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Coherent and non-coherent power combining - RF Cafe Forums

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Post subject: Coherent and non-coherent power combining Posted: Thu May 07, 2009 4:36 pm


Joined: Tue Sep 04, 2007 7:54 am

Posts: 13

Location: Dallas


As per some of the power combiner Application notes, if two non-coherent (Different freq) signals are combined, there will be 3dB loss (For N way combiner 10log(1/N).

For two coherent signals, with no phase difference there will be no loss and 180 out of phase, total loss. Is this correct?


Loren A.

Post subject: Re: Coherent and non-coherent power combiningPosted: Fri May 08, 2009 8:30 am


Joined: Fri Jul 16, 2004 4:12 pm

Posts: 4

Hello ljoseph.

Combining 2 identical in-phase signals will result in a 3 dB increase at the output. There will be an overall loss of whatever the combiner loss is.


Signal Power (each) = +10 dBm

Combiner Loss = 0.5 dB

Output Power = +10 + 3 - 0.5 = +12.5 dB

For 180-degree OOP signals, the two signals would totally cancel each other out.

Suppose you had the following:

P1 = 0 dBm

P2 = +10 dBm (in phase wrt P1)

Combiner Loss= 0.5 dB

Output Power = (1 mW + 10 mW) / 10^(0.5/10)

= 9.80 mW

= 9.91 dB

So, the resulting output will be lower than the larger signal because of the combiner loss. But, it's not always that way and doesn't really matter.



Post subject: Re: Coherent and non-coherent power combiningPosted: Fri May 08, 2009 10:46 am


Joined: Tue Sep 04, 2007 7:54 am

Posts: 13

Location: Dallas



What happens when the two signals are at different frequencies and +10dBm each.

The ouput should be 10dBm-0.5dB(Coupler loss)=9.5dBm. Is this correct?


Loren A.

Post subject: Re: Coherent and non-coherent power combiningPosted: Sun May 10, 2009 11:57 am


Joined: Fri Jul 16, 2004 4:12 pm

Posts: 4

It depends on how you are measuring the power.

If you are looking on a spectrum analyzer that can discern the individual powers of the two separate signals, then there will only be the loss of the power divider for each of the signals. The two signals at different frequencies do not add or subtract from each other.

If you are using a wideband power meter, or even a spec-an whose bandwidth is wide enough to totally encompass both frequencies, then the total power will be measured as a 3 dB gain, minus the power divider loss.



Post subject: Re: Coherent and non-coherent power combiningPosted: Mon May 11, 2009 10:38 am


Joined: Thu Sep 25, 2003 1:19 am

Posts: 50

Location: texarcana

If you combine 10dBm tones at different frequencies, there will be 3dB of loss for each tone. 7dBm each tone.

Using a broadband power meter will measure 10dBm.

(disregarding the internal losses)


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