why 50 Ohms is standard in RF ?

[nasimesahar]

50 Ohms standard

Hi

1-Why 50 ohms is a standard in RF design?
2- where in my circuit I need to have 50 ohms impedance?
3- if the answer is input and output impedance should be 50 ohms to be matched with other circuits, does it mean that I can use any other value for internal stages of my circuit? for example if I am designing a VCO with a buffer, I need to have a bufer with Zo= 50 Ohms only and I don't need to have the same impedance for the first stage (oscillator).
4- Is there any good reference to read?

Thanks

 

[FvM]

50 Ohms standard

The general question has been answered hundred times at edaboard. Regarding your specific question 3, you're free to use any meaningful impedance, as long as you know how to match it to other circuits and available transmission lines, that are needed to connect the circuits.

 

[nasimesahar]

Re: 50 Ohms standard

In RF circuits for a specific transistor we know the S parameters and so the z parametesrs.
But in RFIC design when we draw a layout we don't know these parameters for the transistor!

I mean for example for a VCO in discrete circuits we know s parameters and we can design matching circuits to have oscillation , how we do this is integrated circuits??

Many thanks.

 

[FvM]

50 Ohms standard

Discrete devices s-parameters are representing the device including package parasitics. There is no similar data in IC design. But of course, the elementary transistors have impedances that can be expressed as s-parameters if you need to.

 

[mauloftin]

50 Ohms standard

1-Why 50 ohms is a standard in RF design?

it is about technology use in coax cable. It is about that there is teflon. I am not sure exactly why, but the coax cable is it historical reason.

 

[VSWR]

Re: 50 Ohms standard

**broken link removed**

 

[nasimesahar]

Re: 50 Ohms standard

Hi

Thanks for answers. I found Why 50 ohms but the main question about integrated transistor parameters is remained.

Can any one answer that?

Regards

 

[BigBoss]

50 Ohms standard

Because due to very short distances compared to the wavelength, impedance matching circuit is not necessary for RFIC circuits.Only output and input terminals need matching circuits to work with external world..