# How to Add Phase Noise from Two Sources? - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

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

Top

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

Top

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.

Top

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).

Top

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

Top

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?

Top

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

Top

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.

Top

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

Top

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

RF Cafe Software

Wireless System Designer

RF & EE Symbols Word
RF Stencils for Visio
Calculator Workbook
RF Workbench
Smith Chart™ for Visio
Smith Chart™ for Excel

1996 - 2022
Webmaster:
Kirt Blattenberger,
BSEE - KB3UON

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website:
AirplanesAndRockets.com

Try Using SEARCH
to Find What You Need.
There are 1,000s of Pages Indexed on RF Cafe !