VCO phase noise

EDAboard.com Forum Index -> Analog Circuit Design -> VCO phase noise

Author

Message

eng_islam

Joined: 06 Dec 2005
Posts: 67
Helped: 4
Location: EGYPT

27 May 2006 11:39 VCO phase noise

i m designing a LC vco using the popular configration which use 2 NMOS +2 PMOS as a -Gm pairs and a NMOS current source

the problem is that the phase noise change greatly"become worse" with the control voltage and i m using the smallest available varactor in the kit "this effect become better when i reduced the varactor value"

any one have a solution to my problem Question

elbadry

Joined: 06 May 2005
Posts: 363
Helped: 62
Location: Egypt

27 May 2006 19:01 Re: VCO phase noise

I am not very familiar with VCOs but I tihnk there is a paper by emad Hegazi which discusses this effect and another by Rael ( not sure of the name )

Google
AdSense
Google Adsense

27 May 2006 19:01 Ads

khouly

Joined: 20 Oct 2003
Posts: 2479
Helped: 343
Location: EGYPT

28 May 2006 10:39 Re: VCO phase noise

try to make the varactor is parallel MOS transistors with M devices , this will make the quality factor of the varactor better coz parallel devices will make less gate resistance of the varactor

khouly

eng_islam

Joined: 06 Dec 2005
Posts: 67
Helped: 4
Location: EGYPT

28 May 2006 11:44 Re: VCO phase noise

khouly wrote:

sorry i do not get ur point
can u explain more "how can i make it parallel with M device?"
N.B:i m making a diffrential control voltage VCO so i used 2 varacto @ every node and this make every node connected to:1- a gate of a varactor and 2- the drain of another one

thank u

khouly

Joined: 20 Oct 2003
Posts: 2479
Helped: 343
Location: EGYPT

28 May 2006 12:23 Re: VCO phase noise

i mean that if the varactor is 300u , then make the varactor 30u * 10 devices connected in parallel connection

this will reduce the gate resistyence of the varactor

check the attached file in varactor design chapter

khouly

Sorry, but you need login in to view this attachment

khouly

Joined: 20 Oct 2003
Posts: 2479
Helped: 343
Location: EGYPT

28 May 2006 12:53 Re: VCO phase noise

what is the design kit do u use , and what is the technology

khouly

eng_islam

Joined: 06 Dec 2005
Posts: 67
Helped: 4
Location: EGYPT

28 May 2006 15:52 Re: VCO phase noise

khouly wrote:

what is the design kit do u use , and what is the technology

khouly

i m using a 0.18µ UMC and i make the vco to work in 2.5GHz range and by the way i made the varactor as u said because in the kit it is only available to change fingure # with fixed w and l
thank u

khouly

Joined: 20 Oct 2003
Posts: 2479
Helped: 343
Location: EGYPT

28 May 2006 17:47 Re: VCO phase noise

which type of varactor do u use ?
invrsion mode varactor or accumulation mode varactor

khouly

eng_islam

Joined: 06 Dec 2005
Posts: 67
Helped: 4
Location: EGYPT

28 May 2006 19:06 Re: VCO phase noise

khouly wrote:

which type of varactor do u use ?
invrsion mode varactor or accumulation mode varactor

khouly

accumulation mode varactor
and the circuit consume 3.8mA is that high or enough?
thank u for concerning

khouly

Joined: 20 Oct 2003
Posts: 2479
Helped: 343
Location: EGYPT

28 May 2006 19:27 Re: VCO phase noise

try to increase the current a little , and the see the effect , i think this will decrease the phase noise , but it will affect the power consumption

is there any options in the varator , to select the gate is single terminated or duoble

this also could decrease the phase noise

best wishes in ur design

khouly

eng_islam

Joined: 06 Dec 2005
Posts: 67
Helped: 4
Location: EGYPT

28 May 2006 21:45 Re: VCO phase noise

khouly wrote:

is there any options in the varator , to select the gate is single terminated or duoble

this also could decrease the phase noise

best wishes in ur design

khouly

"single terminated or duoble"
there is no option with the varactor in the kit Crying or Very sad

but can u tell me what is the diffrence between single terminated or duoble
thank u

khouly

Joined: 20 Oct 2003
Posts: 2479
Helped: 343
Location: EGYPT

29 May 2006 5:38 Re: VCO phase noise

i mean in the layout the gate connection is from the two sides or one side

what about the current did u increase it , and see the phase noise performance

khouly

eng_islam

Joined: 06 Dec 2005
Posts: 67
Helped: 4
Location: EGYPT

29 May 2006 12:46 Re: VCO phase noise

khouly wrote:

what about the current did u increase it , and see the phase noise performance

khouly

i will try it tomorrow ISA.
also i found some explaination in Dr. Emad Hegazi book it is great and have a complete chapter talking bout my problem but i could not upload it because it is so big but mainly the problem is from the nonlinearity in the varactor and the flicker noise upconvergence "AM noise" but due to the varactor it become FM and decreasing the varactor value and increasing the current will solve the problem

i will try this solutions tomorrow ISA and i will inform u about the results

thanx for concern

eng_islam

Joined: 06 Dec 2005
Posts: 67
Helped: 4
Location: EGYPT

30 May 2006 19:48 Re: VCO phase noise

my problem become better after i increase the current and adind a 5 pf capacitor parallel to current source
the phase noise become better and changing with respect to control voltage decrease to only 4.5 db for all the control voltage range
thanx khouly for ur support

khouly

Joined: 20 Oct 2003
Posts: 2479
Helped: 343
Location: EGYPT

30 May 2006 20:56 Re: VCO phase noise

good for u eng_islam

wish u the best ,
note
try to simuate ur own varactor , by using PMOS tarnsistors and force them to work in inversion mode

and see the phase noise performance

khouly