Hi all,


I'm designing an active mixer, and i found the flicker noise corner frequency to be over 10 MHz. This seems to unusually high. Isn't the corner frequency supposed to be closer to 1 MHz? Any thoughts on why or how i can reduce it? Also is it possible that the thermal noise is simply lower than normalresulting in a high corner?


thanks,
Aaron

Hi Aaron,

You might initially want to examine the corner frequency of the switching transistors when biased at the balance point as well as for any transistors in your circuit. That can be easily done via AC analysis....

Depending on the frequency planning, i.e., Frf, FLO... the flicker noise of the transconductor stage (TS) might be significant. Using a large-area transconductor device helps but that trade-offs with the TS linearity at a constant bias current. You can, however, significantly reduce the flicker noise contribution of the switching transistors by:

1- Resonating the parasitic capacitance at the tail node (common-source node of the switching transistors) with a spiral at Flo.
2- reduce the overdrive voltage of the switching transistors by running a parallel DC current branch feeding the transconductor stage from the supply voltage. Consult Abidi's paper on noise analysis in active mixers.

-Ibrahim