On 2006-07-04, Mike Monett wrote:
Ben Jackson wrote:

I built a Colpitts oscillator with a plain old 2N3904 and it hasn't
moved by more than a few Hz in the hour it's been on.

Could you do me a favor and post your 2N3904 schematic?

___ +5V
.--|33k|---+
| |
| |/
.-o---o----| 2N3904
| | |
| --- |
| --- .--o----
_-_100p| | |
|___| o---' .-.
- | | | 150
25Mhz | --- | |
| --- '-'
| 100p| |
o-----o------'
|
===
GND

The 33k/150 were chosen to get CMOS compatible output. The circuit will
oscillate with much higher values for each.

The two 100p might be slightly too high for the 18p crystal I'm using,
but I didn't tune those carefully because I want to pull the crystal
slightly, and in my actual circuit the grounded leg of the crystal is
actually in series with a variable capacitance to tune the VCXO.

(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

--
Ben Jackson

http://www.ben.com/