How to plot IP3 in LTSpice

[Minchuu]

Hello,

I have been unsuccessfully searching on how to plot the third order intercept point or IP3 in LTSpice. I know that its the output power vs the input power both on log scale. However, I can't find my way in LTSPice, as I am still a newbie. My schematic is attached.

Any help would be much appreciated. Thanks.

 

[FvM]

Non-operational LNA circuit, drain power supply missing.

LTspice forum https://groups.io/g/LTspice has some hints on IP3 measurement. Generally, you need to perform a large signal measurement with stepped input level and use an appropriate method to measure third order IM products or thir harmonic, e.g. FFT.

 

[Minchuu]

Non-operational LNA circuit, drain power supply missing.

View attachment 170506

LTspice forum https://groups.io/g/LTspice has some hints on IP3 measurement. Generally, you need to perform a large signal measurement with stepped input level and use an appropriate method to measure third order IM products or thir harmonic, e.g. FFT.

Hello,

Thanks for the reply. May I ask what does drain power supply missing mean? Sorry, I am new and this is all of the components I see in resources so I do not know what drain power supply to add. How can I fix this LNA circuit? It's supposed to be within the BW 0.65MHz - 1.45 MHz with input and output impedance of 50 ohms, NF of less than 3dB, power gain ~10sB, and IIP3 of greater than -0.5 dBm.

Thanks in advance.

 

[FvM]

No idea how you arrived at this circuit. How can a MOSFET amplify without drain current? You want e.g. a choke connecting node "M2_drain" to a supply voltage. Gate bias voltages most likely need adjustments, gate series inductor of 0.045 H makes no sense.

 

[Minchuu]

No idea how you arrived at this circuit. How can a MOSFET amplify without drain current? You want e.g. a choke connecting node "M2_drain" to a supply voltage. Gate bias voltages most likely need adjustments, gate series inductor of 0.045 H makes no sense.

I followed this lecture with a mix of this schematic resource. I also attached my computations based on these resources. I'm really sorry, our instructor did not really teach us anything about this design- and computation-wise, so we are going in blind with this. It would be much appreciated any help. Thanks in advance.