Pashtet wrote on May 16th, 2011, 5:30am:Dear, all.
Let's imagine the situation that we have RF receiver working with passive antenna. LNA of this receiver has acceptable parameters(NF, S12, S21, S22, and so on), except poor S11. Suppose there are no filters between LNA and passive antenna. What would be the drawbacks of such curcuit from the point of view of overall receiver performance?
One thing thing is that part of input power would reflect back, re-radiate by antenna and may corrupt input signal for other receivers working near the one we observe. But i doubt if this phenomenon is of big importance.
Any comments?
Thanks.
for your question, I think there are two case:
1. in your case, your receiver is a wireless receiver. then, the reflect power what you say is another description of not max power transmission. but that not max power transition does not affect others, because your receiver antenna radiation power is low, others can't receive it
2. if in cable transmission, LNA input impedance mismatch will affect cable transmission character impedance. thus it will affect transmission efficiency, that means the transmitter should have higher driver capability.
so , in the first case, you may get a NF without matching to 50ohm. however, in the second case, you may loose antenna gain to get LNA better, the total NF way not be good.
the above is just my thinking, just for reference, I am not quite sure about it