Hello All
I have built an oscillator (Twin T oscillator, with just R and C components). However, there seems to be an appreciable drift in frequency. Is there some additional circuitry that could help me stabilize the frequency.

Thanks

 

Originally posted by shreyas_bhat@Feb 9 2005, 11:01 PM
Hello All
I have built an oscillator (Twin T oscillator, with just R and C components). However, there seems to be an appreciable drift in frequency. Is there some additional circuitry that could help me stabilize the frequency.

Thanks

[post=5178]Quoted post[/post]​

What type of capacitors are you using?

 

Normal mica capacitors (not electrolytic), (47nF)..

Originally posted by pebe@Feb 9 2005, 05:46 PM
What type of capacitors are you using?

[post=5180]Quoted post[/post]​

 

hi

can you show your schem?

 

Originally posted by shreyas_bhat@Feb 10 2005, 03:13 AM
Normal mica capacitors (not electrolytic), (47nF)..

[post=5191]Quoted post[/post]​

You don't normally get mica capacitors of that size. Are you sure they are bot ceramic?

 

OH THAT WAS A AN ERROR. THEY'RE IN FACT FLAT DISC CERAMIC CAPACITORS.

Originally posted by pebe@Feb 10 2005, 03:42 AM
You don't normally get mica capacitors of that size. Are you sure they are bot ceramic?

[post=5198]Quoted post[/post]​

 

Originally posted by shreyas_bhat@Feb 11 2005, 08:48 PM
OH THAT WAS A AN ERROR. THEY'RE IN FACT FLAT DISC CERAMIC CAPACITORS.

[post=5250]Quoted post[/post]​

Then change them to 'poly' types, ie. polycarbonate, polyester, etc. Ceramic caps don't give good stability because of their poor temperature coefficient. The types known as High K are the worst. They use a ceramic with a high dielectric constant to get the required capacity into a smaller space. They are OK for decoupling but not for your application.

 

Originally posted by pebe@Feb 11 2005, 03:45 PM

Then change them to 'poly' types, ie. polycarbonate, polyester, etc. Ceramic caps don't give good stability because of their poor temperature coefficient. The types known as High K are the worst. They use a ceramic with a high dielectric constant to get the required capacity into a smaller space. They are OK for decoupling but not for your application.

[post=5258]Quoted post[/post]​

You are correct however there are actually many types of ceramic capacitors. First question is how much capacitance do you need?

Generally you have a tradeoff between stability and (cost and size together) and capacitance. My numbers below are rough......

NP0 ceramic is rock solid but will become large and expensive above 1 nanofarad.

Mica can be excellent but is not so common these days; and I think there may be good and bad mica. Mica is mined and mainly comes from India and Africa (??)

Another very stable type of capacitor is polystyrene; excellent electrical characteristics but this is destroyed by temp over 85 degrees C. I think this becomes huge and expensive above maybe 10 nF.

Polycarbonate is not as stable as polystyrene but is not too bad and you can get more capacitance, maybe up to a few uF although it becomes a large honker. Also polycarbonate can handle higher temp.

Polypropylene, then polyester are the next steps down from polycarbonate although are still much better than "high K" ceramics such as X7R, Z5U.