problem with spectrerf!

I have the following question:

If I do iip3 simulation for an LNA and after choosing the output, the iip3 is -3dBm for example.

If I choose the net of the input as the output for iip3 calculation, it will be -3dBm(ideally it should be infinity), Why does this happen ?

I use PSS followed by PAC for simulation.

thanks in advance,

Mohamed Abouzied

mohamedabouzied said:

If I choose the net of the input as the output for iip3 calculation, it will be -3dBm(ideally it should be infinity), Why does this happen ?

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I guess SPECTRE doesn't accept your choice and still relates to the LNA output. Not sure, however.

Hello
That's not SPECTRE misunderstanding. IIP3 for input would be infinity if your LNA were unilateral device - without any feedback.
In real life, there are gate-drain or collector-base capacitance - so harmonics from output pass to input - so you'd never get infinite iip3 at input point. It would be little less then for output point.

erikl: So how can I tell him about my new output?
Pavel: yes, but with that reverse leakage, all non linearity will be fed back? I say approximately the same IIP3?

mohamedabouzied said:

Pavel: yes, but with that reverse leakage, all non linearity will be fed back? I say approximately the same IIP3?

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Difference between iip3 at input and output would vary from one circuit to another.
It's obvious that for circuits with strong feedback this difference would be small - almost the same iip3, as you said.

pavel_adameyko said:

Difference between iip3 at input and output would vary from one circuit to another.
It's obvious that for circuits with strong feedback this difference would be small - almost the same iip3, as you said.

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What do u mean by strong feedback?
By the way, my operating frequency is 2GHz and the transistors length is .13um. So, its capacitances are not a big issue in this frequency range.
thanks.

Strong feedback - which you create by yourself - for example, shunt feedback.
But my experience show that gate-drain capacitance is usually enough for intermodulation harmonics appear at the input.
By the way, there are some methods like low-impedance input terminations for BJT amplifiers that improve iip3.

1st my LNA is common gate(which your talking also applies on it)
and the question is: does this degrade iip3 total, or just the IIP3 of the LNA is dominant?
And also, is this a simulation issue or when i measure at the input of LNA, its IIP3 will be that degraded value?
thanks

Why do you measure iip3 by pss and pac analyses? Usually mixers are simulating by this way. Why don't you use simple tran analysis.
write me p.m. with explanation of structure of your LNA and pls clarify your question a bit more.

mohamedabouzied said:

... all non linearity will be fed back? I say approximately the same IIP3?

Click to expand...

pavel_adameyko said:

Difference between iip3 at input and output would vary from one circuit to another.

Click to expand...

This triggered an idea: you could make sure by comparing with a totally linear network instead of your LNA, e.g. an ideal resistor network?

I have done this,
Because the resistor is linear, the 3db/dB is not present.
Let the input freq.s, f1, f2
while sweeping the RF input level, and drawing f2 and 2f1-f2, their lines are parallel, meaning there is not IM3
(Although using IPN curves will plot 2 extrapolated lines with slopes 1dB/dB and 3dB/dB.
The latter will be defined by the extrapolation point which is by default at Pin1.)

After all, thanx.

I will explain my original problem,
I design a differential common gate LNA working at 2GHz with LC tank at output(QL = 10 low which corresponds to higher BW)
In order to account for the loading of the following mixers, I modeled its capacitance with some value.
Then, The loading mixers are 3 mixers, direct conversion Active mixers.
3 mixers due to some specs. in the system design.
To simulate the IIP3 with the mixers, I switch off the clock, i.e. clk = vdd and clk_bar=gnd, such that I get their loading on the LNA stage.
Doing this hurts the LNA linearity so much(from 4dBm to -4dBm).
After some debuging, I saw the balun issue i mentioned previuosly.

How do I measure the IIP3?
by PSS and PAC:
f1 is the PSS freq. and f2 is the PAC freq.
These freq.s are inserted in the input port which is connected to an ideal balun that transform impedance from 50 ohm to 100 ohm differential, i.e. sine wave and pac magnitude with same magnitude prf dBm respectively.
then, in PAC direct plot window, i choose ipn curves and 1st harmonic is f2
and 3rd order harmonic is 2*f2-f1.
and I choose differential nets as output and select the LNA output nets.