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Design >> RF Design >> VCO bias current noise reduction
https://designers-guide.org/forum/YaBB.pl?num=1263886358

Message started by macrohan on Jan 18^th, 2010, 11:32pm

Title: VCO bias current noise reduction
Post by macrohan on Jan 18^th, 2010, 11:32pm

Hi, as you know, the low frequency phase noise of VCO almost come from the tail current source, the following method of tail current noise reduction whether be used or not in VCO design? add a RC filter on the bias.

Title: Re: VCO bias current noise reduction
Post by raja.cedt on Jan 19^th, 2010, 12:46am

hi,
mainly 1/f and thermal noise of the tail current source will be the problem, how u r solution reducing that?

Thanks,
Rajasekhar.

Title: Re: VCO bias current noise reduction
Post by Mayank on Jan 19^th, 2010, 8:40pm

Hii,
This R-C Filter will only filter out the noise coming in from the bias ckt, not of the Tail Current Source....Nevertheless, the technique is used sometimes to limit noise injected from the bias cktry. TCS's flicker & Thermal Noise will still be present.

--
Mayank.

Title: Re: VCO bias current noise reduction
Post by ci on Jan 20^th, 2010, 12:47pm

I would like to add a few comments: Besides the flicker and thermal noise you may also have substrate noise and ground bounce/noise. Substrate noise couples into the tail current source primarily through backgate modulation and source /drain junction capacitance. Ground bounce couples primarily through the source, but depending how you connect your RC filter and the values of R and C, you may create an additional coupling path through the capacitor into the gate. Substrate and ground noise can be neglected in simple test chip experiments, but they may become significant in large mixed-signal chips. In those cases you may need to consider additional techniques to reduce the noise coupling effects.

Cosmin Iorga, Ph.D.
NoiseCoupling.com
http://www.noisecoupling.com

Title: Re: VCO bias current noise reduction
Post by smarty on Jan 22^nd, 2010, 10:35am

Hi,
I have just started on the vco and I am posting one more query in the same context, apology for posting one more question.

I have started on an cross coupled vco with the tail source replaced with an simple resistor ( nmos device). I would like to know if people use resistor rather than the tail source, to improve phase noise of the vco. I see from my phase noise and when I extract the noise contribution in the low frequency it is mostly the cross coupled p and n transistor in vco ( more contribution from pmos). I see from the summary that is due to the flicker noise contribution of the above device. Is there any way of improving the low frequency phase noise apart from just device size increase?.

It would be good for me if any one can help me on this.

Thanks,
SBR.

Title: Re: VCO bias current noise reduction
Post by macrohan on Jan 25^th, 2010, 11:58pm

very thanks your reply.
your are right, the R-C filter can fiter out the noise from bias ckt, it can be showed from the following picture.
the yellow line is without LDO and RC-filter;
the red line is without LOD,but with RC-filter;
the pink line is with LDO and RC filter;

R-C filter can limit the noise from LDO and bis ckt ;

Mayank wrote on Jan 19^th, 2010, 8:40pm:

Title: Re: VCO bias current noise reduction
Post by Mayank on Jan 27^th, 2010, 3:59am

Hi,

@ macrohan : Cool !! You got the funda...
@ smarty :

Quote:

I have started on an cross coupled vco with the tail source replaced with an simple resistor ( nmos device).

Passive components always provide better phase noise profile than corresponding active components, mainly because flicker noise is linherently less in a resistor than a MOS Transistor.

Quote:

I would like to know if people use resistor rather than the tail source, to improve phase noise of the vco.

No, i dont think so....People still prefer tail current source as it provides -ve Impedance [ -1/Gm ] control.

@ Dr. Iorga : I agree Gnd Bounce will couple directly if you place a R-C Filter like the once shown in figure. But I beleive inspite of a nearby M-S cktry, a STAR routing + a separate LDO from the PMU [ which usually comes with a large external cap b/w vdd & gnd rails] should take care of that....So Gnd Bounce shouldn't be that big a problem.
Also, wouldn't the cap prevent the effect of Gnd Bounce by creating same variation on both gate & source & keeping vgs constant ?
Acc. to me, this cap will prevent effects of Gnd Bounce & increasing high-freq. gnd rail PSRR.

--
Mayank.