Re: Newbie question.

Brian Howie GM4DIJ

In message <hijh.1609755777674220352.sAJb@groups.io>, "Luc , on4aol" <luc.van.achte@telenet.be> writes
Goodmorning to all,
After experiments with Q100 , i' want to  take a new step in the
microwave region.
First step is receiving the ON0EME   EME beacon . I made calculations (
with VK3UM EMEcalc and others ) if i have enough SNR to take this step
.
Well , it 's difficult ! Only with a bandwidth of 10 Hz there is about
3 dB SNR , not very much.
But , my really concern is the next issue. Think in the next moment we
have NO noise ( theoretical assumption).
If a have a LNA that can amplify a minimum signal that is on a level of
, let's say -160dBm.
Suppose the Gain is 20 dB , then the output delivers =  - 140dBm.
Is this really so ? Or is there a ( physical) threshold before the LNA
" kicks in" .
Can we project this to all the levels  i.e.  -180 dBm gives - 160 dBm ?
( remember , there is  no noise) .
I welcome any comment.
73  Luc  ON4AOL
Ok this might not be quite right.

In the absence of other noise, you hit the RF photon noise limit .ie you are limited by the noise on the signal. At 1296MHz the photon energy is 8.59×10^-25 joules. In a 10 Hz bandwidth, at an average of one photon per 1/B (or is it 1/2B) = 0.1 sec the power is 8.59x10^23 mW == -220dBm. 160dBm is about 10^6 RF photons in 0.1 sec

Of course you need more than one photon to make sense of the signal, so that sets the minimum SNR. Then there's local oscillator shot noise as you have to mix down to a frequency where you can get a 10Hz bandwidth.

At light frequencies single photon detection is possible ( it used to be part of my day job), but not at RF since you can't get the other noise sources low enough.

Anyway the answer to your question is that there is no threshold until you hit the quantum limit in the absence of all other noise.

Brian GM4DIJ
--
Brian Howie

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