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Help LNA single to differential conversion

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mostafa_maarouf

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Hi,

I am designing an RF front end for WCDMA and i designed the LNA but it is single ended to match with the off chip duplexer. Now i am desiging the mixer and it will be differential (Gilbert cell) so i need to convert from single ended to differential.
I thought of building the LNA as differential and use a balun at the start but this will add loss which will build up the noise figure directly..
so i thought that i can make two branches of amplifier (one +ve gain (C-G) and the other is -ve gain (CS)) ) and sense the output differentially which will make the required conversion.
this method is present in a paper in JSSC by rahem Bagheri. but he was implementing a wide band LNA so the C-G worked with him but i need it to be narrow band , will it work with me. ?
Help me with any data i will be appreciating..

Also if there is a simpler method to convert from single ended to differential in mixer or in any place , plz tell me

Best Regrads,
Mostafa El khouly
 

khouly

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Dear Mostafa

in current implemtations , usually there is a SAW filter after the LNA , which is off chip this SAW filter handel the conversion and then after it u just add a matching circuit for the input of the mixer

this will not add noise to the LNA .

the meathod ur talking about is great , but it need alot of tweaking , for gain and phase balance between the 2 paths

the third way , use on chip transformer which will add alot of chip area but u need to optimize this tranformer

wish this help u

Khouly
 

jallem

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Dear Mostafa,

If the differential side of the LNA or the mixer can have
access in single ended mode you can convert the
measurements to differential using the following
paper I wrote once for me as a reminder which is
part of an AN from ANRITSU:
 

mostafa_maarouf

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Dear Khouly,
thanks alot for ur replies..
The SAW filter u r talking about is for image rejection in the super het Rx while i'm designing a ZIF Rx which needs no image rejection nearl at all which is one of the great advantages of theZIF Rx.

Thanks again for your help
 

khouly

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i am talking about Zero IF archetecture , it is for band selection ,

i have used Qulacomm Chip set for CDMA2000 and WCDA they are ZIF and employ the SAW filter

if u don't want to increase ur board space , then u need to think in designing a balun on chip or u need to design it of chip with L and C passive elements , also ur balun can be matching circuit as well

khouly
 

mostafa_maarouf

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Sorry Khouly,
but r u talkinhg about using a SAW filter AFTER LNA (ina single LNA system) to perform band selection..???
i think that the placement of this filter will be before the LNA (between the antenna and the LNA)
Correct me if i am wrong or i misunderstood u..
thanks again for your replies..
 

khouly

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between the LNA and antenna ,there a duplexer which act as a filter as well

then after the LNA there is a SAW filter as well ,
both are used

i have seen other chipsets , without the SAW filter after the LNA , but also i have used ZIF chips with this SAW filter

also check this link and u will get my point

http://www.mumor.org/public/events/workshop_lausanne/Arnaud.pdf


khouly
 

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