Hi,
Because you are using the port in an unmatched system with Rs << Rl, you will get about double the voltage as a result. This is beacuse a port acts a source with an internal source resistance and what you specifiy is the power delivered to the load, when the load = Rs (ie matched).
This is the same way RF signal generators work, if you measure the power of a 50ohm Rs signal generator into a 50ohm Rin spectrum analyser, it will give the correct amaplitude/power level, if you alternatively measure it into a high impedence oscilloscope, you will see twice the voltage.

You can still use the port for characterising you mixer if you take the mismatch into account.

cheers

aw

The port can be trusted in the mismatch case, you just have to be careful what you are trying to measure. If you are using spectreRF, your small signal analyses (pac, pxf etc.) may incur the 6dB discrepency due to the mismatch voltage gain. You can always do transient analysis to compare.

Usually I work completely in the voltage domain, and specify my LNA & mixers in terms of voltage gains (& input referred noise, V1dB etc.), this is because I generally work on integrated RFIC, where signals are volatges or currents and not power. In your case I think this would make things clearer for you.

But yes you are correct, you can always do the LNA mixer design together to clarify the matter (assuming you have a conjugated matched input LNA, else you will have to again accommodate mismatch gain/loss).

anyway good luck with your design...

If a system is mismatched like the mine, how to evaluate the POWER in dBm

in 50 Ohm or others(1K Ohm for example)?

It is quite troublesome, maybe input power is mesured at 1000 Ohm, and output is at 50 Ohm.

Thanks for discussion~!