Spice Crystal Model - 70MHz

[Fred Bloggs]

Wow! I'm shocked. Fred actually made a funny typo. ;-)

Haha- that is funny- must be the brane disease.

 

[Michael A. Terrell]

Haha- that is funny- must be the brane disease.

I normally try to overlook the occasional typo, but that one was too
funny to ignore. ;-)


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Spehro Pefhany]

I normally try to overlook the occasional typo, but that one was too
funny to ignore. ;-)

"Brane" is an archaic spelling of "brain", no doubt used for effect
and to stimulate ye olde brainpan.


Best regards,
Spehro Pefhany

 

[John Larkin]

Okay- well that might have been a stretch, but JT does need to watch for
the traps, like exciting various crystal vibration modes close to the
4th harmonic but not the 4th harmonic, or some tendency to chaotically
switch into a different mode. The previous people cannot be complete
incompetents and these kinds of issues are widely none thanks to authors
like Parzen, so there must be something up with this particular crystal
cut that makes a reliable design particularly challenging. If he doesn't
get into the crystal physics on this one, his product will be no more
than a hunch.

Do crystals have any serious nonlinearities, aside from the case of
gross overdrive? That could make startup interesting.

And even fundamental crystals have gobs of un-advertised resonances,
some of which can be pretty close to the promised one. You just hope
that the rated-frequency resonance has a much higher Q than the
others, and that your circuit doesn't favor the wrong one. Overtone
oscillators are actually nice because they usually (!) have an LC tank
in the loop, which zaps most of the hazards.

Maybe Jim could make his loop gain peak around 70 MHz somehow. He
should at least get a full-sweep plot that shows all the resonances,
rather than just a Spice model of the main one.

Jim: what was wrong about the flubbed design? Did it not oscillate, or
oscillate at the wrong frequency, or what?

John

 

[[email protected]]

Seems a bit like teaching granny to suck eggs but these are my
thoughts.
The parameters could vary al lot from manufacturer to manufacturer
depending on the electrode diameter quality of quartz and parallelism.
I would construct a device assuming scaled from 20 MHz fundemental to
70MHz fundemental as the blank will not be contoured and will have
scaled plateback.
Assume maximum esr of 100R AND ENSURE THAT THE CIRCUIT ACTIVITY WILL
NOT OSCILLATE spurious 60MHz to 300MHz with an esr much above 120 ohms.
The problem of reactance of Co less than esr is reduced due to
fundemental operation and small electrode diameter but it may pay to
design in some active bridge cancellation of 2pF Co . Power level must
be low due say 10uW or if you can measure a crystal raise power till
frequency changes 1ppm.

 

[Michael A. Terrell]

"Brane" is an archaic spelling of "brain", no doubt used for effect
and to stimulate ye olde brainpan.

I was refereing to this from Fred's previous message:

Okay- well that might have been a stretch, but JT does need to watch for
the traps, like exciting various crystal vibration modes close to the
4th harmonic but not the 4th harmonic, or some tendency to chaotically
switch into a different mode. The previous people cannot be complete
incompetents and these kinds of issues are widely none thanks to authors KNOWN

like Parzen, so there must be something up with this particular crystal
cut that makes a reliable design particularly challenging. If he doesn't
get into the crystal physics on this one, his product will be no more
than a hunch.

Wow! I'm shocked. Fred actually made a funny typo. ;-)


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Ken Smith]

Do crystals have any serious nonlinearities, aside from the case of
gross overdrive? That could make startup interesting.

Yes, there is an extra loss at the small signal end. This makes it
posible to adjust the gain of an oscillator so that it will sustain
oscillations, but the won't start on their own.

[...]

And even fundamental crystals have gobs of un-advertised resonances,
some of which can be pretty close to the promised one. You just hope
that the rated-frequency resonance has a much higher Q than the
others, and that your circuit doesn't favor the wrong one. Overtone
oscillators are actually nice because they usually (!) have an LC tank
in the loop, which zaps most of the hazards.

On the SC cut crystals, the mode 10% away is still trouble with a simple
LC. If you make the Q of the LC high enough, its drift starts to matter.

Lucky for JT, he most likely has a AT cut rock.

 

[Gerhart H.]

Anyone have a reasonably accurate spice model for a 70MHz crystal?
(FUNDAMENTAL)

...Jim Thompson

Maybe it helps to mount one (or a couple of them one after the other for
statistics) into an appropriate fixture of a network analyzer to see
which modes are excited and which not. The result should reflect all the
crystal mechanics in the electrical amplitude, frequency and phase
domain. It then should be possible to disentangle the results into
values of a reasonable electrical model. Is this naive? I have never
done it myself but it makes me eager to start tinkering with it too. Has
anyone experience with this?
Regards
Gerhart