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I have an RF IC (ADF4113) with input impedance (@500MHz), given as (85-j263)ohms.

So this could be seen as a 85 ohms resistor in series with a -j263 (1.2pF) capacitor.

The datasheet says the Input sensitivity is minimum -15dBm or about 32uW in 50 ohms load.

Ok, now for the question: So I need to feed minimum 32uW into the input, but the input impedance is complex, so do I just calculate the RMS voltage needed on the input, with 85 ohms as reference? Or do I need to take account for the complex part (-j263)?

I know it is not possible to deliver power the the complex part (-j263), but not sure if it has some other effect.

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  • \$\begingroup\$ Shouldn't that be 85 Ohms in parallel with the capacitor, not in series with it? I don't do this often, so would have to look it up myself to be sure. \$\endgroup\$ Commented Jun 21, 2011 at 13:01

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What source impedance are you driving the IC with? The complex input impedance of the IC should probably be matched to the source impedance for maximum power transfer, with an appropriate matching network. Once you have a conjugate match and the input impedance of the IC appears the same as the real part of your source impedance, it will be easy to calculate what voltage drive you need for the appropriate power into the IC. The easiest way to construct the matching network is probably the Smith chart; take a look here for examples of how to do it.

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