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question - schmitt trigger based oscillator frequency variation with VDD

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cmos_ajay

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Hello,

I have a CMOS oscillator based on schmitt trigger and a chain of inverters. A capacitor is being charged by a current source. The output of the last inverter is fed to the gate of a NMOS which discharges a capacitor. The output of the last inverter is a series of thin pulses at 720kHz which is fed to the D flip flop to generate a square wave at 360kHz.
Supply voltage = 1.4V , total current = 1.5uA, oscillator freq. = 360kHz
If the supply voltage is increased to 2.5V, then the oscillator frequency changes by 40 percent since the schmitt trigger switching voltages change. Is there a circuit that can minimize this change in frequency with supply voltage ? Any suggestions or ideas will be appreciated.


Attached is a schematic of this oscillator.
Thanks and regards.
 

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    oscillator.jpg
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Replace the schmitt trigger with a comparator. The voltage reference for comparator is a voltage based on bandgap.
 
Last edited:
Increasing the supply voltage, decreases the delay of your inverter chain and therefore it increases the output frequency.
 

If you don't care the frequency variation according to process and temperature, changing the theshold of the schmitt trigger to be a NMOS vth will help to suppress the variation according to the supply voltage.
 

Making the current source also supply voltage dependant would cancel the frequency variation.
 

Hello,
Do you have a circuit schematic or some technical paper to show ' how to make the current source also supply voltage dependant '??
Thanks.
 

Simply generate the current using a resistor from the power supply. Do you really have a constant current anyway? Your circuit shows an ideal source.

Keith
 

This cancellation is hard to control. It is difficult to match the current dependency with the threshold dependency of schmitt trigger.
FvM said:
Making the current source also supply voltage dependant would cancel the frequency variation.
Click to expand...
 

Making the current linear dependant on Vdd would cancel the effect in the first order. The other suggestion of making the threshold supply independant would also suffer from supply variation to some extent, e.g. by varying the switching time.
 

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Hello Leo_O2

Thanks for the reply. Do you have a schematic of an oscillator with a comparator instead of a schmitt trigger ? Any technical paper or diagram will be useful.

Regards.
 

To cmos_ajay:

26_1298426196.gif

If both I1 and VR1 are independent from supply voltage, the freq will be independent from supply voltage.

---------- Post added at 03:21 ---------- Previous post was at 02:58 ----------

Switching time influence can be reduced by adding another capacitor. When one of capacitor is charged, the another capacitor is discharged.
FvM said:
Making the current linear dependant on Vdd would cancel the effect in the first order. The other suggestion of making the threshold supply independant would also suffer from supply variation to some extent, e.g. by varying the switching time.
Click to expand...
 

Thanks for your reply. Best regards.
 

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