Why do we always do impedance matching for 50 ohm?

[chacha]

It seems very silly. but still i would like to know the reason.

Why do we always do impedence matching for 50 ohm only.
Is it because that the characteristic imp of the cables which were used is 50 ohm??
if yes, in RFIC design, why do we still follow this tradition?? what is the significance of 50 ohms.

thanks a lot.

 

[tyassin]

Re: 50 ohm

Hi

The tradition of using 50 ohms, comes from a compromise. The best power handling capabilities is done in a 30 ohm system, however the attenuation is lowest in a 70 ohm system. Therfore a compromise is done at 50 ohms.

Hope this helped.

Regards

 

[balaguru thavamani]

Re: 50 ohm

As tyassin said, it is the compramise between 30 and 77Ω.

30Ω-Good power handling capacity and High Attenuation

77Ω-Low attenuation and low power Handling Capacity.

The Average of 30 and 77Ω gives around 53.5Ω which is rounded of to 50Ω which

lies at the center point of Smithchart.

 

[priyankguddu]

Re: 50 ohm

Yes thats true.

One small correction. It is not 50 ohms that lies at the center of the smith chart but it is the characteristic impedance that lies at the center. If the Zo is 75 then that will lie at the center of the smith chart (Γ=0 point)

Regards

 

[Manjunatha_hv]

Re: 50 ohm

Hello,


Their is a good discussion or message on "There's nothing magic about 50 Ohm" from Gary Breed, from High Frequency Electronics journal June 2007 issue page No. 6 & 7...any way I attached the same article for your reference...
I think this refers lot of books for more information on the topic....

MODERATOR ACTION:
https://highfrequencyelectronics.com/Archives/Jun07/HFE0607_Editorial.pdf


---manju---

 

[VSWR]

Re: 50 ohm

**broken link removed**

 

[supersage]

Re: 50 ohm

It seems that I cannot open and download this article...

 

[yashiro32]

Re: 50 ohm

But why TV cables and some other applications uses 75 ohms. It seems that 75 ohms is another common impedance to use.

 

[dingjingfeng]

Re: 50 ohm

Please reference the Book Writen by Thomas Lee,

the 77 Ohm is because it have minimum insertion loss
Engineer like to use round number, so TV equipment is based on 75 Ohm system

30 Ohm is best for power capability

50 Ohm is a tradeoff between 50 and 30 Ohm

But why TV cables and some other applications uses 75 ohms. It seems that 75 ohms is another common impedance to use.

 

[艾我1]

50 ohm

thanks

 

[nchamber]

Re: 50 ohm

dont discount the natural characteristic impedances of antennas and transmission lines . for example, twin-lead is typically 300 ohms, which scales nicely to 75 ohms after a 4:1 impedance transformation. also, a half wave dipole antenna has an impedance with real part equal to 73 ohms. granted, TV and radio sets are sufficiently broadband that the antenna is not strictly half-wavelength. however, twin lead is a lot cheaper to produce than coaxial cable - an important advantage in the early days of broadcast RF electronics. these factors, in addition to the lower attenuation coefficient (which scales approximately as the ratio of impedances alpha1/alpha2 ~= Zo2 / Zo1) are the reason for the dominance of 75 ohm impedances in commercial RF electronics.

 

[ADS_RFMW]

Re: 50 ohm

try this
**broken link removed**

 

[monster528]

50 ohm

Thank you all!