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Question on a VCO design with differential stage (Read 11676 times)

tumeda

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Question on a VCO design with differential stage
Sep 30^th, 2005, 6:47am

I have a problem about a VCO circuit design with differential stage architecture, its schematic is attached here.
I am wondering about why the output signal cannot oscillate ??? I have also done a PZ analysis, and found that there are positive poles in practice. Is anything wrong with my design? Any feedback is greatly appreciated.
I will try to attach the schematic here later.

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Paul Community Fellow Offline Posts: 351 Switzerland	Re: Question on a VCO design with differential sta Reply #1 - Oct 1^st, 2005, 1:16pm Hi, Maybe you should give some more information about your simulation set-up. What are your start-up conditions? In simulation you have to use initial conditions or an external stimulus to trigger the oscillation. Search this forum for more information. Paul
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tumeda Community Member Offline Posts: 37	Re: Question on a VCO design with differential sta Reply #2 - Oct 2^nd, 2005, 1:27pm -Hi Paul Thanks! This is the schematic of VCO and the PZ anaylsis result. The underleft is a voltage input, where 1V is applied. In a transient simulation, two initial conditions are set on the two nets, 1.5 V for one and 0V for anohter, as showed in the picture. could you tell what the wrong is?
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ywguo Community Fellow Offline Posts: 943 Shanghai, PRC	Re: Question on a VCO design with differential sta Reply #3 - Oct 3^rd, 2005, 8:55pm Tumeda, The image is not here. :) Yawei
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tumeda Community Member Offline Posts: 37	Re: Question on a VCO design with differential sta Reply #4 - Oct 4^th, 2005, 1:06am I have updated these pictures. Sometimes, You need still click right mouse to select the "show picture", then you can see its! Finally. I have found a free space to save picture in internet. � Thanks!
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Andrew Beckett Senior Fellow Offline Life, don't talk to me about Life... Posts: 1742 Bracknell, UK	Re: Question on a VCO design with differential sta Reply #5 - Oct 4^th, 2005, 9:46pm Tumeda, there's still no picture. When you use the img tags, you have to give a URL between the open and close tag, where the URL is a location containing the picture. The image also needs to stay at that URL otherwise people won't be able to see it. Regards, Andrew.
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Ken Kundert Global Moderator Offline Posts: 2384 Silicon Valley	Re: Question on a VCO design with differential sta Reply #6 - Oct 4^th, 2005, 9:51pm I also recommend that you use the "Preview" button to assure everything on is correct before you post. -Ken
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Andrew Beckett Senior Fellow Offline Life, don't talk to me about Life... Posts: 1742 Bracknell, UK	Re: Question on a VCO design with differential sta Reply #7 - Oct 4^th, 2005, 10:01pm Ken, Good point. I know that when I first did this, I made a mistake, and so had to have a second attempt at posting the picture. What was worse, I meant to hit the Preview button and my mouse slipped :-/ Regards, Andrew.
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tumeda

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Re: Question on a VCO design with differential sta
Reply #8 - Oct 5^th, 2005, 1:43am

Sorry for my mistakes! I have really previewed these pictures before I posted them. �But the URL address of these pictures seem to be not constant, they vary always. If you open this page, please try to click right mouse to select the "show picture", I hope you can see them. So far, I donot have a better idea of it. The direct URL address of my pictures page are given. You can direcly click the under URL to see it. Sorry again for putting you in trouble!

http://cn.pg.photos.yahoo.com/ph/notting_zj/detail?.dir=1b00&.dnm=d692.jpg&.src=...

« Last Edit: Oct 5^th, 2005, 6:32am by tumeda »

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Forum Administrator YaBB Administrator Offline Posts: 145	Re: Question on a VCO design with differential sta Reply #9 - Oct 5^th, 2005, 8:15am I apologize for the trouble you are having, but the situation will be improving soon. Sometime in the next few weeks we will be upgrading the Forum to the latest version, which supports the uploading of images and files. -Ken
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ywguo Community Fellow Offline Posts: 943 Shanghai, PRC	Re: Question on a VCO design with differential sta Reply #10 - Oct 5^th, 2005, 6:39pm Hi, Tumeda, Have you tried not to set the initial voltage on those two nodes? Yawei
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tumeda

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Re: Question on a VCO design with differential sta
Reply #11 - Oct 9^th, 2005, 1:44pm

As your suggestion, I made a simulation without any initial setup, the circuit can not oscillate yet. But I have tried to set the initial voltages at another levels, for instance, 1V and 0.5V for two nodes, the circuit can oscillate now.
But, a new problem comes up. I have set the transistor design parameters (W/L) in another values, the circuit cannot oscillate again, while there are also poles in right side. Who can tell me, is there any critiera for VCO oscillation?

« Last Edit: Oct 10^th, 2005, 1:00am by tumeda »

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ywguo

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Shanghai, PRC

Re: Question on a VCO design with differential sta
Reply #12 - Oct 9^th, 2005, 6:50pm

Tumeda,

Do you mean the simulator cannot analyze the oscillator again?

Quote:

But, a new problem comes up. I have set the transistor design parameters (W/L) in another values, the circuit cannot simulate again, while there are also poles in right side.

Would you please paste the errors and warnings on the forum?

Best regards,
Yawei

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Paul

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Re: Question on a VCO design with differential sta
Reply #13 - Oct 10^th, 2005, 12:56am

Hi Tumeda,

[quote author=tumeda link=1128088045/0#11 date=1128890675]Who can tell me, is there any critiera for VCO oscillation? [/quote]

There are the so-called Barkhausen criteria, but I'm sure you are aware of that. Frequently you need more gain in a ring oscillator to guarantee start-up than defined by the above criteria. You may have enough gain to obtain RHP poles, but not enough to start it up.

When you change transistor ratios, are you sure your operating point is still correct? In your schematic, I'm also concerned that when you use active loads in saturation, you may need common-mode feed-back to guarantee a proper operating point. Frequently you use MOS transistors in triode region as resistive loads in such schemes, but it seems not to be your case.

Paul

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tumeda

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Re: Question on a VCO design with differential sta
Reply #14 - Oct 12^th, 2005, 5:27am

Hi ywguo
Sorry for tap mistake! I wanted to mean that , the circuit cannot oscillate again. :P

Hi Paul
About Barkhausen criteria, as my personal opinion, it is not very suitful for VCO oscillator. Even we can use a AC simulation to find the gain and phase, the AC simulation depends on the DC points. The DC point of VCO oscillator varies always during the simulation.
Which conditions can accutally trigger a VCO oscillation is still unclear for me? ???
About the PMOS as the load in differential VCO, Which region is best for VCO, triode or saturation region ?

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