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siva Post subject: down mixer Posted: Thu Oct 19, 2006 2:54 am
Lieutenant Joined: Thu Oct 19, 2006 2:42 am Posts:
2 Location: india hi iam a novice to this RF design field.
I am doing a down mixer( active, folded mixer). I am facing a huge
difficulty in meeting gan and noise figure specs. it is a low IF
receiver structure. 1)How can I get a good noise figure with
out losing the gain?? 2) How to measure the output DC offset
for the mixer?? Top fred47 Post subject: Active
mixer noise figurePosted: Thu Oct 19, 2006 3:47 pm General
Joined: Wed Feb 22, 2006 3:51 pm Posts: 104 Hello!
One problem that mixer designers often encounter with noise figure
is that all mixers (except specially-designed "image reject mixers")
quite happily down-convert noise from the image frequency as well as
the desired frequency. In a low-IF system, you can't filter out the
image frequency. That means that even with perfect devices, you'd have
a baseline 6 dB noise figure - and it would get worse from there.
In general, the rule is that resistance is the source of noise.
In a BJT, that particularly means rbb', in FETs you have channel resistance.
A good noise figure in a mixer is hard to get - which is why the standard
design for an RF system establishes the noise figure in the pre-mixer
LNA. DC offset can occur because of device mismatching or lack
of symmetry - are you designing an IC or a discrete circuit? DC offset
can vary with signal amplitudes - what condition drives your spec?
Good Luck! Fred Top Charl Post subject:
Posted: Fri Oct 20, 2006 5:39 pm Colonel Joined:
Fri May 19, 2006 5:01 am Posts: 25 Location: Netherlands Hi,
Just a quick thought: dual conversion? This will yield better
image rejection. Kind regards, Charl Top
siva Post subject: mixer dc offsetPosted: Fri Oct 27, 2006 9:16
am Lieutenant Joined: Thu Oct 19, 2006 2:42 am Posts:
2 Location: india this mixer is for a radio IC. iam using
spectreRF for my simulations. how to measure the DC offsets for my
down conversion mixer?? if u have any paper(s) related to its measurement,
pls post them.. thank u....
Posted 11/12/2012
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