Af Oscllator

Fig. 2. Schematic of Generator

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Fig. 2. Schematic of Generator

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p.ate pick up the signal much beyond the 8Va in. point.

Pretty soon in your receiver 'peaking work you get to that signal that may be but a tenth of a microvolt or so, and you begin dreaming about cryogenic front ends, masers, and such As mentioned, every fraction of a decibel lower in noise figure, every improvement in sensitivity comes out rigorously and relentlessly on that slide dial. You can easily check which of your low-noise transistors is really low, whether that MOSFET will do a better or worse job for you, and in which circuit.

As you go up in frequency you may have to make smaller and smaller oscillators in order to fit in smaller waveguides to get the cutoff effect. (That will not be a problem if you read 73; the May issue described a "postage-stamp-sized" rf generator that is an ideal candidate for the signal source.)

Circuit

A crystal oscillator, an af oscillator, and a simple class A modulator do an excellent job to start with, FigT 2 shows the present unit as used on 6 meters. It must be stressed again that no wire or other piece of metal may be allowed to reach the outside from this assembly. I'm making up another for 2 meters soon (still my favorite band) and will try one on 450 a little later.

Audio

A controlled feedback transformer-coupled af oscillator does a good job in furnishing a sine wave. A Motorola HEP55 is used for the oscillator, with feedback to the base from the collector through transformer T1, controlled by resistor R2. Audio output is taken off the 5kQ winding of T1, is fed through R4 the modulation control, and then to the base of af modulator Q2. Transistor Q2 is set up for low-power class A operation because not much modulation is needed for the signal generator. Transformer T2 is an old 5W unit from "tube-type portable" days. The secondary of T2 feeds a modulated + 9V signal to Q3, the crystal-controlled 50 MHz oscillator.

This rf oscillator is one of my negativefeedback jobs with phase reversal in the crystal. A V/i in, square plate is tied onto the collector, radiating energy to the receiver pickup plate facing it inside the waveguide. This energy is rapidly attenuated as you move the plates apart, and should be impossible to detect after some 9 or 10 in. of separation.

Once again, do not bring any wires or any other metal or conductor out from the oscillator assembly. If you want an outside controlled switch or other control, bring it out as a wooden dowel handle.

That's about it Tune everything up outside the waveguide on the bench and when you're satisfied, plug your best 6 meter receiver into J1, push the oscillator plank along the waveguide (or rather 1 should say pull it along) away from J1 You'll get a surprise1 Hope this helps vou with your low-noise receiver uork It did a lot for me.

William E. Hood W2FEZ 116 I'/esf Park St. Albion NY 1441J

itubes

Itrctronii flash has long since come into its own with a wide variety of applications ranging from photography to theatrics. In spite of its wide range of exploitation, however, it seems to have attracted relatively little interest to the experimenter. Considering the simplicity of the circuitry, it is something oi' a shame that this field is so neglected.

Whether taking a picture, activating a rub> laser, or freezing motion, the principle and circuit is basicallj tiie same, ยก he lamp itself consists of a glass or quartz tube filled with a rare gas. In photographic applications, the gas is usually xenon, which produces a spectrum approximating that of daylight.

The tube is connected across a large capacitor which is charged to a high voltage. When a high voltage pulse is applied to a triggering electrode a conductor in close proximity with the lamp envelope - the gas inside ionizes, i Ipon ionizing, the resistance of the gas drops to almost a short circuit and the capacitor discharges producing a brilliant flash of light.

The power consumed by a flash tube is usually measured in watt-seconds - the number of watts drawn by the load times the number oi" seconds the lamp is lit. The average amateur flash unit runs about 25 watt-seconds. This may seem small until you

units average around I X>00 BCPS. A thousand beam candle power seconds crowded into 250 microseconds has an instantaneous brilliance of four million candle power!

Milk drop making crown effect.

consider the flash only lasts about 250 microseconds. A 25 watt-second lamp, if it burned steadily, would consume 100,000 watts! You don t believe it? Okay. Here's where I got the number. 2 50 microseconds equals 1/4000 seconds. To deliver 25 watt-seconds in that amount of time, the instantaneous power consumption must be 25 x 4000 or 100,000 watts!

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