QL With this circuit, crystal dri\e is reduced to less than 10 microwatts (less than 1/100 of what it would be without AGO,

03 is an additional follower to isolate the following stages from the oscillator itself. Because of AGC action, the DC level on the emitter ol Q2 operates near ground, so Q3 must be a PNP device to provide proper DC operation.

Q4 and Q5 are high-gain amplifiers and follower Q6 provides a low source impedance for the AGO detector circuit, Dl and 02^ Q7 and Q8 provide a TTL-compaii-blc output signal. The output is a nearly symmetrical 5 volt square wave.

D4 and CI6 perform as a variable capacitance diode to allow electrical fine-tuning of [he oscillator frequency. The control voltage is provided by the divider consisting of R26 and Rl. Rl is a 10-Uirn precision pot with a counter dial, mounted on the front panel of the unit. Increasing the resistance of Rl increases the frequency. Although the adjustment is somewhat nonlinear the tuning rate is roughly one part in 10^ per turn.

The Ovens

To be effective, the crystal and oscillator must be kept at a constant temperature, as said before. This means ihey must not be affected by changes in the surrounding environment. While testing my original oven unit, which housed everything in a minibox, I noticed that even a few degrees change in room temperature affected frequency. I finally decided that the bcM and most easily available housing for this unit would be a thermos bottle. The mode! I chosc is the one-pint Food Jar model 7221. The top cup is not used.

The oscillator circuit is housed inside the thermos bottle and the oven heater consists of four 10 ohm 10 wail resistors (Figure 2, R26-29). Temperature control is provided by a thermistor (Figure 2, RTl) which is connected to the bridge circuit of Figure 2,

Even with the use of the thermos bottle housing and the proportional control scheme, outside temperature changes can still affect the inner temperature. This is solved by a second outer oven which is also proportionally controlled. The housing for the entire unit consists of a 3/4"~thick particle board box attached to the back of a 12"-high relay rack panel. The box dimensions are: 14" wide by 10" high by 10" deep. The inner surface of the front panel is lined with styrofoam insulation. The thermos bottle is

"To be effectivey the crystal and oscillator must be kept at a constant temperature fastened by its handle to a shelf which fastens to the rear panel.

Some of the oven control components (Q3. Q4, 1C4. 1C5. and associated components) arc mounted on a 3" by 7" metal plate that is fastened to the rear panel. Resistors R3Q-33 mount on terminal strips on the shelf in front of the thermos bottle. (See photos.) Front panel connections to the frequency control pot and the meter selector switch are made through an 8-pin Cinch-Jones connector. A connector assembly is also used on the cabling from the thermos bottle so it can be easily disconnected and removed. F\>wer and the 1 MHz output are connected to a barrier strip on the rear panel. If you use an external meter rather than an internal one as 1 did, you will need a larger barrier strip than one with only four screws.

Photo C. Detail of the inner oven heater assembly. Note the heater resistors* crystal, an J thermistor. The assembly is mounted to the to ft of the thermos bottle and to the end of the oscillator board prototype.
Photo D. Looking inside the box, (Photos by WB9DOF.)
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