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I have a radio signal source (USRP N210 with a WBX daughterboard) that can supply a maximum power of 100mW. I am designing a power amplifier for it and I would like to simulate the radio signal source in Spice. What is the equivalent circuit ( maybe the Thevenin equivalent) of the radio signal source (USRP N210) so that I can simulate it in Spice?

I know that for maximum power transfer, the internal resistance of the signal source must be 50 ohms (typical resistance of an antenna). Does that mean to supply a 100mW sine wave, the signal source behaves like an ac voltage source with a peak voltage of 6.3V with a series resistance of 50 ohms so that the power across a 50ohm load would be 100mW?

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The USRPs are not calibrated power/signal sources, but software defined radio devices. That 100 mW is not your fixed full-scale output power, but it's the maximum observable power of that device, over its whole operational range, plat highest gain, for one specific frequency where power output is strongest. On other frequencies, you get less power.

Your question reflects a lack of understanding of impedances in Transmission line theory: while 50 Ω indeed is a ratio of volt and ampere it describes the relationship between E- and H-field, not just between potential difference and current. This lack of understanding doesn't bode well for your amplifier design: you're lacking the basics of microwave engineering! Maybe start with a commercially available amplifier with SMA connectors. What you plan to do has little to do with things you can simulate in SPICE (though that should also be part of your system design, but not the RF part).

Also, you're asking for a model that's far too complex to ever be built: this is a piece of hardware that allows you to generate any sufficiently bandlimited signal.

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  • \$\begingroup\$ Can you point out what exactly is wrong with the question? I plan to build a Class C power amplifier that works in the commercial FM band (88-108 MHz). I believed that I could design it if I know the voltage variations of the source as that decides the bias and the working region of the transistor. \$\endgroup\$ – Kevin Selva Prasanna Feb 26 '18 at 10:28
  • \$\begingroup\$ I am also not asking for a model that will generate any signal, but a model that amplifies signal with a bandwidth of a few 100 KHz in the commercial FM band. \$\endgroup\$ – Kevin Selva Prasanna Feb 26 '18 at 10:39
  • \$\begingroup\$ microwave devices aren't properly represented by simple voltage variations. As said, transmission line theory is a bit different, because you don't deal with currents flowing in and out of components, but with the transport of energy in coupled electrical and magnetic fields. \$\endgroup\$ – Marcus Müller Feb 26 '18 at 12:00

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