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https://designers-guide.org/forum/YaBB.pl Design >> RF Design >> why 50 ohms impedance matching for low noise amplifier https://designers-guide.org/forum/YaBB.pl?num=1284194476 Message started by suryakumari on Sep 11th, 2010, 1:41am |
Title: why 50 ohms impedance matching for low noise amplifier Post by suryakumari on Sep 11th, 2010, 1:41am why 50 ohms impedance matching in designing LNA,,why not 75 ohms..is there any specific reason for choosing that number??? can anyone tel me waiting for reply |
Title: Re: why 50 ohms impedance matching for low noise amplifier Post by pancho_hideboo on Sep 11th, 2010, 3:43am See the followings. http://www.designers-guide.org/Forum/YaBB.pl?num=1195659245/5#5 http://www.designers-guide.org/Forum/YaBB.pl?num=1195659245/3#3 http://www.designers-guide.org/Forum/YaBB.pl?num=1195659245/1#1 http://www.designers-guide.org/Forum/YaBB.pl?num=1065493598 |
Title: Re: why 50 ohms impedance matching for low noise amplifier Post by suryakumari on Sep 12th, 2010, 1:10am Thank u. I read that discussions but i am not clear . I am new to rf :( please can anyone clarify my basic Q'ns so that i can move in rf world (1) matching impedance of LNA depends on the type of antenna we are using..is my statement is correct??? and 50ohms for which antenna??? (2)If there is no filter between antenna and LNA then what about the impedance matching for LNA ??? (2) If there is a filter between antenna and LNA then what about the impedance matching for LNA and filter ??? (3)what is the effect of transmission line or co-axial cable on impedance matching??? Thanks Suryakumari |
Title: Re: why 50 ohms impedance matching for low noise amplifier Post by pancho_hideboo on Sep 12th, 2010, 1:28am saam wrote on Sep 12th, 2010, 1:10am:
With understanding only these easy questions, you can not still even touch a door of RF World. saam wrote on Sep 12th, 2010, 1:10am:
saam wrote on Sep 12th, 2010, 1:10am:
http://www.designers-guide.org/Forum/YaBB.pl?num=1245527790 saam wrote on Sep 12th, 2010, 1:10am:
saam wrote on Sep 12th, 2010, 1:10am:
So you have to apply it as matching for antenna-filter and filter-LNA. saam wrote on Sep 12th, 2010, 1:10am: If line is perfectly lossless, Γ keeps a constant magnitude(that is a constant VSWR) and changes its phase. |
Title: Re: why 50 ohms impedance matching for low noise amplifier Post by suryakumari on Sep 12th, 2010, 2:07am Thank u loss of rf coaxial cabel is minimum around 50ohm characteristics impedance....can anyone tel me the reason?? suppose if v use 50 ohms co-axial cable for minimum loss then do v have to use the antenna having radiation resistance of 50 ohms also do v have to match the input impedance of LNA as 50 ohms??? If there is no feeder line between antenna and LNA then as you said we have to match i/p impedance of LNA to the o/p impedance of antenna. If If there is no feeder line between antenna and LNA then what about the matching of LNa??? Thank u suryakumari |
Title: Re: why 50 ohms impedance matching for low noise amplifier Post by pancho_hideboo on Sep 12th, 2010, 2:12am saam wrote on Sep 12th, 2010, 2:07am:
Rc value for true minimum loss is 76.7ohm not 50ohm. But losses of them are almost same. See any beginner text book on microwave engineering. http://en.wikipedia.org/wiki/Coaxial_cable saam wrote on Sep 12th, 2010, 2:07am:
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Title: Re: why 50 ohms impedance matching for low noise amplifier Post by suryakumari on Sep 12th, 2010, 2:47am Rc value for true minimal loss is 76.7ohm not 50ohm. But losses of them are almost same. then why don't they use 76.7 ohms ??? as you said conventional commercial antenna impedance is 50 ohms ..is it answer for my Q'n??? Thank u so much :) |
Title: Re: why 50 ohms impedance matching for low noise amplifier Post by pancho_hideboo on Sep 12th, 2010, 2:51am saam wrote on Sep 12th, 2010, 2:47am:
Again from practical point of view, losses of Rc=50ohm and Rc=76.7ohm are almost same. |
Title: Re: why 50 ohms impedance matching for low noise amplifier Post by suryakumari on Sep 12th, 2010, 5:03am Thank u :) |
Title: Re: why 50 ohms impedance matching for low noise amplifier Post by loose-electron on Sep 14th, 2010, 6:14pm 50 (or for the fussy types, 52 ohms) is compromise between 77 ohms and 32 ohms Best power handling is 30-32 ohms, and has been used for high power broadcast antenna systems in the past. Best cable loss characteristic is at around 74 - 80 ohms and is commonly used in 75 ohm CATV systems to minimize system losses -- See: http://www.microwaves101.com/encyclopedia/why50ohms.cfm for a pretty good treatment of the subject. |
Title: Re: why 50 ohms impedance matching for low noise amplifier Post by pancho_hideboo on Sep 15th, 2010, 8:31am loose-electron wrote on Sep 14th, 2010, 6:14pm:
Input impedance of folded-dipole antenna is 300ohm. This is a reason why Rc of parallel feeder is 300ohm. If we feed this folded-dipole by single-ended, it is easy to use 1/4-balun. So Rc of coaxial cable is chosen as 75ohm historically. |
Title: Re: why 50 ohms impedance matching for low noise amplifier Post by loose-electron on Sep 15th, 2010, 1:48pm Or use a 6:1 to go from 300 ohms of a folded dipole to 50 ohms. Parallel wire (aka twin lead) comes in 300 and 200 ohm variations. Go research the history of it. pancho_hideboo wrote on Sep 15th, 2010, 8:31am:
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