Compression point

The compression point is used for the quantitative description of the non-linear behavior of an active element ( e.g. an amplifier ) e.g. B. in audio components or in high frequency technology . It indicates the greatest amplitude of the input signal at which the distortions caused by non-linearity do not yet exceed a specified level.

The cause of the non-linear behavior is the saturation that occurs in every amplifying component ( transistor , electron tube ) above a certain input amplitude. The designation compression point indicates that the saturation always leads to a smaller, ie "compressed" (compressed) output signal compared to the linear case.

definition

The 1 dB compression point , often referred to as P1dB , is that value of the input amplitude at which the power of the output signal at the fundamental frequency deviates by 1 decibel from the ideal linearly extrapolated characteristic of the component

${\displaystyle P_{1\mathrm {dB} }=10\log \left({\frac {P_{\mathrm {real} }}{P_{\mathrm {ideal} }}}\right)=-1.}$

( and are linear quantities here.) ${\displaystyle P_{\mathrm {real} }}$ ${\displaystyle P_{\mathrm {ideal} }}$

In data sheets the P1dB is given as input power in dBm . In addition to the 1 dB compression point, you can also use any other compression point, e.g. B. Define P3dB for a deviation of 3 dB.

Calculation of the amplitude

Schematic representation of the P1dB

The characteristic of a non-linear amplifier can be expressed in a Taylor series

${\displaystyle U_{\mathrm {out} }=U_{0}+\alpha \cdot U_{\mathrm {in} }+\beta \cdot {U_{\mathrm {in} }}^{2}+\gamma \cdot {U_{\mathrm {in} }}^{3}+...}$

describe.

In the 1 dB compression point, the following applies

${\displaystyle -1=20\log {\frac {\alpha \cdot U_{\mathrm {1dB} }+{\frac {3}{4}}\gamma \cdot {U_{\mathrm {1dB} }}^{3}}{\alpha \cdot U_{\mathrm {1dB} }}}.}$

${\displaystyle U_{\mathrm {1dB} }}$ is the amplitude at the 1 dB compression point

The following results are changed:

${\displaystyle U_{\mathrm {1dB} }={\sqrt {(10^{-{\frac {1}{20}}}-1)\cdot {\frac {4\alpha }{3\gamma }}}}}$

or logarithmic:

${\displaystyle A_{\mathrm {1dB} }=20\log \left({\sqrt {(10^{-{\frac {1}{20}}}-1)\cdot {\frac {4\alpha }{3\gamma }}}}\right)=10\log \left({(10^{-{\frac {1}{20}}}-1)\cdot {\frac {4\alpha }{3\gamma }}}\right)}$

literature

• Otto Mildenberger (Ed.): Information technology compact. Theoretical foundations. Friedrich Vieweg & Sohn Verlag, Wiesbaden 1999, ISBN 3-528-03871-3 .
• Hans Dodel, René Wörfel: Satellite frequency coordination . Regulations - link design - system technology. Springer Verlag, Berlin / Heidelberg 2012, ISBN 978-3-642-29202-6 .
• Johann Siegl, Edgar Zocher: Circuit technology - analog and mixed analog / digital. 5th edition. Springer Verlag, Berlin / Heidelberg 2014, ISBN 978-3-642-29559-1 .
• H. Meinke, FW Gundlach: Pocket book of high frequency technology. Volume 3: Systems. 5th edition. Springer Verlag, Berlin / Heidelberg 1992, ISBN 3-540-54716-9 .
• Frieder Strauss: Basic course in high frequency technology. An introduction. 2nd Edition. Springer Verlag, Berlin / Heidelberg 2016, ISBN 978-3-658-11899-0 .
• Manfred Thumm, Werner Wiesbeck, Stefan Kern: high frequency measurement technology. Procedures and measuring systems. 2nd Edition. Springer Fachmedien, Wiesbaden 2008, ISBN 978-3-519-16360-2 .

See also

• Intercept Point

Web links

• Description of the 1-dB compression point on page 8 (PDF file)
• Description of the 1 dB compression point on page 9 (PDF file; 76 kB)
• Description of the 1-dB compression point on page 37 (PDF file; 900 kB)