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Design >> RF Design >> why cmos relaxation RC oscillator is frequency stable over varying temperature?
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Message started by kanz on Apr 4^th, 2014, 3:29am

Title: why cmos relaxation RC oscillator is frequency stable over varying temperature?
Post by kanz on Apr 4^th, 2014, 3:29am

cmos relaxation RC oscillator was found to be frequency stable when i swept temperature ranging from -40 degree to 100 degree.when i plotted propagation delay of the cmos inverters which i used for this design,it was found to be varying with temperature sweep.Why the change in propagation delay not affecting the frequency of operation ? In the case of ring oscillator ,propagation delay is varying much and it is affecting the frequency ..what point am I missing here ? please help :(

Title: Re: why cmos relaxation RC oscillator is frequency stable over varying temperature?
Post by boe on Apr 7^th, 2014, 5:26am

Kanz,
the frequency depends mainly on the RC time constant, I suppose...
- B O E

Title: Re: why cmos relaxation RC oscillator is frequency stable over varying temperature?
Post by analog_rf on Sep 22^nd, 2014, 5:38am

There could be some kind of cancellation hapenning. I suppose you are using comparators, could be possible that comparators(I/C or RC kind of osc's) are slowing down at 100c and your reference current is increasing at 100c,effectively ending up compensating each other.

Title: Re: why cmos relaxation RC oscillator is frequency stable over varying temperature?
Post by raja.cedt on Sep 22^nd, 2014, 7:13am

Hi,
what's your architecture? is it based smitt-trigger and inverting amp or some capacitor charging and comparator?? I didn't understand why inverter delay comes into picture?

@analog_rf..Can you explain your reply bit more clear..Usually compactor is much faster than actual oscillator delay, so I don't think that makes lot of diff.

Thanks,
Raj.

Title: Re: why cmos relaxation RC oscillator is frequency stable over varying temperature?
Post by analog_rf on Sep 24^th, 2014, 6:03am

Faster is a relative term. As the Osc frequency reaches say tens of Mhz Comparator delay begins to play a role (say 20Mhz or more).Basically if without applying any cancellation techniques, you end up getting very good freq accuracy, then you are probably seeing some cancellation (eg:comp delay more at 100c causing output freq to decrease and Iref increasing at 100c). One method to isolate causes in such a scenario would be to plot :osc freq vs Temp for atleast 5 temperature points and you would see the freq variation peaking at a point other than 100c.