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Design >> RF Design >> RF Matching question
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Message started by flywill on Mar 8^th, 2007, 10:17am

Title: RF Matching question
Post by flywill on Mar 8^th, 2007, 10:17am

A basic question is still not clear to me:
Why RF front-end need impedance matching while IC design internal circuit we did not care even for GHz speed circuit? I understand max power tx/noise matching for LNA etc, but on the other hand why internally we mostly talk about voltage gain and never mention impedance matching? Is it because the signal level is much higher than noise level?

Title: Re: RF Matching question
Post by ACWWong on Mar 9^th, 2007, 2:44am

As you correct say RF Front-Ends deal with signal power. As such impedance matching is needed to transfer maximum power or optimise noise figure (SNRin/SNRout) where SNR is signal to noise POWER ratio.
For an integrated LNA, once on chip (after impedance matching) the signal power is converted to either signal voltage, and so is then processed as such. So now we are concerned with voltage gains (for signal voltage and noise voltage)
For a PA signals on chip are say voltages, the output match converts this output voltage into a output signal power to be delivered to the load (i.e antenna).

Title: Re: RF Matching question
Post by aaron_do on Mar 9^th, 2007, 4:30am

Impedance matching also makes the circuits general purpose since they can always be connected to any other 50 ohm circuit without risking reflection and loading. Also, circuits often are connected by transmission lines which are usually 50 ohm (50 ohm is a tradeoff between power handling capability and something else...i wanna say attenuation but it doesn't sound right). When you design on IC you know the load so you don't need to worry, and the lengths of the metal lines are too short to be considered transmission lines.

cheers,
Aaron

Title: Re: RF Matching question
Post by mg777 on Mar 9^th, 2007, 9:10am

Impedance matching becomes important when the receiver of a signal is located some wavelengths away from the source - wired or wireless. That's when the source sees a resisitive load line proportional to the impedance of space η₀. It's this resistive load line that gets us tangled with power transfer - you can't develop a darn TEM voltage without sourcing a darn TEM current.

For connections within an IC the wavelengths for our typical signal frequencies are much larger than the IC dimensions, ergo....

M.G.Rajan
www.eecalc.com