Hi,

Would like some help...
i am building an osc for Experimental purpose. For my design, i have use a colpitts oscillator.

My required freq is 700~800khz, but my measure freq is only about 800hz~3khz.

I've use the following components:

Cvdd = 10uF
CB = 1nF
R1 = 21k ohm
R2 = 12k ohm
RE1 = 1-10k ohm Potentiometer
RE2 = 470 ohm
L1 = 1.5uH
C1 = 0.1uF
C2 = 33nF
Q1 = 2N2222A Bipolar Transistor

Voltage measure at Base of Q1 is 4.4v, at Collector is 3.8v. Therefore the transistor is working.

Does anyone know why i am getting the wrong freq?
I've build it on a breadboard. Some practical issue that i have overlooked?

i've tried changing the L1,C1, C2 but it seems to get the same result.
Anyone have any clue?

Thx in adv.

 

Hello,

Did you read this article on the colpitts oscillator?
http://en.wikipedia.org/wiki/Colpitts_oscillator

Greetings,
Bertus

 

Yap... can't seems to find any link to my problem..
i am using the same eqn listed on the page to find my freq.

 

The top of L1 should be connected back to the bottom of C2 via something like a 1uF ceramic or plastic film cap.

As shown, the main tuned circuit, whic consists of L1 in parallel with C1+C2, is 'broken' by having the electrolytic decoupling cap in series with it.

Electrolytics are great for bulk power storage and audio stuff, but useless at RF.
You need to bypass the power circuit with a cap that has very low impedance at the frequency you are working with.

Keep all connections absolutely as short as possible!