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down mixer - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.


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??


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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


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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


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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....



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