silentgun wrote on Apr 8th, 2008, 1:29am:hi, all
for most receiver, there need IQ down mixers. for active mixer design, there's two way to
achiev the IQ down conversion
one way is designing a seperated mixer in I-path, and just putting a copy of mixer in Q-path ;
another way is designing an IQ combined mixer which shared one transconductance stage.
it seems IQ combined mixer has lot of advantages over seperated mixer, like less power, better NF and
better linearity.
but it seems to me that seperated IQ mixer is more popular in curcuit design.
does IQ combined mixer have any disadvantage which make it unpopular?
thanks
[Merit]
I-Path Gain and Q-Path Gain are well-matched.
If it is used as image rejection mixer, NF is better than separate transconductance amplifier.
Local leak to RF will be 4 times frequency of Local.
[Reference]
A merged CMOS LNA and mixer for a WCDMA receiver
Sjoland, H.; Karimi-Sanjaani, A.; Abidi, A.A.;
Solid-State Circuits, IEEE Journal of
Volume 38, Issue 6, June 2003 Page(s):1045 - 1050
[Demerit]
If Local signal doen't have enough amplitude and then gilbert switchs are operated in finite transition time,
I-Local and Q-Local have some interference via common transconductance amplifier.
It causes degradation in image rejection performance.
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IQ dwon mixer design (Read 4713 times)