little problems In Razavi's microelectronics

[hjzs18]

Hi guys:
I am reading Razavi's Microelectronics now, I got two problems in Chapter 6 , and think maybe here i can get some useful help.
1. In equation (6.3), where does the 12dB of the output SNR system required come from?
2. How does the loss of the image-reject filter, noise figure and IP3 of the mixer govern the gain of LNA? any formula here ???


Thanks In Advance.

 

[nael_ee]

hi,

in regards to the question given:
1- i think that the 12dB of the SNR is given. for a certain design criteria it is normally governed by BER required in the digital receiver, Razavi provides it as an input to the problem so he shortened the step of getting it from BER carves for a certain modulation scheme.

2- it is not totally clear, bit I think it is driven by linearity requirement of the block characteristic of the receiver.
Let me try yo combine all of these parameter it:

a- cascade equation of the NF: from it we find that if the LNA-gain increased the effect of the next stages noise is negligible so from this point of view it is better to maximize gain.

b- iip3 cascade equation: it is better to minimize the gain of the first stage to get the best iip3 performance .which I think will govern the maximum gain of the LNA. this is important in block characteristic.

c- where does the imge-reject come in this picture, there is a combrimize iip3 and the seletvirty of the filter check APP 749 @ maxim-ic.

so combining all the previous point we find that there is an rules that govern image rejection filter selectivty (I think the loss has effect you should cascade your system and see), NF, iip3 of all the system.

hoping this helped you to understand.