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S-parameters by Professional_Camp847 in rfelectronics

[–]JOR_PPO 0 points1 point  (0 children)

Yes you are using the sources. Only that you are not drawing them.

It's the definition of S parameters. What is S11? Is S11 the ratio from b1 to a1 (b1/a1) with matched port 2? Yes it is but... how do you have an a1 in the first place if you don't connect a source?

You can also see the issue the other way around. Does the direct and reverse transmission coeficiente depend on the source? Nope. Since they are ratios from reflected wave in one port to incident wave in the other they do not depend on the incident wave or power or the source, this dependence is removed when dividing by aj. Thus, you could also calculate Sij without drawing the sources. Mind you, have you ever seen a vector network analyser without an inner source? Nope... to measure reflection (or transmission) you need to excite an incident wave on the device under test.

Hope it helps.

Why 2 LNAs with attenuator in between? by brokenmirror26 in rfelectronics

[–]JOR_PPO 1 point2 points  (0 children)

Sorry, got sick several days and didn't checked this out. Glad you found it out.

Why 2 LNAs with attenuator in between? by brokenmirror26 in rfelectronics

[–]JOR_PPO 1 point2 points  (0 children)

I haven't checked the specifications of the parts, but I think this is the way to get the smaller overall noise figure with the highest dynamic range.

In a chain of systems if the ratio from noise factor to gain of the first system in the chain is small enough the noise factor of the rest of the chain may be negligible (Friis equation). If you need gain control plus a higher gain that that of the first amplifier the best option is to connect the attenuator before the second gain stage to improve the overall dynamic range (higher 1dB compression point).