Why three stages of VCO show small swing and high frequency?

[neoflash]

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?

 

[sunking]

VCO amplitude

please attached the full schematic.

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

 

[tuki]

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 .

 

[mady79]

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 ..

 

[neoflash]

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 ..

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:

please attached the full schematic.

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

Sunking:

Yes, they are all negative feedback in DC.

 

[mady79]

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..