I don't understand that.  Why can't I build a high Q TCXO?  I don't need to
change the compensation very fast.  Are good crystals high enough Q that it
would take too long?

What's the time constant?  I'd guess it's Q/freq, maybe with factors of 2pi
or e or ???

That seems small relative to how fast temperature changes.  (but maybe fast
relative to FCC smearing or things like that)

--
These are my opinions.  I hate spam.

On Wed, November 1, 2017 3:38 pm, Hal Murray wrote:

Compensation implies pulling the frequency away from the natural
resonance.  High Q implies that the frequency cannot be pulled very far
away from natural resonance.
The two items are directly contradictory.

--
Chris Caudle

I don't need

Hi

A high Q crystal by design is very difficult to tune. Putting it in a circuit that will
swing it far enough to compensate it degrades the Q. In addition, thermal noise
will come into the compensation circuit (even if it is noise free) and degrade things.

Bob

Bob,

This discussion is getting really interesting.  In thinking about the
crystal Q versus
tuning range conundrum, two (presumably-overlapping) concerns come to mind:

1. The motional parameters of a high-Q crystal are such that the external
   network
   needed to pull it very far would be wholly impractical.

2. Varactors themselves probably have pretty limited Q over much of their
   range.

Is my thinking on the right track at all?

Dana  K8YUM

On Wed, Nov 1, 2017 at 5:13 PM, Bob kb8tq kb8tq@n1k.org wrote:

On 11/1/2017 3:44 PM, Dana Whitlow wrote:

Sorry your thinking is NOT on the right track.

What determines the pullability of a crystal is the ratio of the
motional capacitance to the static capacitance, commonly denoted
as C1/C0.  The Q of the crystal has nothing to do with it.  The
only thing significant about the Q is that it limits the
how QUICKLY you can change the crystal frequency.

What determines the noise of a crystal is the intrinsic
flicker of frequency noise.  The Q has nothing to do with it.
If the Q is degraded somewhat by adding varactors to pull the
frequency, it doesn't affect the noise.  It is true that if
varactors are used, it is possible that the noise will be
degraded if the tuning voltage is not clean enough.  The
HP smart clocks were always limited by this problem because
no realizable voltage source was good enough, at least 20
years ago.

In the 5071, I modified the 10811 to increase its tuning range
by an order of magnitude.  This did not affect its noise
at all, AKAIK.  The zener diode reference in the 10811 is
actually quite good.  This modification was done to eliminate
the need to tweak the coarse tuning of the 10811 as it aged.

Having said this, with currently available technology, I recommend
using frequency synthesizers to do a "virtual pull"
on crystal oscillators, rather than trying to pull them
with varactors.

Rick N6RK

Hi

As mentioned in Rick’s post, it’s not really Q, it’s the motional capacitance that is the issue.
Even if you resonate out C0, you still have to deal with Cm. The only practical high Q designs
for crystals are very low Cm resonators. Yes, if you could do a design that had Rm of 0.1 ohms
it could be high Q without a high Cm. That’s not the way it works out.

Since the electrical equivalent circuit is related to the physics of the resonator, simply changing
this or that is not trivial. You might well have to change the basic material to impact some of this.
Simply as a side note, SC’s are “worse” (lower) for Cm than AT’s of similar frequency and overtone.

Depending on you definition of impractical … this is TimeNuts …. In the real world, yes indeed
impractical.

They have a very finite Q. That turns into loss in the oscillator circuit. Loss goes up as the
pull increases. The varactor is a bigger part of the circuit as the pull increases.  The Q of a real oscillator
is only partly from the crystal. Tossing more loss into the oscillator will impact the overall Q.
You might go from crystal / 2 to crystal / 5 (very arbitrary numbers and only for illustration … )

Pretty much.

Bob

On 11/1/17 1:38 PM, Hal Murray wrote:

Think of the oscillator as an amplifier and a high Q mechanical filter -
it gets the Q from being mechanically stiff.

In order to move the frequency, you have to electrically push it, which
is counter to the mechanical stiffness.  You just don't have the tuning
range available.

We struggled on a project to build a high Q, but adjustable DRO, and
that was the fundamental problem.