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AMP_B 3rd Order AM/AM Slope
This project demonstrates issues with the 3rd Order AM/AM curve when using the AMP_B or the AMP_B2 models. A common question is why is the slope not equal to 3.
The Amp Test System Diagram is used to demonstrate the use of the fundamental and 3rd order intermodulation measurements. It contains the AMP_B model and the VNA. The amplifier's input power is swept from -30 to +16 dBm.
The OIP3 vs. Input Power graph displays the OIP3 shown to be constant up to roughly the Input 1 dB compression as expected. It uses the IPn measurement.
The Signal and Distortion vs. Input Power graph shows the fundamental signal and the IM3 product. Note the effect of the noise floor at the lower power values. Noise can be disabled (set to Noiseless) using the NOISE parameter of the amplifier or the "RF Noise Modeling" setting in the System Options dialog box if the block's NOISE parameter is set to "Auto". When you turn off the System Option, the IM3 slope be 3 at the lowest powers.
The Slopes graph shows the slope of the fundamental and 3rd order intermodulation products. They are calculated by using the der() function in the Output Equations document.
When the IP3 parameter is around 10 dB (or lower) above the P1dB parameter, then you can expect a reasonable slope of 3 in the linear region of the amplifier. If this slope is not 3, the typical cause is IP3 is much more than 10 dB above P1dB. What is happening is the amplifier has a fairly small linear region, ending far below P1dB. This is because of the effects of the 3rd and 5th order terms of the polynomial, which are relatively large in order to model the specified IP3 and P1dB.
System Diagram - Amp Test
Graph - Signal and Distortion vs Input Power
Graph - OIP3 vs Input Power
Graph - Slopes