[DSP-10] 10MHz Crystal osc question

Sat, 23 Feb 2002 19:12:12 -0700

Nando,

Which I think I did.

The explaination was for the benefit of the DSP-10 list.  I am sure there
are those out there that would like to understand more about how their
DSP-10 works.

- Steve, N7HPR
  ([email protected])

> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of H.G.
> Sent: Friday, February 22, 2002 9:29 PM
> To: [email protected]
> Subject: RE: [DSP-10] 10MHz Crystal osc question
>
>
> Steve:
>
> I am, and quite aware, about the Pierce Oscillator in addition of several
> other types since I have been using them for more that 38 years as an
> Electronic Design Engineer in areas of R.F.  --
>
> I wanted you to do the clarification of your statement.
>
> Regards
>
> Nando
>
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of Steven Bible
> Sent: Friday, February 22, 2002 9:33 PM
> To: [email protected]
> Subject: RE: [DSP-10] 10MHz Crystal osc question
>
>
> Hello Nando,
>
> You are correct in stating if the capacitors are in parallel you add them.
> I made a misleading statement.  Please allow me to clarify,
>
> In a Pierce oscillator, two load capacitors are placed on each side of the
> crystal.  They have a dual purpose, they provide reactance and thus phase
> shift and they provide impedance matching between the amplifier (which is
> onboard the LMX1501A) and the crystal.  Recall that the
> Barkhausen criteria
> states that for an oscillator to start, you must have gain exceeding unity
> and N x 2 X Pi radian phase shift in the loop.  The two load capacitors
> provide part of the necessary phase shift.  The best explaination
> of Pierce
> oscillator operation I have read is in R.J. Matthys, "Crystal Oscillator
> Circuits."
>
> I made the mistake of saying the load capacitors are in parallel with the
> crystal.  Physically maybe, but electroly they are in series.  Thus the
> formula I gave (capacitors in series) is correct, my description was not.
>
> Does this help clear things up?
>
> Sorry about the mistyping :-)
>
> - Steve, N7HPR
>   ([email protected])
>
>
>
>
>
> > -----Original Message-----
> > From: [email protected]
> > [mailto:[email protected]]On Behalf Of H.G.
> > Sent: Friday, February 22, 2002 11:42 AM
> > To: [email protected]
> > Subject: RE: [DSP-10] 10MHz Crystal osc question
> >
> >
> > STEVE:
> >  >If C108 is 22 Pf and the 37 Pf you added to C110, in parallel
> > that's 13.8
> > Pf.
> >
> > If the capacitors are in parallel, the values need to be added, if the
> > capacitors are in series the capacitors need to be added with
> the inverse
> > value and the result the inverse value.
> >
> >  59 Pf if in parallel  or 1/22 + 1/37 = 13.79 Pf if in series,.
> >
> > Regards
> >
> > Nando
> >
> > -----Original Message-----
> > From: [email protected]
> > [mailto:[email protected]]On Behalf Of Steven Bible
> > Sent: Thursday, February 21, 2002 10:22 PM
> > To: [email protected]
> > Subject: RE: [DSP-10] 10MHz Crystal osc question
> >
> >
> > Hello Lee,
> >
> > The 10 MHz crystal supplied with the TAPR kit should be an Epson
> > CA-301 Type
> > (DK SE3420).  There's information about how Bob selected this
> > crystal on the
> > DSP-10 web pages.
> >
> > In looking at the data sheet, the load capacitance is 18 pF.  I
> > assume that
> > when you say you are adding 37 pF you are stating the total parallel
> > capacitance around the crystal.  If that's the case, then it
> > sounds like you
> > are in the ball park.  If C108 is 22 pF and the 37pF you added
> to C110, in
> > parallel that's 13.8 pF.  Add 5 pF for stray capacitance and
> the total is
> > 18.8 pF.  So you see, you are there.  Since U104 (LMX1501A) is using a
> > Pierce Oscillator, it would be best to balance the load
> capacitor values.
> >
> > If I made a bad assumption about your parts placement, can you
> let us know
> > what capacitors you added where?  What values you have for
> C108, C109 and
> > C110?
> >
> > - Steve, N7HPR
> >   ([email protected])
> >
> >
> >
> > > -----Original Message-----
> > > From: [email protected]
> > > [mailto:[email protected]]On Behalf Of Lee Scott
> > > Sent: Thursday, February 21, 2002 12:02 PM
> > > To: [email protected]
> > > Subject: [DSP-10] 10MHz Crystal osc question
> > >
> > >
> > > I have my DSP-10 up and running but have had to pad quite a bit on the
> > > 10MHz crystal to get it to the right frequency.  As originally
> > > constructed,
> > > the DSP-10 was transmitting 10KHz high so I padded C109 first
> > with a 5pf,
> > > then a 10pf cap.  I was still too high and I had to add a 27pf
> > > NPO disk cap
> > > from C110 to gnd before the low end of the C110 would tune the
> > > osc down low
> > > enough to bring it on freq.
> > >
> > > Am wondering if the 10MHz crystal TAPR is kitting is the
> right one as I
> > > have had to pad a total of 37pf to pull the osc down.
> > >
> > > Lee Scott - AA1YN
> > >
> > > _______________________________________________
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