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Design >> RF Design >> AM_PM Conversion, Power Amplifiers
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Message started by Faisal on Oct 31^st, 2007, 11:03am

Title: AM_PM Conversion, Power Amplifiers
Post by Faisal on Oct 31^st, 2007, 11:03am

Hi everybody,

I am working on narrow band power amplifiers. I make a AM_PM plot in SpectreRF using a PAC modulated analysis. But I have been unable to understand its significance/utility? (I am sweeping frequency from the carrier center-frequency to some offset around)

Kindly explain a bit in detail, i am a newbie in RF

Title: Re: AM_PM Conversion, Power Amplifiers
Post by didac on Oct 31^st, 2007, 1:11pm

Hi Faisal,
You should think in terms of digital modulation. Consider a 16-QAM modulation, you have the 16 sixteen dots with their detection bounds clearly defined, if you have an excessive AM to PM conversion one of the symbols can drift to a position near the bound of their neighbour, in other words you are starting to degrade BER of the system just in the transmitter and you are reducing the margins for fading and noise that would be added by the channel and the receiver.
Hope it helps,

Title: Re: AM_PM Conversion, Power Amplifiers
Post by Faisal on Nov 1^st, 2007, 3:00am

Hi Didac,

Thanks for the reply. I have a binary PSK modulation i.e. constant envelope and its narrow bandwidth too. How to relate the results of the PAC modulated analysis AM-PM or PM-PM with the theoretical calculations in the text books

Kind Regards,
Faisal Mateen.

Title: Re: AM_PM Conversion, Power Amplifiers
Post by didac on Nov 1^st, 2007, 10:01am

Hi,
Long time since I've performed this kind of things but If my memory not fails the procedure is the following(please double check with other sources):
1)From data of AM-AM,AM-PM calculate Error Vector Magnitude(EVM).
2)Convert EVM to SNR(I think that the relationship is something like EVM=1/sqrt(SNR))
3)Convert SNR to Eb/No given you bandwidth and data rate(Eb/No=SNR*BW/Rb)
4)Apply the Eb/No to the standard equation and compare the new BER with the theoretical BER without AM-AM,AM-PM errors.
Hope it helps,
PD:as I said please check with other sources, I've did this things time ago but I'm not working a system level engineer...
PD2:I think that in Matlab simulink(Commstoolbox) exist a block that enables easy simulation of this things http://www.mathworks.fr/access/helpdesk/help/toolbox/commblks/ug/bp863a1-2.html]http://www.mathworks.fr/access/helpdesk/help/toolbox/commblks/index.html?/access/helpdesk/help/toolbox/commblks/ref/memorylessnonlinearity.html&http://www.mathworks.fr/access/helpdesk/help/toolbox/commblks/ug/bp863a1-2.html[/url].

Title: Re: AM_PM Conversion, Power Amplifiers
Post by RFICDUDE on Nov 2^nd, 2007, 5:34am

If you have a perfect constant envelope signal then AM-PM probably doesn't matter much because there is no AM in the input signal. However, the modulator before a PA may introduce a small amount of AM due to the imperfect amplitude balance, phase balance, and nonlinearity in the modulator (this can convert PM-AM). If the input signal contains enough AM then the AM-PM of the amplifier will interact with the signal plus AM to generated more distortion at the output. The end result is spectral regrowth and worse phase error since now you get extra deviations in phase which are dependent on the instantaneous amplitude variation.

I don't think simulating AM-PM with PAC analysis is the 100% correct thing to do. You need the large signal results to obtain an accurate representation of the AM-PM characteristic. So, you might try measuring the AM-PM of the carrier signal from the PSS simulation. Just measure the large signal complex gain from input to output while sweeping carrier power.

Here are some references that may be helpful to you

Jones, A.E.; Wilkinson, T.A.H.; Gardiner, J.G., "Effects of modulator deficiencies and amplifier nonlinearities on the phase accuracy of GMSK signalling," Communications, Speech and Vision, IEE Proceedings I , vol.140, no.2, pp.157-162, Apr 1993

Hyunchol Shin; Walker, B.; Dongling Pan; Dunworth, J.; Jaffee, J., "Analysis of spectral spreading in a phase-modulated system for 1.75-GHz GSM RF transmitter design," Custom Integrated Circuits Conference, 2003. Proceedings of the IEEE 2003 , vol., no., pp. 103-106, 21-24 Sept. 2003

Title: Re: AM_PM Conversion, Power Amplifiers
Post by Faisal on Nov 5^th, 2007, 9:11am

Hi

Thank you RFICDUDE for the explanation and the links.