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I have someone else's design that I have been asked to choose matching components for. shown here

The antenna is a chip antenna and the filter is a band pass SAW filter in a single package. The SAW filter is a DC short to ground, so at DC, all the components are tied to ground, but at it's operating frequency, it acts like a proper transmission line network.

My question is, since this is on a PCB and I've got an Agilent/HP 8753 VNA with S11 only, what is the proper way to measure and select matching components for this?

More specifically, I have read from some App notes from TI that the way to calibrate the 8753 is to do OPEN, SHORT, and LOAD measurements on the actual board itself...But, what do you remove from the board to test the load? Where should the VNA measurement point be? Do I have to do each filter separately? Can I leave the SAW filter in or should I replace it with a 0ohm resistor?

I've made some component changes (namely only using L1, L2 and C2) and have got a reasonable match by measuring at the L1,C2,BPFilter node and ground (~-27dB@fo), but my range of my antenna is still spotty. I'm not sure what to compare it to or if I'm doing this measurement right.

Thanks!

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  • \$\begingroup\$ What frequency (roughly) does this run at? \$\endgroup\$
    – The Photon
    Aug 5, 2015 at 14:44
  • \$\begingroup\$ 2.44GHz (bluetooth) \$\endgroup\$
    – testname123
    Aug 5, 2015 at 14:51

1 Answer 1

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If you have -27 dB S11, then the match is definitely not your range problem! However, SAW filters often have a good bit of loss, so if there is no amp between it and the antenna, I might be a bit suspicious of that.

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  • \$\begingroup\$ I agree. .. -27bB is pretty matched, but my question is whether I'm measuring correctly or not. Should I calibrate on the PCB and how would what/where should I replace things with opens, shorts and 50ohm SMT resistors? \$\endgroup\$
    – testname123
    Aug 6, 2015 at 13:05
  • \$\begingroup\$ understood - ideally you would physically solder the short on the board at the point of matching. But that would also require you to be able to disconnect it from all further circuitry so that you could measure that as an open, and solder on a 50 ohm resistor for the load. \$\endgroup\$
    – MikeP
    Aug 6, 2015 at 14:11
  • \$\begingroup\$ And what about the SAW filter? Currently, I'm doing all those things...shorting at the test point, removing the antenna and matching components (subsituting L1 with a 0ohm resistor and leaving the C1 & C2 open), but leaving the filter and the other pi filter components and the RF_IN on the microcontroller connected (since they're pretty difficult to get to). Should this be done this way? It doesn't feel right since it is an additional parallel circuit...So, Do I remove the SAW filter and match both ends separately? \$\endgroup\$
    – testname123
    Aug 10, 2015 at 17:40
  • \$\begingroup\$ yup, you need to be able to isolate that from your calibration load when you calibrate - sorry, i'm sure it's a pain! \$\endgroup\$
    – MikeP
    Aug 10, 2015 at 17:45

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