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# Description of linear characteristics of microwave power amplifier

by：Gisen
2020-01-02

Among the many technical parameters of microwave power amplifier, two are very important, one is output power and the other is non-linear distortion.
For an ideal linear power amplifier, its input-output response should show a linear relationship, that is, the output should be a linear function of the input, while the actual power amplifier deviates from this linear feature and presents a nonlinear relationship, this is nonlinear distortion. Nonlinear distortion has different description methods. Commonly used are (1)1 dB compression point (P1dB)(2)Third-order intermodulation coefficient (IP3)(3)Amplitude modulation-phase modulation (AM-PM)Conversion and amplitude modulation-Amplitude Modulation (AM-AM)Conversion etc.
These indicators can describe the nonlinear distortion characteristics of microwave amplifiers individually or simultaneously as needed. (1)
1dB compression Point, near the receiving point of the power amplifier input and output characteristics, when the signal power gain drops by 1 dB, the corresponding output power is called 1dB compression point power P1dB.
When the output power exceeds that point, the amplifier will quickly enter the saturated workspace. (2)
The third-order AC modulation coefficient, AC modulation distortion is a hybrid component generated by two or more input signals with different frequencies passing through the power amplifier, which is also caused by the non-linearity of the power amplifier.
The stronger the non-linearity of the power amplifier, the greater the AC component.
Assuming that two signals with frequencies f1 and f2 are input in the operating frequency band of the amplifier, due to the nonlinear nature of the amplifier, in addition to the frequencies of these two input signals, there are also their harmonics and frequency difference between harmonics, mf1, nf2, mf1nf2, etc, the ratio of the component with frequency 2f1-f2 or 2f2-f1 to the component of fundamental wave f1 or f2 is called the third-order alternating modulation coefficient ip3.
Since the frequency of the third-order AC component in the AC component of each order is very close to the frequency of the fundamental wave signal, it is impossible to filter it out directly from the Channel, the third-order AC component becomes the main interference signal.
Therefore, the third-order alternating modulation coefficient becomes an important indicator to measure the linearity of microwave power amplifiers. (3)AM--PM conversion and AM--AM conversion definition and nature.
For a single carrier, due to the non-linearity of the transmission channel, the channel gain is compressed and harmonics are generated. The amplitude and phase of the harmonics are related to the amplitude of the input signal, so that the amplifier gain is related to the amplitude of the input signal, so that the phase and amplitude of the output signal change with the amplitude of the input signal, and the AM-PM is defined as the ratio of the phase change of the output signal to the amplitude change of the input signal.
AM-AM is defined as the ratio of the amplitude change of the output signal to the amplitude change of the input signal. AM--
PM is the direct expression of gain compression. The more gain compression, AM--
The stronger the PM effect, after the circuit enters the saturation zone, AM--PM effect will be very serious.

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