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Design >> RF Design >> Limiter for driving mixer in CMOS
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Message started by Jan on Feb 20^th, 2007, 10:06am

Title: Limiter for driving mixer in CMOS
Post by Jan on Feb 20^th, 2007, 10:06am

Hi,
I am trying out limiter (1G-10G) which will drive max load of .5pf of mixer in CMOS design.
The idea of the limiter is to limit the o/p of VCO to a fixed value(.25V peak)so that mixer gain does not change..
Please let me know if there is any publication in this area.
Thanks in advance.

Title: Re: Limiter for driving mixer in CMOS
Post by ACWWong on Feb 23^rd, 2007, 2:32pm

Hi Jan,
I guess what you are describing is commonly termed a VCO or LO Buffer. If so the simplest design is differential pair (cascoded/cascaded or other variant) based with resitive load, such that the "limited" output swing is I*Rload related. Given your high frequency requirement and large capacitive load, the current needed to "limit" at 10GHz would be large.
If you wish truly to control the VCO amplitude, then you can use ALC (automatic level control) techniques which compare the VCO amplitude to a known (bandgap) voltage in a feedback system which controls the VCO current in order to keep the vco amplitude fixed. This would be suitable for driving your mixer VCO port directly if you had no requirement for isolation/ mixer on~off loading.
Anyway, I would check with the mixer design whether the mixer gain would actually benefit from limiting or whether the key parameter is slew (which is usually the case is switching LO type mixers), I think getting the same gain over your frequency range from you mixer would be tough even if you did have limited VCO signal...
Cheers
aw

Title: Re: Limiter for driving mixer in CMOS
Post by Jan on Feb 26^th, 2007, 2:26am

Hi aw,
Thanks for your comments/suggestions. You are right, its actually a LO Buffer. I am yet to check how the mixer gain varies with frequency. BTW, I can not put ALC because it degrades the phase noise of the oscillator. I am worried about current consumption for simple diff. pair with resistive load., so searching for different topologies.
Thanks for your feedback.
Jan