In a message dated 10/10/2007 11:13:21 Pacific Daylight Time, jra@febo.com
writes:

Hi John,

interesting anecdote!

Would you know if the thermistors are still available? Who made  them?

Thanks
Said

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SAIDJACK@aol.com said the following on 10/10/2007 02:17 PM:

We bought them as a replacement part from Yaesu USA.  This data is 10
years or more old, but here you go:

Part Name Yaesu Part Number Murata Part Number
Posistor G9090019 PTH507A01BG330N020

They used to be about $7.00 each.  Yaesu's parts-order phone number at
the time was (800) 255-9237.  I'm not sure if there's another source for
the Murata part; back then, we couldn't find one.

Amazingly, I still have a web page documenting all that we learned about
those cursed radios, including the schematic of our original homebrew
heater design: http://www.febo.com/hamdocs/d4art.html.  The quality of
the heater schematic isn't very good, but it's all I have.

John

From: SAIDJACK@aol.com
Subject: Re: [time-nuts] Improving the stability of crystal oscillators
Date: Wed, 10 Oct 2007 14:17:24 EDT
Message-ID: bd4.1a3ab729.343e7134@aol.com

John,

Agree!

Thanks!

Cheers,
Magnus

I bought 10kohm nominal NTC thermistors  from Digikey, P/N 490-4653-ND. They
are very small, 1/10th of an inch long or so.

I have used those (or similar parts) in projects both at home and at work
(in military equipment...) for about 15 years.

Didier KO4BB

I have made several ovens for oscillators over the years.
The recipe is:
get a piece of aluminium big enough to contain the oscillator,
voltage regulator and first stage amplifier.
With a mill remove the shapes of each component.
Bolt a large power transistor, large power fets are best, to the
outside of the block as a heater, and it is run
off the unregulated input power. Judicious selection of a component
decides the start-up current.
Make a plate to cover the excavation for the components, and bolt it
down.
The circuit can be made with discrete transistors in the most
unstable looking amplifier ever seen,
alternating NPN and PNP transistors, connected directly to each other
with load resistors.
The main temperature sensor is a resistor bridge with a high value
glass encapsulated thermistor.
These are available a several trade houses. The amplifier is also
temperature sensitive, but is within the thermal loop.
The thermistor bridge gives a very large signal ~ 50mV per degree.
Gain may have to be backed off if thermal
oscillations do not die down, but the metal block acts as an
integrator and the circuits are very easy to get
high gain and sensitivity.
The whole block is packed in two inches of foam insulation, my 1MHz
oscillator only draws about 80 mA at 12 volts.
The temperature is set to 40 C.
The stability of the oscillator is very good, but as I have not yet
got a disciplined oscillator going I dont know which is drifting,
the HP 10811 in my frequency counter or the 1MHz oscillator. After a
year, the difference is currently 0.3 ppm.
cheers Neville Michie

On 11/10/2007, at 9:24 AM, Didier Juges wrote:

Neville Michie wrote:

Neville

A correctly tuned PID control loop should allow even tighter temperature
control.
A "boostrapped" oven like that used by Wenzel should be even better.

(http://www.wenzel.com/documents/Sub-pico%20Multiplier.pdf).

Despite Wenzel's claims this type of oven isn't new it was used for
portable standard cell enclosures decades ago.

Bruce

Hi All,
I am not trying to start an argument, but I would like to point out
that PID controllers are only
good at controlling a certain class of system.
For a system that has a coal truck that must dump its coal down a
hole, the system has mass, velocity
and distance, all the qualities to get a perfectly damped system with
PID control.
For thermal control, the function is more likely to be a Bessel
Function, and a Z transform filter
is more likely to find a match.
In any case, PID controllers are often to be found in totally
unsuitable situations giving worse control
than even a bang-bang controller.
The thermal block controllers work well because of the dominant
integrating effect of the block,
the time delay for a heat front to propagate through the block is the
only concern for instability.
When instability is a problem I relocate the thermistor closer to the
heater, giving a marginal degree of under-
control.
Because the block is well insulated it soon becomes very close to
isothermal.
cheers, Neville Michie

On 11/10/2007, at 3:08 PM, Bruce Griffiths wrote:

Neville Michie wrote:

Neville

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

Neville Michie wrote:

I would like to point out that the E1938A uses a PID controller and
has a transient thermal gain of many 1000's not to mention a static
gain that has in some cases exceeded 1,000,000 for a single oven.

Your last statement is not generally true.  The block cannot be well
insulated because of the thermal overhead of the oven circuitry (the
heat has to escape).  I explained in my 1997 FCS paper how to achieve
the isothermal condition, which is achieved by symmetry rather than
high amounts of insulation.  The E1938A oven works quite well if the
insulation is omitted or replaced by poor insulation, except that it
consumes more power.

Rick Karlquist N6RK