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

Subject: [DPRG] Re: Need help debugging a XTAL colpitts oscillator
From: Ray Xu rayxu123 at gmail.com
Date: Wed Jun 23 00:07:42 CDT 2010

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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