Hi all,


I would like some opinions on which is better: voltage-mode or current-mode passive mixers. For voltage-mode passive mixers, the switching core is followed by a high impedance, while for a current-mode passive mixer, the switching core is followed by a low impedance. I will start with a few.

voltage mode
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1) Smaller devices means less driving strength needed by frequency synthesizers.
2) Higher gain before baseband amplification. Potentially less noisy?
3) Easily coupled with Gm-C type filtering for low noise, low power, and wide bandwidth.

current-mode
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1) Better linearity due to low voltage across the switches and the LNA.
2) Easier to get a well defined gain as the load impedance is easier to define at baseband.
3) Smaller resistance in series with the signal path. Potentially less noisy?
4) Easily coupled with op-amp based filtering for high linearity.

Overall, my opinion is that a current-mode passive mixer will get you a more robust system, however, I believe potentially the voltage-mode passive mixer has better NF. I have been assured by a very experienced designer that all real systems use current-mode passive mixing. What are your thoughts?


thanks,
Aaron

Hi RFICDUDE,


thanks for the reply. I have a feeling that you are mistaken in the way you model the noise of the current-mode mixer. I believe that the mixer stage is treated as a resistor in series with the signal path. Therefore, the smaller the resistance, the better. Correct me if I'm wrong here.

Actually, I've beed doing a little analysis and I'm starting to have doubts about the voltage-mode passive mixer. I will post it if I have the time.


regards,
Aaron