Re: [time-nuts] Improving the stability of crystal oscillators
The 10811 has an ANALOG oven control loop. The gain is set
to be just below the oscillation point. This is due to the
stability limits dictated by the oven mass and (believe it
or not) the size of integrator capacitor that can physically
fit. If you want to "soup up" at 10811 oven, externally wire
in a larger capacitor in parallel and change the resistors to
increase the gain. The 10811 designers did the best they
could with what they had to work with, but you don't want to
blindly copy them in new applications.
BTW, do not use a "metalized" plastic integrator capacitor.
Must be "foil" type.
I am extremely happy with the PII^2D control loop on the E1938A
(I didn't design it, only tested it). I can't imagine anything
being better.
Rick Karlquist N6RK
Bruce Griffiths wrote:
If a purely proportional control loop has such great performance why
does the 10811A use a PI temperature controller and the E1938A use a
PII^2 D controller?
Surely the finite offset between the setpoint and actual temperature
achieved by a proportional controller is a source of long term
temperature instability?
If one uses resistive heating then some linearisation improves the
performance as the heat from the heating element is proportional to the
square of the voltage across the heating element.
A state space controller may give improved performance but PI(10811A),
PID and PII^2 D(E1938A) controllers seem to work well when used to
regulate crystal oscillator temperatures.
Bruce
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Hi Tom:
Very interesting. How about building a circuit to drive those GR crystals?
The same man is the inventor of all four of the early GR patents, James Kilton
Clapp, of "Clapp Oscillator" fame.
http://en.wikipedia.org/wiki/Clapp_oscillator
In the 1930s GR patents he's showing an inductor in series with the crystal and
Wiki dates the "Clapp Oscillator" 1948 where a capacitor is in series.
Have Fun,
Brooke Clarke
http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Cam
Tom Van Baak wrote:
Maybe because it was written back in 1930's, but one of the better
discussions of quartz temperature control, including considerations
of insulation and isothermal layers (attenuation and conduction)
are in the patents for the old double oven General Radio frequency
standard:
http://www.leapsecond.com/museum/gr676b-50kc/
http://www.leapsecond.com/museum/gr676b-50kc/US1967185.pdf
/tvb
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On 11/10/2007, at 10:23 PM, Bruce Griffiths wrote:
If one uses resistive heating then some linearisation improves the
performance as the heat from the heating element is proportional to
the
square of the voltage across the heating element.
If you use a transistor as a heater, the full supply voltage is
across the
element all the time, so heating is proportional to the current flowing.
Neville
Neville Michie wrote:
If you use a transistor as a heater, the full supply voltage is
across the
element all the time, so heating is proportional to the current flowing.
Neville
Neville
However using a transistor has the disadvantage of a small area heat
source rather than the large area heat source possible with a heater
winding.
Using a small area heat source produces significant temperature gradients.
Bruce
We went through this tradeoff on the E1938A. Resistive heaters
can be distributed. However, it is very inefficient to drive
them with transistors, because then you waste a lot of power
heating the transistors, which is waste heat if resistive heating
is used. Prior to the 10544, they just put up with this. The
10544 used a switching regulator for up the efficiency, but it
put a 1 kHz spur on the oscillator. The 10811 used two transistors
on opposite sides to try to sort of distribute the heat. On
the E1938A, we looked at an array of small surface mount transistors
to have the best of both worlds. However, this turned out not to
be manufacturable and we settled for resistive heaters (back to
1970!).
Rick Karlquist N6RK
Bruce Griffiths wrote:
); SAEximRunCond expanded to false
Errors-To: time-nuts-bounces+richard=karlquist.com@febo.com RETRY
Neville Michie wrote:
If you use a transistor as a heater, the full supply voltage is
across the
element all the time, so heating is proportional to the current flowing.
Neville
Neville
However using a transistor has the disadvantage of a small area heat
source rather than the large area heat source possible with a heater
winding.
Using a small area heat source produces significant temperature gradients.
Bruce
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From: Bruce Griffiths bruce.griffiths@xtra.co.nz
Subject: Re: [time-nuts] Improving the stability of crystal oscillators
Date: Fri, 12 Oct 2007 10:08:34 +1300
Message-ID: 470E90D2.6000304@xtra.co.nz
Hej Bruce and Neville,
Neville Michie wrote:
If you use a transistor as a heater, the full supply voltage is
across the
element all the time, so heating is proportional to the current flowing.
Neville
Neville
However using a transistor has the disadvantage of a small area heat
source rather than the large area heat source possible with a heater
winding.
Using a small area heat source produces significant temperature gradients.
Using a single transistor yes. Spread out transistors can provide significantly
less gradients.
Cheers,
Magnus