Why we have to use always 50 Ohms at the input and output port

Dear Friends,
Why we we have to use 50 ohms at the input and output ports of the filters, amplifiers, mixers. What is its significance. The ports, we are always using 50 0hms, why we have to use 50 Ohms. Why can't we use other values such as 100 ohms, 200 ohms, 300 ohms, 10 ohms.

Your previous posts suggest that you already know something about RF circuits. So I presume you know that RF components are connected by transmission lines, either cable or embedded in a PCB. Here you are asking about single ended ports, connected by coxial lines respectively PCB microstrip and striplines. They have a so-called characteristic impedance https://en.wikipedia.org/wiki/Transmission_line

By basic physical laws, there's a limited characteristic impedance implementation range, 10 to 100 ohm is feasible under extreme conditions. Depending on the primary design goal, maximum power handling or minimal loss, and also considering reasonable cable geometries, you end up in a small impedance range between 50 and 75 ohm. By convention, 75 ohms has been assigned for baseband video and TV antenna cables and 50 ohms for general RF instrumentation and transmitter antenna cables.

There are additional aspects like typical antenna impedances and feasible impedances of RF circuits.

During some development, source and load impedance was standardized to be used in test equipment. Most signal generators, power meters and recently Vector Network Analyzers use 50 Ohms.
By doing that we can easily compare all components and circuits.

Microwave101 has a good explaination.
http://www.microwaves101.com/encyclo...why-fifty-ohms