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Power for MCM and Continous Wave - RF Cafe Forums

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venkataravi
Post subject: Power for MCM and Continous Wave Posted: Wed Aug 30, 2006 7:38 am

Captain

Joined: Thu Jul 27, 2006 7:36 am
Posts: 11
Location: Bangalore --India
Hi.


If the CW power of a Power amplifier is 3 KW. What will be its MCM power .

Iam of the opinion of following two approaches

1. MCM power is 1/3 of CW power

2. MCM power is 3-5dB down of CW power inorder to keep IMD in limits


Please correct

Regards

Ravi


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IR
Post subject: Posted: Tue Sep 05, 2006 2:25 am

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hi Ravi,

the MCM is dependant on several factors: Number of carriers, modulation scheme, filter used (e.g. Root Cosine...), these all determine the CCDF of the signal (Distribution of peaks) and hence the Peak to Average Ratio (PAR). So the peaks will be at the P1dB power of the amplifier, but the average power will be of course lower and will be determined by the parameters listed above. In order to give an absolute answer to the output power of your PA, you need to provide more information like:
# of carriers, modulation scheme etc. By the way there is a SW from Rohde & Schwarz called WINIQSim (I think it is a freeware), which can calculate the PAR and CCDF based on these paramaters.

Hope this helps!

_________________
Best regards,

- IR


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venkataravi
Post subject: Posted: Tue Sep 05, 2006 2:49 am

Captain

Joined: Thu Jul 27, 2006 7:36 am
Posts: 11
Location: Bangalore --India
Hi Itay,

1. Modulation is OFDM iam not sure about the filter mostly it will be cosine.

2. Regarding number of carriers it can vary but the total bandwidth is 2.3 Mhz.


With Regards

Ravi


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IR
Post subject: Posted: Tue Sep 05, 2006 2:54 am

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
OK. This gives some light.

From my recent experience with OFDM signals (Up to 256 carriers), the PAR can reach up to 10dB, so your average output power will be around 300W or +55dBm. The accurate output power however is still dependant on the exact setting of your application.

_________________
Best regards,

- IR


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venkataravi
Post subject: Posted: Tue Sep 05, 2006 3:11 am

Captain

Joined: Thu Jul 27, 2006 7:36 am
Posts: 11
Location: Bangalore --India
Hi IR,

Now iam having the exact information. Iam having 2 Bands, A and B with A taking care of 17 Channels and B taking care of 23 channels.


For a Terrestrial Broadcast i require MCM with seperate ( Amplifier + Txm) for each band.

In order to have a 1 KW Txm i will be opting for 3.2 KW which will take care of 5dB backoff.

1.Can you suggest me as a thumb rule for deciding MCM depending upon CW power.

2. How the number of carriers will decide the performance wrt to IMD

With Regards

Y.V.Ravi


Top

IR
Post subject: Posted: Tue Sep 05, 2006 4:32 am

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
The number of carriers will increase your PAR. So when you know your PAR you can tell the maximal average output power: P (CW)-PAR.

On top of that in order to to meet the linearity requirements, you will have to back-off your input power. I worked on commercial applications and there you back-off the input power by 3-5dB. Take into consideration that this will degrade your efficiency - I don't know how much this plays role in your spec.

_________________
Best regards,

- IR




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