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[DPRG] RE: [SeattleRobotics] Re: Need help debugging a XTAL colpitts oscillator

Subject: [DPRG] RE: [SeattleRobotics] Re: Need help debugging a XTAL colpitts oscillator
From: Ted Larson ted at larsonland.com
Date: Thu Jun 24 15:29:04 CDT 2010

Let me start by saying..I am no analog expert..with that said.my 2 cents..A
quickie examination of this page explaining how it is supposed to work..


.suggests that the oscillator capability is quite dependent on the impedance
across the two capacitors.  These equations assume you don't have a bunch of
stray impedance from your wiring of the circuit itself.  How do you have
this circuit constructed?  Is it just on a breadboard?  Or did you layout a
PCB with maximum effort to minimize trace length impedance, with the whole
thing laid down over a big ground plane?


If it is on a solderless breadboard, I would suggest this is a likely
candidate for the problems.





From: SeattleRobotics at yahoogroups.com
[mailto:SeattleRobotics at yahoogroups.com] On Behalf Of Ray Xu
Sent: Tuesday, June 22, 2010 10:08 PM
To: SeattleRobotics at yahoogroups.com; dprglist at dprg.org
Subject: [SeattleRobotics] Re: Need help debugging a XTAL colpitts



And also one more thing I forgot to mention (sorry about this...late at
night): I probed the power rails of the oscillator circuit and only showed
small stray ringing noise.  I placed two 0.1uF decoupling capacitors to try
and get rid of that ringing, but its still there.  I do understand that a
circuit cannot be noise-less; so I dont know if this small ringing noise is
of importance to my problem.  I then probed the base pin of the transistor,
and it did show that low-frequency component.  I also tried adding a small
capacitor in series in between crystal and ground, but that did not work.

Thanks again,

Ray Xu 

On Wed, Jun 23, 2010 at 12:03 AM, Ray Xu <rayxu123 at gmail.com> wrote:

Hey Guys,

Long time since my last post...

Anyways, I'm building a crystal oscillator-driven Colpitts Oscillator.  I
got all the capacitor/resistor values correct and the output is oscillating
at its frequency stably, however there is a parasitic low-frequency
component in the output.  In other words, the output is not biased at a
fixed voltage, but rather more like at a 10-ish KHz sine wave signal.  Note
that I'm not talking about a AM-envelope-like interference, but rather as if
the center of my signal-of-interest is moving in a sine wave pattern of a
fixed frequency.  My goal is just to get rid of this low-frequency component
by finding the source of this parasitic signal.  Building a high-pass filter
would be my last resort. 

So far, I've been talking with my mentor (Randy M Dumse).  Among the things
we've tried are: decoupling the circuit, using a battery as a power source,
and I even tried building a LC high-pass filter however that severely
loaded/attenuated both signals.  My circuit is like this
http://en.wikipedia.org/wiki/File:Cc_colp2.svg where L is replaced with a
crystal oscillator, Ic is a resistor, and with a LC filter circuit (L=4.7mH,
C=150uF) at the transistor's collector.  The crystal oscillator's output is


Ray Xu 



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