When is matching LNA and mixer necessary?
what situations is and isn't needed
just if u are going out of the Chip, for an IRF for instance then u need to match to 50 Ohm both in the LNA's o/p and Mixer's i/p
if they are in the same chip , and the distance is near 1/4 λ then u need matching network , but if they are near so u donot need matching circuit
khouly
hello khouly,
how r u?
i was wondering if they are on the same chip , what value do i match to? 50 Oms too?
hello safwatonline
i am fine , thanks , how are u ?
about the on chip matching , no it is not 50 Ohm , but the matching is to transform the output impedance of LNA to the input of Mixer
this if needed
khouly
sounds with sense,
so for example if mixer's S11 is measured with circuit matched at 50Ω, and later a LNA is atached to the same chip before this mixer, then the mixer's S11 will be the same if LNA output impedance and mixer input impedance are the same ?
hello again,
i am fine too khouly, so i tried a quick estimation for a mixer working at 2.2G this will give lambda/4 =114 pm which seems tooooooo small , but i don't see many poeple working at this high frequency making any matching in direct Receiver
regards
The purpose of external matching is not only to prevent unwanted reflections but also for maximum power transfer.
So if the LNA output impedance is high and the mixer input inpedance is low, you may want to match so that you get maximum power transfer from the mixer to the load, and thus have better P1dB and IIP3.
Greg
lambda=c*f,where c is the speed of the light, that is 3e8
thus, the lambda/4=3e8/2.2e9/4=2.4cm
remember , lambda in space and lambda in time, what concern matching is lambda in space ; lambda*freq=C, C= 300*10^6
lambda=c/f only applies in free space, e.g. waveguide circuits. When the signal propagation is done over a substrate (or PWB), the guided wave length, λg is used in stead. For example, in microstrip configuration, λg=c/f/sqrt(ε_eff)
matching to 50 ohm is needed when we use an off chip filters such as image reject filter between the lna and mixer.
but if we don't use such as filters the power matching isn't neccessary after lna generally. but noise matching is neccessary.