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How to Add Phase Noise from Two Sources? - RF Cafe Forums
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mmaassel
Post subject: How to Add Phase Noise from Two Sources?
Posted: Thu Jun 04, 2009 2:36 pm
Colonel
Joined:
Mon Apr 10, 2006 12:43 pm
Posts: 25
Good day.
I am trying
to remember (without any success) how to get the resulting phase noise
when two sources are mixed together. I know that if both sources have
the same phase noise, the resulting phase noise is increased by 3 dB.
Can anyone provide me with some information (equation, magazine
article, website, etc.) on how to do this?
Thank you for your
time and help.
Michael
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yendori
Post subject:
Re: How to Add Phase Noise from Two Sources?Posted: Mon Jun 08, 2009
10:38 am
General
Joined: Thu Sep 25, 2003 1:19 am
Posts: 50
Location: texarcana
Hi,
You need to use a 90°
splitter. In this case you are just left with the DC component of one
signal, after the LPF.
Rod
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Noise
Post
subject: Re: How to Add Phase Noise from Two Sources?Posted: Wed Jun
10, 2009 8:33 am
Captain
Joined: Tue May 02, 2006
4:59 am
Posts: 15
convert it to absolute number, sum it and then
converte back to logarithm number.
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madengr
Post subject: Re: How to Add Phase Noise from Two Sources?Posted:
Fri Jul 24, 2009 5:00 pm
Captain
Joined: Fri Feb
02, 2007 5:22 pm
Posts: 5
Location: Overland Park, KS
With
3 dB you are assuming the phase noises are uncorrelated. It could be
as high as 6dB if they are correlated (running off the same reference).
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biff44
Post subject: Re: How to Add Phase Noise
from Two Sources?Posted: Wed Jul 29, 2009 1:20 pm
Colonel
Joined: Wed Apr 08, 2009 11:07 am
Posts: 33
If they are correlated,
it could be higher OR lower, depending on the block diagram.
For instance, If I had a 1 Ghz reference and a 1.1 Ghz reference, and
they were both phase locked to a lower frequency crystal reference,
they would have correlated phase noise inside their loop bandwidths.
If I then mixed the 1 GHz signal with the 1.1 GHz signal, and generated
a 100 MHz signal, it should have better phase noise than either of the
starting two frequencies.
_________________
Rich
Maguffin
Microwave Consulting
www.MaguffinMicrowave.com
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mmaassel
Post subject: Re: How to Add Phase Noise from Two Sources?Posted:
Wed Jul 29, 2009 3:32 pm
Colonel
Joined: Mon Apr
10, 2006 12:43 pm
Posts: 25
Good day. Thank you all for your
responses. I really appreciate it.
Let me ask my question again,
this time with a little more information.
I am upconverting
915 MHz to 1715 MHz using a mixer, so my LO is at 800 MHz.
The
two RF sources are uncorrelated.
What is the resulting phase
noise if source #1's phase noise is higher then source #2 by 5 dB?
When I looked at the responses, I thought I had the answer,
but I am not sure -
This is the method I am using.
1) The
phase noise of each source is measured at the same offset frequency.
2) Now I convert the dBc into the ratio (Pnoise/Pcarrier1) and (Pnoise/Pcarrier2).
3) Then I will add them together and convert them back to dBc.
My question - Pcarrier1 and Pcarrier2 are not equal. Can I still
use this method or is there something else I need to do?
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biff44
Post subject: Re: How to Add Phase Noise from
Two Sources?Posted: Wed Jul 29, 2009 6:05 pm
Colonel
Joined: Wed Apr 08, 2009 11:07 am
Posts: 33
The phase noises
would then simply add. The somewhat confusing thing is that you have
to convert from log to linear, add, and then convert back to log.
Lets say Source 2 had SSB phase noise of -100 dBc/Hz at 10 KHz offset.
And, Source 1 had SSB phase noise of -95 dBc/Hz at 10 KHz offset.
Your resulting phase noise would be added linearly:
(10^(-100/10))
+ (10^(-95/10))=
10E-10 + 3.16E-10 = 4.16E-10
Then to convert
back, 10 * LOG(4.15E-10)= -93.8 dBc/Hz at 10 KHz offset
_________________
Rich
Maguffin Microwave Consulting
www.MaguffinMicrowave.com
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mmaassel
Post subject: Re: How to Add Phase Noise
from Two Sources?Posted: Thu Jul 30, 2009 11:01 am
Colonel
Joined: Mon Apr 10, 2006 12:43 pm
Posts: 25
Good day.
Thank you for your reply.
Final question (maybe). What is the
resulting phase noise if the two (uncorrelated) sources are subtracted
from each other. Is the same procedure (described by biff44) used?
For example downconverting using a mixer (RF = 1715, LO = 800 MHz,
and then IF = 915 MHz)
For discussion purposes let us use biff44
phase noise numbers.
Source 1 = -95 dBc/Hz at 10 kHz offset
Source
2 = -100 dBc/Hz at 10 kHz offset.
Thank you for all of your responses.
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biff44
Post subject: Re: How to Add Phase Noise
from Two Sources?Posted: Fri Jul 31, 2009 9:17 am
Colonel
Joined: Wed Apr 08, 2009 11:07 am
Posts: 33
Nope, just because
you are "subtracting" the frequencies (which both have random uncorrelated
phase noise) there is no stable phase relationship between the two sources
that would allow phase jitter from one to cancel the other partially.
If they both had the same phase noise spectral density, after mixing
the total combined phase noise would be 3 dB higher.
I like to
think in terms of frequency dither. Lets say source 1 suddenly jumped
up 1 Hz, and source 2 was correllated (lets say thru a X2 frequency
multiplier). Then source 2 would suddenly jump up 2 Hz. In that case
the mixer output frequency would suddenly jump either UP 3 Hz if you
were adding the two frequencies, or UP only 1 Hz if you were subtracting
the two frequencies. The FM noise is either enhanced or reduced, depending
on the specific block diagram.
Same example, but sources were
uncorrelated in time. Source 1 jumps 1 Hz in frequency, and Source 2
jumps 2 Hz in frequency, but you do not know (statistically) which way
(up or down) either source jumped. So it is equally likely that the
resultant jumped 1 Hz or 3 Hz (or somewhere in between) depending on
randomness.
_________________
Rich
Maguffin Microwave Consulting
www.MaguffinMicrowave.com
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mmaassel
Post subject:
Re: How to Add Phase Noise from Two Sources?Posted: Thu Aug 06, 2009
9:45 am
Colonel
Joined: Mon Apr 10, 2006 12:43 pm
Posts: 25
BIF44 - Thank you very much for your answers. I really
appreciate the time it took you to give me a good explanation.
Thank you to everyone else who responded to my question. You raised
some good points that I needed to know about.
Michael.
P.S. BIF44 your website is impressive.
Posted 11/12/2012
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