Question about system gain planning in a wireless receiver

[Ghost Tweaker]

Hi everyone,

a small question about system gain planning: in a wireless receiver, I should ideally all the gain in the first stage for the best noise figure but also all the gain at the last stage for the best linearity, am I right?

Thanks for your help

 

[mamali]

Re: IP3 queation

Hi everyone,

a small question about system gain planning: in a wireless receiver, I should ideally all the gain in the first stage for the best noise figure but also all the gain at the last stage for the best linearity, am I right?

Thanks for your help

well, destributed gain is better for linearity; each stage has a limited range to be linear, so distributing the gain will remain the linearity the best.

.m

 

[Ghost Tweaker]

Re: IP3 queation

Yes but in the IIP3 formula, the gain of the last stage doesn't appear which means that it doesn't degrade any of the stage's IIP3. ->Ideally all the gain would be "placed" at the last stage. Am I wrong?

 

[vfone]

Re: IP3 queation

Yes, the last stage gain would not affect the over all Input IP3, and also would not affect the total noise figure.
The gain distribution of each stage is important to don’t saturate following stages. For example too much gain on the LNA, at high input levels could saturate the first mixer.

 

[Ghost Tweaker]

Re: IP3 queation

Thank you vfone this was helpfull. I have one last question: is the IIP3 value found the IIP3 of the first component of the receiver or that of the complete system?

Thanks for your help

 

[ashaheer]

Re: IP3 queation

IP3 queation...........?

 

[bolin]

IP3 queation

Tweaker, The firts stage IIP3 is not the system IIP3.System IIP3 is determined by every IIP3 of each stage and the gain allocation. Normally , in a receiver the IIP3 of mixer is most important!

 

[Ghost Tweaker]

Re: IP3 question

Bolin, I fully understand that they are not the same but which one should I use in this case? Should I consider each stage's IIP3 individually with the signals present at its input or can simply consider the receiver's IIP3 with the signals present at the input of the receiver and not care about what happens inside?

Thanx for your help

 

[designtech]

Re: IP3 queation

hi,

i think iip3 is somewhat like parallel resistors the minimum dominates, the mixer is typically most important because it doesnt have a very high linearity and there is lots of LNA gain before it..but for some reason if mixer IIP3 input referred (ie divided by gain of LNA) is still larger that than LNA iip3, then the latter will dominate, so i think all stages are equally important, except for baseband gain stages, because you can insert a filter (RC) after mixer to crush higher order terms that may cause iip3 effects in following stages