Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Effects on S/N ratio combining in-phase sources with independent noise  (Read 965 times)
NOCALLSIGN9000
Member

Posts: 8




Ignore
« on: November 09, 2012, 09:52:47 PM »

If I have two similar antennas with similar preamps having similar noise figure, what is the effect on the S/N ratio when they are merged in a combiner? The signals would be (negligibly) perfectly identical but their noise would be independent of each other.

Does the S/N stay the same, or does the signal double and the noise increases by some other amount? Or does the noise technically not increase at all while the signal increases?

Rationale: In phase signal will always result in increase while noise would self-cancel 50% of the time but self-build 50% of the time.
Logged
K2DC
Member

Posts: 1372


WWW

Ignore
« Reply #1 on: November 10, 2012, 03:35:08 AM »

The signal (voltage) will double and the noise should stay the same.  Even if the preamp outputs are combined in phase, the noise sources are independant and are not synchronized in time so they don't add.

73,

Don, K2DC
Logged
G8HQP
Member

Posts: 124




Ignore
« Reply #2 on: November 10, 2012, 05:21:10 AM »

You might get a net improvement of 3dB in S/N ratio. The signal will add linearly, but the noise will add quadratically if it is uncorrelated. This assumes that the noise comes from the preamps (e.g. VHF and up in frequency). If the noise comes mainly from the antennas (e.g. LF, HF) then there may be little or no improvement as the antennas may be picking up exactly the same external noise.
Logged
WX7G
Member

Posts: 6129




Ignore
« Reply #3 on: November 10, 2012, 07:15:28 AM »

The S/N improvement is 3 dB for isotropic noise.

An antenna at a particular frequency receives X noise power given isotropic noise (the same in all directions). It does not matter the antenna gain, the noise power does not change with gain, given identical antenna efficiency. The signal power can double with two properly phased antennas combined. So, the S/N ratio can increase 3 dB.

Now let's combine the output of our two phased antennas to a single preamp that is rather noisy. The S/N ratio can increase by 3 dB.

Now let's connect each antenna to the same type of preamp and combine the preamp outputs. The S/N ratio can increase by 3 dB.

There is no S/N improvement to be had by using two preamps vs one preamp.

« Last Edit: November 10, 2012, 07:35:53 AM by WX7G » Logged
K4SAV
Member

Posts: 1847




Ignore
« Reply #4 on: November 10, 2012, 07:23:51 AM »

If the two noise sources are uncorrelated, that is, not coherent or totally random relative to each other, then the resultant RMS noise voltage from the sources is the RSS sum of the two (root of the sum-of-their-squares).  Vsum = sq rt (Vn1^2 + Vn2^2).  

If two signal sources are the same amplitude, and in phase, the resultant voltage will double.  

White noise, thermal noise, Johnson noise, Gaussian noise, shot noise, excess noise, flicker, pink noise, etc would qualify as random noise and would sum by the RSS technique.   Atmospheric noise received by two antennas would not qualify as random relative to each other.  

Signals received by two separate receiving systems may not be in phase, unless steps are made to guarantee that, and if they are not in phase the sum of their voltages will not double.

You may want to convert the noise voltages to noise power for evaluating the effect.

Jerry, K4SAV

edit:  It's possible to gain some amount of signal to noise ratio by using two phase locked receivers with separate antennas in a situation where atmospheric noise is the dominate source.  Separate antennas with different patterns make part of the atmospheric noise correlated and part uncorrelated, while the signal may double.  The exact amount of improvement is difficult to calculate.
« Last Edit: November 10, 2012, 07:47:02 AM by K4SAV » Logged
NOCALLSIGN9000
Member

Posts: 8




Ignore
« Reply #5 on: November 10, 2012, 11:54:46 AM »

Great, this answered my question and then some. Thanks!
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!