HP E1938 Ovenized Crystal Oscillator

The E1938A was sold to at least 3 major customers and we played games with the connector to make it compatible with them. I don't remember exactly why that funny connector was used. Either the connector it had to mate with had coax inserts already for historical reasons, and the E1938A connector had to at least have holes in it without inserts to allow mating, or there was some other arcane reason like the connector with inserts was a different height or something. Some versions had the connector on the other side of the board. Whatever the reason was, you don't need to worry about it because no version of the E1938A ever used those coaxial positions. I think you can put a regular connector in there if you clip off the unused pins. I vaguely remember shenanigans like that.
-Rick-

Hz off freq. after warm up:
This is a measure of how well the reasonating capacitor was selected by the factor to center 10 MHz in the EFC window.

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.

(referring to another temperature control system) 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.

Refering to vacuum oven: http://rfdesign.com/vlf_to_uhf/time_and_frequency/709RFDF1.pdf
At HP, in the 90's, we did a lot of brainstorming about vacuum ovens. This never seemed to make sense to us. If you actually achieve high amounts of thermal resistance, then you can't get the heat out of the oven. And if you don't, why bother with a vacuum. Also, a vacuum only helps if you do everything else you need to do to make a true Dewar (thermos bottle), like having mirrored surfaces, etc. Finally, having a vacuum means that nothing that outgasses can be used in the oscillator. Maybe Vectron has figured out something we didn't think of or has sufficiently difference constraints that a vacuum makes sense for them.

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-

The E1938A project started out as a Meacham bridge oscillator and the number was chosen because it was the date of invention of that oscillator. Eventually, we realized that design wasn't going to work for us and we had to invent our our bridge oscillator.

The lamp thing is interesting because there is a lot of HP folklore that has grown up around the HP garage, the invention of the 200A oscillator, etc, and it seems that Bill Hewlett has gotten credit for the lamp stabilization idea. Possibly, he independently invented it, since he filed 6 months after Meacham did, but long before the Meacham patent was granted. Hewlett also copied or reinvented the idea of a bridge oscillator. His real contribution was to harness Meacham's previous technology to enable him to eliminate the inductor from the oscillator, which allowed him to raise the impedance level high enough to allow air variable capacitors to be used. It was a great design, whoever invented it.

(According to tradition, the model number 200 was used instead of 100 to give the impression that this wasn't the first HP product).

The original Hewlett patent is on display at Agilent headquarters where I work. I was kind of surprised that HP ("HP Invent") let Agilent have it. I am also pleased that in the HP Archives museum, on the first shelf, in the center, is a *working* 5071A. There is also, of course, a 200A oscillator on display.

The above are my own opinions and don't represent Agilent or HP.

Rick Karlquist

Calls:

4155257	May 1979	Wittke
4283691	August 1981	Burgoon
4358742	November 1982	Ferriss
4518930	May 1985	Rozema et al.

Called By:

7,230,503	Imbalanced differential circuit control
6,885,534	Electrostatic discharge protection device for giga-hertz radio frequency integrated circuits with varactor-LC tanks
6,504,443	Common anode varactor tuned LC circuit
6,218,885	Circuit and method for providing temperature stability in an FM quadrature detector

Calls:

4157466	June 1979	Herrin
4216371	August 1980	Marotel
4317985	March 1982	Wilson
4396892	August 1983	Frerking et al.
4839613	June 1989	Echols et al.
5025228	June 1991	Gerard et al.
5041800	August 1991	Long et al.

Called by:

7,113,051	Frequency characterization of quartz crystals
7,102,220	Multiple cavity/compartment package
6,784,756	On-board processor compensated oven controlled crystal oscillator
6,606,009	Self-compensating ovenized clock adapted for wellbore applications
6,362,700	Temperature controlled compensated oscillator
6,208,213	Thermostatically controlled crystal oscillator
6,166,608	Thermo-electric cooled oven controlled crystal oscillator
6,127,661	Dynamic thermal control for ovenized oscillators
6,060,692	Low power compact heater for piezoelectric device
6,049,256	Low profile ovenized oscillator packing having a high thermal conductivity substrate
5,919,383	Package for a temperature-sensitive optical component with inner and outer containers and resistive element therein
5,917,272	Oven-heated crystal resonator and oscillator assembly