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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..

http://en.wikipedia.org/wiki/Colpitts_oscillator

.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.

 

-Ted

 

 

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
oscillator

 

  

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
16.6MHz

Thanks,

Ray Xu 

 

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