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down mixer - RF Cafe Forums

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Post subject: down mixer Posted: Thu Oct 19, 2006 2:54 am


Joined: Thu Oct 19, 2006 2:42 am
Posts: 2
Location: india

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??


Post subject: Active mixer noise figurePosted: Thu Oct 19, 2006 3:47 pm


Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104

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!


Post subject: Posted: Fri Oct 20, 2006 5:39 pm


Joined: Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands

Just a quick thought: dual conversion? This will yield better image rejection.

Kind regards,


Post subject: mixer dc offsetPosted: Fri Oct 27, 2006 9:16 am


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....

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