[SI-LIST] Re: Crystal osciiator not oscillating
- From: Eddy <eddyvk@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Sat, 26 May 2007 12:00:24 -0700 (PDT)
Adding capacitance right across a crystal is indeed
the worst you can do. The crystal itself will have a
holder capacitance in the range 3-5pF and adding
12-14pF of a high resistance (10MOhm) passive probe
will certainly have impact. However, most crystal
oscillators use a CMOS inverter for amplifier and in
this setup both leads of the crystal will have a
signal. You would need a differential probe to really
measure "across" the crystal. Touch either lead of the
crystal with just the ground side of a passive probe
and you'll kill the oscillator for sure.
When you connect the passive probe ground to the
circuit ground and probe each crystal lead, the
oscillation will most likely not stop. However, it may
change so much that there is no point doing it other
than to answer the question "does it run?". The added
capacitance will lower the amplitude and move the
frequency. Touch the inverter input side with a FET
probe and you'll kill the oscillator for DC bias
reasons, not capacitance reasons. From each crystal
connection to ground there is already something like
20-50pF on the chip or you need to add it external. If
adding another 12-14pF from a probe kills the
oscillation, I'd say the design is marginal.
All this applies to 'regular' crystal oscillators in
the range 10-50MHz. Real time clock oscillators (RTC)
at 32.768KHz are horribly sensitive to anything.
Breath in the direction of the circuit and it dies.
;-)
I sometimes use a differential FET probe to measure
the voltage swing across a crystal, to get an idea of
crystal drive. To not get in trouble with the DC load
of the probe I add a capacitor (100pF) in series with
each of the two probe tips. Works like a charm!
Eddy van Keulen
PhaseLink Corp.
--- art_porter@xxxxxxxxxxx wrote:
>
> Probing crystal oscillators right across the crystal
> is a tough challenge. Any high-resistance scope
> probe has too much capacitance. Even a tiny amount
> of capacitance will kill the oscillator. And active
> probes with lower capacitance have 100K or lower
> resistance, which changes the bias, which also kills
> the oscillator.
>
> Art Porter
> Agilent Technologies
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of
> Eddy
> Sent: Wednesday, May 23, 2007 6:50 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Fwd: Re: Re: Crystal osciiator
> not oscillating
>
>
> This is what I was thinking too. My first thought
> was
> actually that the probe used to measure the signal
> might be killing the oscillator. If the feedback
> resistor is 1MOhm and you are loading it with a
> 100KOhm FET probe, then you pull the bias completely
> out of its linear region where the AC gain exists.
> So
> the oscillation stops.... The general advise is to
> probe the inverter output where this sensitivity for
> DC load does not exist. If you want to see the
> waveform on the input, add a capacitor in series
> with
> the probe tip to remove the DC load.
>
> Eddy
>
>
> > --- Andrew Ingraham <a.ingraham@xxxxxxxx> wrote:
> > >
> > > > 2) By keeping the load capacitance value
> > > same as 15pF, a 100K
> > > > externalresistor between the XTAL &EXTAL pin
> of
> > > the microcontroller,
> > > > population starts oscillations.
> > >
> > > I don't know much about xtal oscillators, but
> I'm
> > > wondering if there might
> > > be a DC bias problem with the circuit. With the
> > > circuit not oscillating, is
> > > the input (one of the pins) biased so that the
> > > inverters are in their linear
> > > (amplifying) region? Perhaps the 100K resistor
> is
> > > needed to help make that
> > > happen.
> > >
> > > Andy
> > >
> > >
> >
>
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