[amsat-bb] Signal to Noise ratio

Tue Jul 28 13:28:49 UTC 2020

An EXCELLENT topic of conversation, and one I keep circling back to over and over and over.......

But to keep this one short, if interested in the math behind why a preamp close to the antenna terminals is important, I invite you to read this article from Wikipedia: https://en.wikipedia.org/wiki/Friis_formulas_for_noise

The key point here is the G1 value (gain of first device) is in the denominator of every term in the equation......this is why you want a high gain preamp, to keep those terms small. The Noise Factor of the first device, F1,  (linear version of noise figure) is the first term in the equation and is NOT normalized by any other gain term.......this is why you want a low noise figure for the preamp.  The first device in the above equation tends to set the *system* noise figure.....this is why you want that first device to be a high gain, low noise amplifier.  The antenna is not included in the above equation, but requires some more description to get to *system* noise figure (required for things like link budget analysis and computing SNR).

The Friis noise equation gives you what I like to call the 'component' noise factor (which can be converted to noise figure and then to noise temperature), referred to the INPUT of the preamp (output of the antenna).  Then there's the antenna noise temperature (insert lots of other math here.....a good first cut for antenna noise, without doing all the integrals....is around 200 Kelvin, a bit less than 'warm earth' at 290K).  'component' noise temp, plus 'antenna' noise temp = 'system' noise temp  (all in Kelvin).  system noise temp can then be converted to system noise figure, NF_sys (dB).  

When computing noise floor (aka Minimum detectable signal, MDS), the equation is: MDS [dBm]= -174 dbm/hz + 10log10(receiver/signal bandwidth in Hz) + NF_sys [dB].

As a quick side note, it is common to refer things to the antenna terminals.  This is somewhat arbitrary, but its important that the reference point be tracked.  This is also why you do not include things like preamp gain, coax loss, IF gain, etc. in the figure of merit calculation (also galled G/T).  The Friis Equation above already accounts for all of that in the computation that is referred to the input of the preamp.  So once you get noise temperature, all you have to do is take antenna gain over the *system* noise temperature to get *system* G/T.  G/T is a simple way to compare one ground station to another, whoever has a large G/T has a larger SNR (and it doesn't matter how they achieved that G/T....whether they have cryogenic LNAs, or huge antenna gain......G/T is G/T).  If your station has 3 dB more G/T than my station, you've got 3 dB more SNR than my station.

There's lots of details I skipped.....but fun topic!  Excellent question.  I once asked a similar question (about Eb/No....digital version of SNR) on a research contract of some of my colleagues...and got to see three PhDs argue over what the 'meaning' of SNR (and Eb/No) is for over an hour......super informative and fun to watch!

-Zach, KJ4QLP
-- 
Research Associate
Aerospace & Ocean Systems Lab
Ted & Karyn Hume Center for National Security & Technology
Virginia Polytechnic Institute & State University
Work Phone: 540-231-4174
Cell Phone: 540-808-6305

-----Original Message-----
From: AMSAT-BB <amsat-bb-bounces at amsat.org> On Behalf Of W3AB/GEO via AMSAT-BB
Sent: Monday, July 27, 2020 11:10 PM
To: Zach Metzinger <zmetzing at pobox.com>
Cc: amsat-bb at amsat.org
Subject: Re: [amsat-bb] Signal to Noise ratio

To add to the excellent info Zach gave, the LNA should be mounted as close to the antenna as possible.

⁣___
Sent from my two way wrist watch
73 de W3AB/GEO

On Jul 27, 2020, 17:39, at 17:39, Zach Metzinger via AMSAT-BB <amsat-bb at amsat.org> wrote:
>On 2020-07-27 19:04, Albert Ozias via AMSAT-BB wrote:
>> Do preamps improve the signal to noise ratio or simply amplify signal
>and noise?
>
>Hello Albert,
>
>No, a preamp cannot improve the SNR of the signal at its input
>terminal(s).
>
>All amplifiers add noise to the signal that they are amplifying, and 
>that added noise can't be removed.
>
>That's the key point: An analog signal's SNR only goes down, never up.
>
>The RF energy intercepted by your antenna is immediately combined with 
>other sources of noise, mostly thermal noise, on the way to your radio 
>for eventual demodulation.
>
>A LNA (low-noise amplifier) does this while adding a minimum of noise
>to 
>the signal. This prevents the SNR from degrading to a point where
>useful 
>information can no longer be recovered from the demodulated signal, be 
>it analog (voice) or digital (data).
>
>Also note that your radio also adds its own measure of noise to the 
>incoming signal, degrading the SNR. However, if the SNR has been 
>preserved from the antenna through the LNA and coax, then the noise
>that 
>the radio adds will not degrade the SNR appreciably.
>
>This topic is involved and sometimes complex, but rewarding!
>
>--- Zach
>N0ZGO
>
>_______________________________________________
>Sent via AMSAT-BB at amsat.org. AMSAT-NA makes this open forum available
>to all interested persons worldwide without requiring membership.
>Opinions expressed
>are solely those of the author, and do not reflect the official views
>of AMSAT-NA.
>Not an AMSAT-NA member? Join now to support the amateur satellite
>program!
>Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb
_______________________________________________
Sent via AMSAT-BB at amsat.org. AMSAT-NA makes this open forum available
to all interested persons worldwide without requiring membership. Opinions expressed
are solely those of the author, and do not reflect the official views of AMSAT-NA.
Not an AMSAT-NA member? Join now to support the amateur satellite program!
Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb