Noise of the front-end!

well, folks, i got a problem when i designed a RF front-end, i design the LNA and the Mixer seperately, the NF of the LNA is 3.6(output impedance is 300ohm differential, and the power gain is 16), and the Mixer is nearly about 10(input port is set to 300ohms differential), but then i combined them together, i found the cascade NF is not 4, the simulation result is nearly 8.8dB, if the NF of the LNA is not affected by the cascading (confirmed by the noise simulation in spectre), then it is undeniable that the mixer NF is up to 23dB!
why?

Do you have an image reject filter between the LNA output and mixer? Without it you will have 3 dB higher noise figure.

of course it is a DC receiver, i think!

thanks, wan, that is correct!

so, your question is answered? what is the real reason?

no, i donnot mean that, the problem is still there!