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Suppose I have a device which needs to have its impedance matched to some incoming generator at one specific frequency. This device is placed on a PCB and a trace is connecting it to a 50 Ohm SMA connector. Does it make a difference to put the matching network before the connector or after it (on the PCB)? enter image description here

My idea is that with the connector, I'm just adding a short piece of 50 Ohm transmission line. Depending on the wavelength, this might not be much of a discontinuity and therefore the position of the matching network would not make a difference?

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3 Answers 3

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It depends.

If the operating frequency is low enough, then the errors produced by moving the matching network from the PCB onto the cable may be small enough to not matter.

Or, if the signal frequency band is narrow enough (maybe less than 1/8 octave), you may be able to move the matching network onto the cable, but the design of the matching network will change. You might even find that by moving the matching network to the right location, you can simplify the design down to a single capacitor or inductor, or a simple tee-stub of transmission line. (Design of matching networks and how they change as the matching network is moved away from the load and toward the generator is typically taught when the Smith chart is introduced, if you want to look for more details online)

But if you're just using a matching network design recommended by the device manufacturer, then you should keep the matching network on the board as close to the device as possible, where they designed it to be placed.

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  • \$\begingroup\$ Thank you! Can you explain why the signal bandwidth matters in this case? I thought it is more about absolute frequency. Or do you mean it will be easier to match if the bandwidth is smaller? \$\endgroup\$
    – Bread123
    Commented May 17, 2023 at 6:45
  • \$\begingroup\$ @Bread123, yes it's easier to match if the bandwidth is smaller. Particularly if you're moving the matching network further from the load. \$\endgroup\$
    – The Photon
    Commented May 17, 2023 at 16:13
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The SMA connector is usually 50 \$\Omega\$ so your matching should be on the PCB between your device and the transmission line.

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if your source is 50Ω, your SMA is 50Ω, then your blue MN must be just a 50Ω transmission line (i.e., no matching required). So, the only thing where you could and must match is between device and SMA connector.

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