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Design >> RF Design >> LC-VCO design
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Message started by AZADBAKHT on Jan 20^th, 2015, 11:36am

Title: LC-VCO design
Post by AZADBAKHT on Jan 20^th, 2015, 11:36am

Hi all,
I designed a LC-VCO for tuning range of 4.8 to 5.8 GHz.
when i do sweep PSS & PNoise analysis for give best Phase Noise, when I-Tail is teriod region(M4 in picture), Phase Noise of VCO is the best more then Saturation region.e
can anyone tell me why?

Best of Regards,
Azadbakht.

Title: Re: LC-VCO design
Post by raja.cedt on Jan 21^st, 2015, 3:36am

Hi..
generally tail current noise will be up converted and appears mostly in -30db/dec region. Amount of up conversion depends on tail current source gm* tank resistance. Triode current source will have less gm this could be the reason why you are getting low noise. Some low phase noise vco will not have any tail current or they will use some resister to minimize the contribution, but this comes at the cost of supply sensitivity. Please refer the following pap
A Filtering Technique to Lower LC Oscillator
Phase Noise

Title: Re: LC-VCO design
Post by AZADBAKHT on Jan 21^st, 2015, 9:17am

Hi,
tnx a lot sir raja.cedt for reply my question.
i have any question of you!
in your opinion if M4 is teriod region, not problem for layout and fabricate this circuit? or i must design I-Tail in saturation region?

Title: Re: LC-VCO design
Post by raja.cedt on Jan 21^st, 2015, 1:22pm

that's fine as long as your models are correct. If you you have a stable vdd better go with a resister...

Title: Re: LC-VCO design
Post by totowo on Jan 21^st, 2015, 6:46pm

I guess it's because you increase the bias current so as to push the tail in triode....

Generally, more power, less noise.
So you should compare the results with the same power consumption.

AZADBAKHT wrote on Jan 20^th, 2015, 11:36am:

Title: Re: LC-VCO design
Post by AZADBAKHT on Jan 21^st, 2015, 10:09pm

Hi raja.cedt,
tnx a lot dear, i got it.

Title: Re: LC-VCO design
Post by AZADBAKHT on Jan 21^st, 2015, 10:18pm

Hi totowo,
Ok, thank you for your attention.

Title: Re: LC-VCO design
Post by raja.cedt on Jan 22^nd, 2015, 8:14am

Hi,
Phase noise improves with power consumption provided oscillator works in current limited region, as soon as it enters into voltage limited range phase noise will not change with power consumption any more.

Tail transistor enter into triode region if it is small in size, then it could easily enter during the peaks of the signal swing

Thanks,
Raj.

Title: Re: LC-VCO design
Post by AZADBAKHT on Jan 22^nd, 2015, 11:16am

Hi dear Raja,
Ok, i got it...
tnx a lot for your attention dear.