# 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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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.

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.

(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

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