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Hboue & Beyond continued from page 63

oven are inserted into a con-trolled-temperature chamber. In many circuits, the device used is nothing more than a sophisticated custom-made Thermos1 M bottle with a hole through the sealing cap for bringing leads out to run the circuitry,

Actual construction of the device containing the oscillator would look like a three dimensional sandwich containing the inner oscillator circuity protected with some insulation, and covered by the first inner oven. Then more insulation such as Styrofoam™ and the outer oven and still more insulation like a thermal blanket to hold the unit snugly in the Thermos bottle. Lfs kind of like constructing a ship in a bottle, bul making sure that everything is insulated (thermally) from each other, and that a!t voids are minimal.

Can we duplicate 'he circuitry or is this too complex? Yes wre can, if we start out with an oscillator circuit that is not a simple unit but has several stages to isolate the oscillator from the output, and incorporates voltage regulation! Wcran into a suitable unit, a 10 MHz TCXO oscillator that had internal temperature adjustment and 10-volt voltage regulation. The oscillator was made to function on +12 volts and ran on 35 mA of current so as to not produce much heal on its own account There is a variable capacitor access hole on the oscillator side to set the unit to frequency.

Construction can be as simple as placing an insulated unit on its own in u Thermos bottle and letting the ambient temperature do its trick. Or you could create a very slow time-constant single oven to maintain some preset temperature above a normal high ambient, say 90 degrees. Why slow time-constant temperature control? Because a fast temperature control will over-shout its mark and be running continually, either trying to catch up? or {when it cools down), trying to reheat the unit.

A very slow control reacts over a longer time period and will ultimately remain quite fixed at some mean temperature point you select. This will continue when inserted in a Thermos bottle, as it. and all the components in the bottle, will maintain die same temperature.

If you have an oscillator already, or have put together something similar to this, I

Fig. I. Schematic diagram of simple one-stage temperature control circuit using a J130 op amp. RI is selected to reflect the Temperature of the thermistor s resistance at the calibration point selected. The heater can be a large number of turns of #36 enamel wire or nichrome resistance wire to provide a suitable load (¡0 to 20 Ql for ihe T/P-120 Darlington transistor The thermistor selected was a 10 k Unicurve™ devicet but others will work. It should be in direct thermal contact with the oscillator metal container for best results* Ri = thetmistbr temperature at regulation point. For example, if thermistor used is 67'8 £2 @ 100^, then Rl - 67H Q.

Fig. I. Schematic diagram of simple one-stage temperature control circuit using a J130 op amp. RI is selected to reflect the Temperature of the thermistor s resistance at the calibration point selected. The heater can be a large number of turns of #36 enamel wire or nichrome resistance wire to provide a suitable load (¡0 to 20 Ql for ihe T/P-120 Darlington transistor The thermistor selected was a 10 k Unicurve™ devicet but others will work. It should be in direct thermal contact with the oscillator metal container for best results* Ri = thetmistbr temperature at regulation point. For example, if thermistor used is 67'8 £2 @ 100^, then Rl - 67H Q.

outlined previously how you ensure you have a good accurate unit.

There are several methods to accomplish the necessary calibrations for achieving a specific point of stability in your oscillator. Time and frequency standards are broadcast by the government's National Bureau of Standards, now renamed NTST. The calibration of your oscillator can be accomplished by comparing it with one of these transmissions. The simplest of them all is WWV, which broadcasts frequency standard information at 5, 10, \5, and 20 MHz exactly. Their frequency is compared at the source to extremely accurate reference oscillators.

To compare your oscillator, you only need a receiver capable of receiving these frequencies and to make an audible beat note adjustment minimizing the difference between your oscillator and WWV, This method is capable of making an adjustment that is accurate to 1 part per MHz. Not too bad, but this is just the start of w hat could be a very accurate comparison.

The reason you are not able to make exact reference is that on these frequencies transmission via the atmosphere affects the exact frequency and changes it slightly, A more accurate transmission is at the VLF or

Very Low Frequency of 60 kHz. Here ihe atmosphere does not impart as much of a change on the signal and better resolution can be obtained. Other sources include LORAN, GPS (Globally Positioned Satellite), or even calling up N1ST on the telephone and obtaining voice lime readings. There is even a service on the Internet that will adjust the clock o: your computer to exact or nearly exact time.

More information can be obtained from the NIST information published on their internet page at [http://rio.bldrdoc.gov/ timefreq/T This is the best informational page covering most of the frequency standard information that is available—and it's tree, Can't beat that for good information, so give it a look-see.

Just in case some of you can't locate a suitable oscillator for trying some of these methods that I have covered, I will be g!ad to provide one for experimentation. I have come across a suitable oscillator with specs that are quite good. I have a large number of Epulis" that were rejected for one reason or another. I suspect these units fail to meet original temperature specs over a great range. I have short-term tested them and junked any that fail to function, are way off frequency, or have any other malady, instead of

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