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high power amplifier linearity

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saba

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Hi,
I am learning about linearity improvement of wideband high power RF amplifier.
I would be glad to find out person who interested to interchange information on this topic.
As an start I would like to know more about practical aspects of "Two Tone Test" which is ,aimly used for finding some linearity benchmarks of wideband Amps.

Regards,
SABA :grin:
 

odyseus

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The two tone test consists of two signal sources set to the same power and are fed to a 3dB power combiner (Eg Wilkinson) and a stepped attenuator..
The attenuator is initially set high so that the amplifier is well into it's linear region. Then monitoring the outputs on a spectrum analyser you record the two injected tones,and the 3rd and sometimes 5th intermodulation products that will eventually appear either side of the two tones.
You then plot input power against output power,ploting one of the output tones the IM3 & IM5 output signals against the input signal.
You'll then have a linear graph (gradient is the linear gain of the amplifier) that eventually saturates (horizontal trace).
The input power that causes the linear gain to drop by 1dB is known as the 1dB compression point. The IM3 linear region will have a gradient of 3 and the IM5 linear region a gradient of 5.
If you extrapolate the all the linear regions you will find they intercept at a particular point known as the intercept point and is typically 10dB'd above the 1dB compression point....
 
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m229

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the two tone test is an almost universally accept method of assessing amplifier linearity and can illustrate both amplitude and phase distortion present in an amplifier
for most information refer to book linearity high power amplifer
I thank you for choise that topic
Best Regards

_________________


<font size=-1>[ This Message was edited by: m229 on 2001-07-29 15:48 ]</font>
 

vanderspunk

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Beware that a two-tone measurement cannot completely predict the output spectrum for wideband signals. It cannot for example replace an ACPR (Adjacent Channel Power Ratio) measurement, if ACPR is of any concern to you.

There was an interesting article in the May issue of MTT, titled Prediction of a CDMA output spectrum based on intermodulation products of two-tone test, where the authors try to give an upper and lower limit on the spectrum, based on a two-tone measurement.

<font size=-1>[ This Message was edited by: vanderspunk on 2001-07-29 15:09 ]</font>
 

dobricic

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Hi
Beside 2 tone test in TV tehniques is very usual using of 3 tone test which much better simulate real conditions and predicts results.
First signal simulate vision carrier Fv, second tone simulate sound carrier Fs, and 3rd colour carrier Fc. Their levels may differ from test to test but one of the very usual is:
vision carrier -8db (simulating negative amplitude modulated signal), sound -10 dB (sometimes -13dB) and colour carrier -16 dB.
One of most problematic distorsion is Fv+Fs-Fc which is generated inside used band around 1 MHz apart of Fv giving so called "moire" effect on TV screen.
Depending of WB amplifier aplication it is possible to conduct many other different test of linearity, and some of them are already mentioned by other guys.
Best regards
DD
 

R110

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Hi
Do any body know what kind of test is usual for digital transmitter witch uses more that two carries ( like OFDM based transmitters)? best wishes

<font size=-1>[ This Message was edited by: R110 on 2001-10-20 09:46 ]</font>
 

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