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I'm trying to understand the recommended impedance match for a TA0168A SAW filter input. The datasheet is https://www.rfmw.com/datasheets/taisaw/ta0168a%20_rev.2.0_.pdf In my first circuit I'm trying to match to a 50 Ohm antenna and 50 Ohm SDR input.

The datasheet states that the SAW input is equivalent to 390 Ohm // 300pF:

electrical specs equiv circuit

The recommended matching to 50 Ohms is: enter image description here

I don't really understand what C1+L1 accomplish.

IIUC 390Ohm // 300pF is about Z = 0.027 - j3.25 Ohm

If I enter the circuit into SimNEC I get an input impedance of 0.094 -j85 Ohm: enter image description here

I understand that this form of matching circuit is likely recommended for its DC blocking and ESD protection characteristics. Experimentally, in my PCB using a VNA I get the best S21 using C1=8.2pF and L1=68nH and the resulting S21 is -7.1dB, which seems OK given SMA connector losses, etc. But I'd like to understand a little better why this matching is appropriate and especially how to derive it from the specs.

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    \$\begingroup\$ I'll +1 the question. I'd like to see an answer. I'm ignorant about the SAW filter component, designing them, or designing around them. This is more my speed. ;) \$\endgroup\$
    – periblepsis
    Commented May 1 at 0:52
  • \$\begingroup\$ 300 picofarads?,,,really? slipped a decimal place perhaps? Or they mean 300 ohms reactance? \$\endgroup\$
    – glen_geek
    Commented May 1 at 3:56
  • \$\begingroup\$ It appears twice in the datasheet (the images I posted are from the datasheet). \$\endgroup\$
    – TvE
    Commented May 1 at 4:06

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The datasheet must be wrong. 390 Ohm || 300pF is the same as 300pF in series with 0.03 Ohm @ 163MHz. There is no practical way to match that to 50 Ohm.

If you assume the LC-matching networks given in the datasheet are correct, the input has approximately 183 Ohm || 11.6pF and the output 348 Ohm || 13.5pF.

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  • \$\begingroup\$ Assuming that the datasheet has a little error does make the calculations fall in place. With my actual matching of C1=8.2pF and L1=68nH I got an input impedance of 55 -j19 at 166Mhz. That suggests about 184 Ohm || 5.8pF for the filter plus my PCB, in which case C1=12pF and L1=68nH should give me a bit better match. Have to try (although in practice I don't think it will improve things that much). \$\endgroup\$
    – TvE
    Commented May 1 at 4:56
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    \$\begingroup\$ Abracon, Golledge and RFM sell the same SAW filter. RFM is the only company that omitted the weird impedance specs in their datasheet but kept the info for the matching networks. Try measuring the filter in a 50 Ohm system, save the data as a S2P file, load it into a RF simulator and vary source and load impedance until you get the correct filter response and return loss. \$\endgroup\$
    – Raonoke
    Commented May 1 at 13:13

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