scooby-doo
Newbie level 1

Hey all,
I want do design a current starved inverter based vco.
to achieve a linear voltage-to-frequency transfer characteristic i built already a linear voltage to current converter which supplies the input current for the vco.
this current is mirrored by simple current mirrors into the vco.
i designed the pmos transisotrs to be 10/1 and the nmos transisors to be 5/1.
(1 is the shortest lenght in process)
this allows me equal delays for rising and falling edges and an equal current in the sourcing/sinking device during transition.
as far as i know the frequency is determinded by fosc=I/(N*C*Vswing)
where I= current through inverter
N=number of stages
C=parasitic capacitance of inverter
Vsing=Voltage swing of inverter.
therefor I assumed a linear relation between my input current and the oscillation frequency.
but if I look at my transfercaracteristic I see a perfect quadratic behaviour between current and frequency. it´s looking close to linear but there is still a visible arc within my transfer characteristic.
does anyone know where that could come from and what i could do to make in more linear?
I want do design a current starved inverter based vco.
to achieve a linear voltage-to-frequency transfer characteristic i built already a linear voltage to current converter which supplies the input current for the vco.
this current is mirrored by simple current mirrors into the vco.
i designed the pmos transisotrs to be 10/1 and the nmos transisors to be 5/1.
(1 is the shortest lenght in process)
this allows me equal delays for rising and falling edges and an equal current in the sourcing/sinking device during transition.
as far as i know the frequency is determinded by fosc=I/(N*C*Vswing)
where I= current through inverter
N=number of stages
C=parasitic capacitance of inverter
Vsing=Voltage swing of inverter.
therefor I assumed a linear relation between my input current and the oscillation frequency.
but if I look at my transfercaracteristic I see a perfect quadratic behaviour between current and frequency. it´s looking close to linear but there is still a visible arc within my transfer characteristic.
does anyone know where that could come from and what i could do to make in more linear?
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