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nonlinear model for a microwave FET such as NE72218

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Newbie level 6
Aug 16, 2007
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nonlinear fet models

How can I allocate nodes 1,2,3 for a non linear model of a microwave FET using MWO? One way that is acceptable to MWO is to use ports but what should the impedance of each port be 50 Ohms or the resistance of the corresponding gate, drain and source resistance?
Please help.

+microwave +fet

The port impedance is irrelevant when using a subcircuit in a heirarchy. The port impedance is only used when running a simulation directly on that subcircuit's schematic. Just leave it at 50 ohms and it will be fine when simulating in a circuit.


Thanks. I was on a bus that wnet past Kansas City once!
What I am doing is to repeat the non linear analysis of a DRO in the application note of California Eastern Laboratories AN1035. I think I get the resonance curve and the phase OK, but when I connect the OSCAPROBE to get the frequency and the harmonics, I get the message; 'failed to detect start-up conditions in the specified frequency range'
I widen then naroow the bandwidth but still get the same message.
Can someone offer me a step by step procedure to get OSCAPROBE to work properly in this case.
Please help.

fet microwave detect

I think this means you have to tune your circuit.
Startup condition for oscillations are
1) loop gain must be>=1
2) total phase shift must be 2Π

You may have to re tune your circuit.

schematic nonlinear model fet

Attach your MWO file and maybe we can look at it.

ne72218, replace

madengr and abhishebabs,
I believe that I tune the oscillator to have a mag.>1 and phase=zero.
I think that I may do something wrong with the setting up the simulation. I hope that you can shed some light on my shortcomings.
Many thanks.

You were not getting a real, negaitve resistance looking into the gate so I tuned TL8 and TL4 to fix that. It now osciallates with 7 dBm at 10.2 GHz, which is of course below the 11.2 GHz of the DRO due to loading.

Look at the MWO examples and they break the problem into gain stage and resonator, analyzing them independantly using different test benches Putting everything on one schematic is tough since there can be several things wrong.

Actually I'm wrong. It oscillates at 10.2 GHz but that's a different mode. The resonator is not having effect here.

Added after 4 hours 34 minutes:

OK, this one works. That circuit in the appnote has too much crap on it (as in distributed elements). What the app note does not show is that all those 1/4 chokes form an additional -R down at 4 GHz which messes up the circuit. I removed most of it and it's simplified now. Breaking up the resonator and Negative Resistance Amplifier into two subciruits makes it easier to look and loop gain and free-runing oscillation. Now I'm not sure why the loop gain peaks at 180 deg instead of zero, but the oscillation mode is definetly the resonator this time.

Thank you. You are great. I download the file but it does not seem to show anything in my MWO which is a version 2002. Do you any reason for not being able to open it?
Best regards,

Added after 3 hours 33 minutes:

I am happy because I have had glimpses of it working.
What I have achieved has been to run the early version of the file i.e. and have had conversion at a few frequencies like 10.2 GHz, 14 GHz, 16 GHz even though the fend is 12GHz. Enough to give me comfidence that with your valuable help it will work.
However I could not make any sense out of the second file: It has two files, one under and the other is latter can not be opened. The former shows different circuits for the DRO but I can not run any of them. The file is looking for data.
During uploading all files seem to be jumbled up. For instance the OSCAPROBE becomes horizontal and imbedded with the trnasmission line. I replace it with the one on my MWO that has an oscillator symbol but this did not help.
I wonder if you could send me all files to my e-mail addres?
I still have a lot to learn and what you have done has eased a lot of the brainache in my head. I am getting there.

Added after 58 minutes:

I forwarded the two files to my email but they were the same.
Could you kindly send me copies of the files without zipping? my e-mail address
Best regards,

The files are not being corrupted. I have MWO version 7.5 (2007) and you have 5.x (2002). I saved the file in two different versions for you. I think there must be problems trying to save in such an old version.

Try these and see if it works.

Here it is in PDF. Yoiu can re-create it from this. Notice that each simulation uses different frequency ranges; you need to set this in documnent frequencies instead of project frequencies.

Many thanks for all your problem.
I will see if I make it work today. I had glimpse of it working albeit at not the correct frequency.

I have got the DRO to work @11.2GHz. The quality you suggested seems very high being R=50,000, when I reduce R to 5000 as it should, I experience non convergence! Is there anyway to overcome this problem?
One change I would like to make is to change the frequency to 10.5 GHz with L=0.00535nH and C=42.72pF, without any changes made to the microstrips, I can only have a frequency of 11.8 GHz. Pls help me reduce the freuency to 10.5 GHz.

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