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VCO amplitude

 
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neoflash



Joined: 02 Jul 2005
Posts: 346
Helped: 7
Location: Asia
Post14 Nov 2005 15:19   VCO amplitude
Folks:

I'm designing a voltage controlled oscillator.

Circuit is the same as pic attached. Control voltage is generated by another scheme as attached.

Both three and four stages cascaded are tested. Four stages looks fine in freqneucy and swing; However, three stages structure exhibit very small swing and ultra-high frequency.

Any inputs?

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sunking



Joined: 25 May 2004
Posts: 914
Helped: 46
Post15 Nov 2005 2:26   VCO amplitude
please attached the full schematic.

make sure that they have phase shift 2*pi*n
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tuki



Joined: 05 Oct 2005
Posts: 12
Post15 Nov 2005 8:57   VCO amplitude
To neoflash:

You use this type of VCO which has some drawbacks. It's swing will follow the controlled voltage to change . After all , it's gain will decease under unit then it is limited frequency .
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mady79



Joined: 30 Mar 2004
Posts: 88
Helped: 6
Post15 Nov 2005 10:09   Re: VCO amplitude
If the swing is not good then
check if replica bias circuit but assuming it is working fine as same design might be used in 4 stage as well .only thing to check is linearity of Ron Vs Ictrl ,this is the one which would give swing variations .If swing is small and constant and frequency is larger than expected then MOS Ron measured in testing are much lower than Ron seen in designing the circuit .

Hope this helps ..
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neoflash



Joined: 02 Jul 2005
Posts: 346
Helped: 7
Location: Asia
Post15 Nov 2005 14:50   Re: VCO amplitude
mady79 wrote:
If the swing is not good then
check if replica bias circuit but assuming it is working fine as same design might be used in 4 stage as well .only thing to check is linearity of Ron Vs Ictrl ,this is the one which would give swing variations .If swing is small and constant and frequency is larger than expected then MOS Ron measured in testing are much lower than Ron seen in designing the circuit .

Hope this helps ..


mady:

Replica circuit is the same between 3 stage and 4 stages. So, for same current, the voltage generated is the same.

If we assume the circuit is correctly connected in negative dc feedback, thus we can see same Ron for both 3 stage and 4 stage designs.

thanks,
Neoflash

Added after 7 minutes:

sunking wrote:
please attached the full schematic.

make sure that they have phase shift 2*pi*n


Sunking:

Yes, they are all negative feedback in DC.
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mady79



Joined: 30 Mar 2004
Posts: 88
Helped: 6
Post15 Nov 2005 18:55   Re: VCO amplitude
If there are no issues with the linearity then if swing is small and oscillation frequency is large then it is satisfying this equation Fosc = Iss/(Vdd-Vref).Cl
small Vdd-Vref large Fosc .

Hope the 3 stage has different gains and bandwidth than 4 stage if you are targeting at same Fosc .

Hope this helps..
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