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Design >> RF Design >> Distortion in sine wave at different nodes of lna
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Message started by s_ss on Feb 28^th, 2013, 2:58am

Title: Distortion in sine wave at different nodes of lna
Post by s_ss on Feb 28^th, 2013, 2:58am

Hello,

I am designing a wideband common gate lna. I have checked the s parameters, nf , etc all are ok but when i checked the waveforms at the various nodes of the lna, i did not find exact sine waves? Can someone explain what is the cause of this? Is this because of phase delay ? and how to trouble shoot such errors? I have inductors in my design so I was thinking that this may be the cause of distorted wave?

Can someone help? I can provide more details if someone needs?
thanks

Title: Re: Distortion in sine wave at different nodes of lna
Post by Ken Kundert on Feb 28^th, 2013, 12:54pm

A linear system driven with pure sinusoids will eventually, after all transients decay away, respond only with pure sinewaves. If you are in steady-state and are seeing distorted sinusoids, it is due to a nonlinearity in the circuit. Inductors are generally pretty linear and so are unlikely to be the root cause of the problem.

-Ken

Title: Re: Distortion in sine wave at different nodes of lna
Post by loose-electron on Mar 20^th, 2013, 4:52pm

Suggest that you look at some papers and textbooks on RF LNA ***linearity***

Keywords to search: rf amplifier linearity, IP2, IP3, Linearity LNA

Plenty of papers out there on the subject.

Title: Re: Distortion in sine wave at different nodes of lna
Post by s_ss on Apr 28^th, 2013, 11:17pm

Thanks a lot for your replies. Now I understand that its the linearity which is responsible for the distortion.

My confusion actually is....

1)As far as I know, LNAs are usually designed as class A amplifiers..... So I expect a sine wave at input and a sinewave at output. Now because of lineairty if the wave forms at different nodes appear distorted or the wave totally looses its shape (no more sine/ very thick positive part and thin negative part) and S parameters including NF and gain are excellent. Then can we report this LNA to be working fine??

2) In my design S parameters are very good especially NF for which I wanted to report my work but looking at the transisent waveforms which do not remain in sine wave shape, leaves me confused. Although the waves are badly distorted, the transistors are all in saturation.

If some one has some idea please reply soon.

Title: Re: Distortion in sine wave at different nodes of lna
Post by RFICDUDE on Apr 29^th, 2013, 4:55am

Hi s_ss,

The linear results, S and NF, do not account for the nonlinear performance of your design. S and NF analysis assume the circuit is perfectly linear around the bias point the circuit is operating at. In addition to S and NF, you need to run large signal simulations that measure the nonlinear performance of your design.

A LNA design is not complete without measurements of the nonlinear performance. Nonlinear metrics like the third-order intercept point are commonly used to quantify the nonlinear performance of LNA designs.

However, just simulating the performance does not mean very much unless you understand what performance goals your design is supposed to meet. If there is no expectation for the linear performance (no specification or goal) of your design then the nonlinear performance has no meaning, but as a practical matter, any communication system with interfering signals present the nonlinear performance matters.

Does your project have a requirement, or are you just trying to learn what is important for the design you are working on?