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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 20:28:26 CDT 2010

Hey Kip,

Thanks for the info.

I found out that this signal of mine is indeed a "ghost signal".  In a short
wrap, my scope was aliasing the signal at higher time scales...

So, basically, I've been trying to solve a problem that doesn't exist for
the past week...Wow, I feel stupid of myself that I got myself into this
kind of situation...

Sorry for wasting your time

Thanks,

Ray Xu

On Wed, Jun 23, 2010 at 7:47 AM, Kipton Moravec <kip at kdream.com> wrote:

> I think you are on the right path with the decoupling capacitors.
> However 0.1 is not enough.  I would add a 1, 10 and 100 MFD in parallel.
> If that takes care of it, then you can start playing to see if you can
> reduce them.  Because the frequency is around 10 kHz it makes me think
> 0.1 MFD is not enough.  Putting different values in parallel also lowers
> the capacitor's series resistance.
>
> Kip
>
> On Wed, 2010-06-23 at 00:07 -0500, Ray Xu wrote:
> > 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
> >
> > _______________________________________________
> > DPRGlist mailing list
> > DPRGlist at dprg.org
> > http://list.dprg.org/mailman/listinfo/dprglist
> --
> Kipton Moravec AE5IB .- . ..... .. -...
>
> "Always do right; this will gratify some people and astonish the rest."
> --Mark Twain
>
>
>
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