and get a better 1st trial result. If you really want a perfect match, you'll need final adjustments with a VNA.

author: rfclown
author_rank: Frequent Contributor
timestamp: September 14, 2023, 02:13:05 am
content: See if there are S parameters for the part. I use QUCS (a free S parameter simulator) which will let you simulate using a Touchstone file (the standard format used for S parameters). Going by the datasheet, you should be able to duplicate their response using the 1.8pF and 68nH shown. Then if you want to match directly to something other than 50 ohms, you start with the simulation (not guessing on a PCB).