I can not understand what you mean by "source" at all.1.) An ADC is known for its horrible NF, say 30dB.
A conventional receiver would add LNA & gain in order make the overall NF much lower (through Frii's), say 5 dB.
However, these gain stages add noise themselves.
So if I assume the maximum input swing of my ADC is 2Vpp (13 dBm) and I can pick my input power level of the receiver arbitarily, is there any way to beat connecting the ADC straight to the source, despite having a high NF of 30 dB?
Signal source (modeled as voltage source) with 50 ohm in seriesI can not understand what you mean by "source" at all.
If amplitude level of source is enough large comparable to FullScale of ADC, you can connect it to ADC directly.
Don't consider NF.
Consider input referenced noise voltage at ADC input.
About your questions 2), 3), 3a) and 3b), describe more clearly.
It is very difficult to find out what is your main question.
I agree, your overarching question is confusing. From the title and your first comments, perhaps you are asking the difference for maximum power transfer in the far-field and near-field case?
See the followings.What does that mean in practice for a circuit?
Does it just mean that the amount of energy transferred is lower (or there is a certain signal attenuation)
or can I expect distortion (due to reflections) or other side effects
if I do not match?
This can never be true for RF.The classical example: Signal source (voltage),
Rs ohm in series and Rin of my receiver.
Suppose I make Rin=1G.
Then the efficiency would be 100%.
Right.Furthermore, with arbitrary Rs & Rin, the signal and noise power is given as (is this correct?):
Psig = Vsource^2(Rin/(Rs+Rin))^2/Rin
Pnoise = 4kT*Rs(Rin/(Rs+Rin))^2/Rin
I can not understand what you want to mean at all.The SNR Psig/Pnoise stays constant regardless of Rin & Rs if I take Vsource as the actual signal source.
I can not understand what you want to mean at all.On the other hand, Pnoise becomes smaller, the larger Rin (larger mismatch).
Again, I can not understand what you want to mean at all.So again, by making Rin=1G I can effectively eliminate the source noise while I can leave the signal level unchanged.
Do I miss something?
This can never be true for RF.
If Rin is high such as 1Gohm, parasitic capacitance Cin can never be ignored.
The SNR Psig/Pnoise stays constant regardless of Rin & Rs if I take Vsource as the actual signal source.
I can not understand what you want to mean at all.
Psig/Pnoise=Vsource^2/(4kT*Rs) is dependent on both Vsource and Rs.
I mean, for a transmitter I do understand I want to deliver the maximum power because that's just required for a certain distance. But for a receiver, I am interested in the information anyway, so wouldn't it be better if I just look at the voltage?
SNR is constant regardless of Rin value.I want to know if I need to set Rin to 50 Ohm (as the source resistance)
and what the implications are if I do not.
Wrong. SNR is constant regardless of Rin value.Then I want to maximize SNR, i.e. minimize the noise voltage which is given as 4kT*Rs(Rin/(50+Rin))^2.
If Rin=50, this is kT50.
If Rin=10k, this is ~kT200 - a factor of 4 worse.
Contrary to what I said before, the best SNR would not be if Rin is large
No. If Rin=0, both signal and noise voltage at node x are zero.but if Rin approaches zero.
Is this true?
Again surely see the followings.So why do we care about maximum power transfer (and hence matching) in a receiver?
See the followings.If Rin=infinity, the receiver would just sense the voltage (at the gate of a MOSFET e.g.) while not drawing any power at all (maximially efficient).
Again surely see the followings.
Network theory: travelling waves vs power waves
Network theory: travelling waves vs power waves
Your confusion is no more than your lacking of understanding simple math.
Reconsider simple math befire posting.
[...]
SNR is constant regardless of Rin value.
[...]
Wrong. SNR is constant regardless of Rin value.
[...]
No. If Rin=0, both signal and noise voltage at node x are zero.
If Rin=0, you have to define SNR by current instead of voltage.
Here SNR is constant even if Rin=0.
See the followings.If Rin=infinity, the receiver would just sense the voltage (at the gate of a MOSFET e.g.) while not drawing any power at all (maximially efficient).
https://www.designers-guide.org/Forum/YaBB.pl?num=1065493598/1#1
https://www.designers-guide.org/Forum/YaBB.pl?num=1195659245/5#5
There are four options for matching.
(1) Power matching(=Conjugate matching)
(2) No reflection for voltage wave
(3) Noise matching
(4) Maximum condition for voltage at LNA input
You mean (2) ? If so you should match 3+0j to 300-50j.
If you mean power matching, you should match 3+0j to 300+50j.
There seems to be no feeder line between antenna and LNA. And since your application is low frequency,
so I think (4) is preferable than other three.
Reconsider simple math before posting.
SNR is constant regardless of Rin value.
BTW, answer my quetions in your nonsense thread.
Can you understand "Noise matching" correctly ?Advantage 3: Best SNR
Obviously wrong.I do not see a problem in the equations I wrote in my attached picture.
I think it is valid and it is obvious that SNR changes if I keep signal constant but not noise.
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