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Design >> RF Design >> a question about the PSD of UWB
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Message started by Lilylily on Sep 24^th, 2008, 7:28pm

Title: a question about the PSD of UWB
Post by Lilylily on Sep 24^th, 2008, 7:28pm

Hi, I have a problem when I design a PA used for UWB transmitter.According to the FCC limits,the PSD of UWB system is below -41.3dBm/MHz.If I want to design a PA ,does that mean the output power should be -41.3dBm?Thanks

Title: Re: a question about the PSD of UWB
Post by didac on Sep 25^th, 2008, 12:05pm

Hi,
If I recall correctly the calculation is as follows:
P_radiated=-41.3-10*log10(1e6)+10*log10(BW).
If you are using the 7.5GHz full bandwidth the number it's:-2.549dBm, if it's the Multi-Band approach(and if they didn't change the BW of 528MHz since last time I checked it):-14.073dBm.
This numbers doesn't take into account other factors so you must check the on/off times of the transmitters and system specific details like if it's impulse radio without time-hopping it will produce spectral spikes that could sent the signal out of the mask, the type of antenna and this kind of things.
Hope it helps,

Title: Re: a question about the PSD of UWB
Post by Lilylily on Sep 26^th, 2008, 6:41pm

Hi,Didac,I appreciate your reply very much!But There is still something confusing me! I plan to design a class-C PA. I think the power is too small to specify the match load resistor. And how can I ensure that the output power of -2dBm distribute over the total band and the PSD is below -41.3dBm simultaneously. Do I need to specify a central frequency?
Please forgive me for so many problems.I'm really a green hand.Thanks!

Title: Re: a question about the PSD of UWB
Post by didac on Sep 29^th, 2008, 6:50am

Sorry for the delay in the answer. My experience with impulse radio UWB is that it's better to work in time domain with the pulse that you are going to use in reality, it's complicated to translate narrowband specs that assume the use of a carrier to a carrierless system. So the best testbench for design I think it's to use your impulse signal(generated from the modulator or feeded with a pwl source) and your circuit, use transient simulation and for post-processing I prefer the use of Matlab and the PSD estimators that come with it.
Hope it helps,

Title: Re: a question about the PSD of UWB
Post by Lilylily on Oct 2^nd, 2008, 1:58am

sorry for delay!Didac,I think my problem is how to design a widband PA!I don't know how to balance the bandwidth and resistor matching!
Thank you!very much!You have been a lot of help!

Title: Re: a question about the PSD of UWB
Post by didac on Oct 2^nd, 2008, 6:53am

Hi,
I'm glad it helped a little bit. Just few ideas I could think right now:
-if you have access to ADS it has an impedance matching wizard that could be helpful(other tools like Genesys have it also,Ansoft Designer or Microwave Office I'm pretty sure that they also have this tool but I don't have this programs installed at this moment).
-check ieee for modified butterworth/chebyshev matching network-->they are based in the polynomial filters but with unequal load/source impedance.
-a couple of references that I have at hand:
*"Microwave Filters,Impedance-Matching Networks,And Coupling Structures",George L.Matthaei,Lee Young,E.M.T Jones, Artech House(I don't know the availability of this book I have a copy from 1980...), I found it a very good book on filters and matching networks, although a bit "hard" to read. I also remember founding in IEEE a paper from Matthaei that deals with the lumped ladder matching network but I cannot find it right now, I will search it.
*"Broadband Microwave Amplifiers",Bal S.Virdee,Avtar S. Virdee, Ben Y.Banyamin,Artech House. The problem is that it deals basically with travelling wave amplifiers,multistage ones...
*I remember than in "Microwave Engineering" from Pozar reviews the concept of broadband matching, but I think that only with transmission lines transformers, and give some references.
Hope it helps,

Title: Re: a question about the PSD of UWB
Post by Lilylily on Oct 5^th, 2008, 6:29pm

Hi,Didac,I appreciate your kindness very much!You give me a lot of help!I'll search these books as soon as possible!