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# oscillator simulation in ADS

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Dear Cray,

> Under what circumstances does the negative resistance approach fail?

First fails because the start up condition is guaranteed by a S21>0dB at the zero crossing of the ang of S21. A positive mag of S11 is a necessary condition but not sufficient. In addition the phase noise got from the real circuit is unpredictable and generally poor.

>....to guarantee the start up conditions. A negative resistance at the input terminal doesn't means your oscillator will necessarily start up. You need to analyze S21 in open loop instead.

See above.

> why a negative resistance at the input terminal doesn't mean the oscillator will start up? isn't the startup can trigger by surrounding noise?

Yes, but you have to have enough gain to sustain the oscillation. And the negative resistance approach doesn't offer this kind of analysis.

>...to predict the loaded Q of the resonator used and so to estimate the phase noise. The loaded Q is readily available analyzing the group delay of S21 in open loop.
how to find loaded Q from group delay of S21?

Ql=(W*Td)/2 => Please, refer to the book by Rhea (Oscillator Design and Computer Simulation)

>If you try to use the open loop approach instead of the negative resistance approach you will get an oscillator with a reasonable agreement with your simulations.
What simulation you use to compare with your measured result? transient or harmonic balance?

Normally I use time domain simulation

Last words:
By using the virtual ground approach described in previous post you are able to analyze any topology using the open loop even if you are using strays or intrinsic properties of the active device.
Please refer to the articles mentioned in previous post.

NandoPG

### tony_lth

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### MOHSEN15

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thanks nandopg,boy,
i finally understand your analysis nandopg, the discussion was very helpful indeed. my school library doesn't have the following book "Oscillator Design and Computer Simulation" by Rhea, does anyone has the ebook? and paper on "Analysis Method Characterizes Microwave Oscillators" by S. Alechno

cray

MOHSEN15

### MOHSEN15

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Dear Cray
the book by rhea can be found at
**broken link removed**

I dont have those articles.

Verification with transient simulation is always needed to ensure oscillation. Osc-port under HB inside @ds is recommened for initial design.

RC13 said:
Verification with transient simulation is always needed to ensure oscillation. Osc-port under HB inside @ds is recommened for initial design.

hi RC13,
i discovered that when we use an ideal "DC-block" element in ads, the transient simulation doesn't show the oscillation waveform(it gives a DC waveform at steady state). when i replace it with a capacitor (for DC-blocking). both the HB and transient shows oscillation occur. anyone has similar problem?

p/s: thanks boy for pointing me to the osc book

thanks
cray

I still have a problem to include intrinsic and parasitic into the feedback loop. Any idea? For low frequency ( say hundreds MHz), the result is ok but for 10 GHz, the result from HB, and openloop sinificantly disagree.

boy said:
I still have a problem to include intrinsic and parasitic into the feedback loop. Any idea? For low frequency ( say hundreds MHz), the result is ok but for 10 GHz, the result from HB, and openloop sinificantly disagree.

do you mean that u transistor model was a bare chip model? and u want to include the parasitic element? then, u need to optimise ur transistor model (plus ur self-added parasitic element) with measured s-parameter. At high frequency, parasitic effect cannot be neglected.

cray

Hi guys, this is the first part os Alecno's paper.

Analysis Method Characterizes Microwave Oscilators. The rest is on the way. ( My libraian said so )

tony_lth

### tony_lth

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so much useful informations in this topic,thank you all

Added after 1 hours 10 minutes:

would anyone share the book: Analysis Method Characterizes Microwave Oscillators, please

MOHSEN15

### MOHSEN15

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Hello....i want design a ring oscillator by using ads .....but i canot....please guide me....if every ones have paper or article please send to me

iranutiran@yahoo.com

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