Amateur Radio Radio Amateur 03-1967 Mohican Product Detector

range switch. With the values shown in the schematic, this circuit provides gains of 3, 10 and 30, which extend the 1.5 volt scale of the VOM to 500, 150 and 50 millivolts lull scale.

There are two zero controls which must be adjusted when using this unit; first the 500 kilohm pot (HI) in the base bias lead to transistor QI is adjusted to zero the output with no input and the base isolated from ground. The 1.5k zero adjust pot (R2) is then adjusted for an output zero with the input leads shorted together.

it will detect and amplify signals from 20 Hz to above 432 MHz. Since this circuit only requires about 100 microamperes, no on-off switch is provided. This very small current drain insures that the life of the battery will be nearly that of its shelf lite. By using miniature components and a little care in layout, it is possible to mount this complete injector/tracer in an old pen light case or metal cigar tube.

VOM range extender

Most volt-ohm-mill¡ammeters are not too suitable for use with transistor circuits because their lowest voltage scales are either L5 or 3 volts full scale and they are not sensitive enough to accurately measure the base to emitter voltage of a transistor which may he 120 millivolts or so. The low voltage dc preamplifier shown in Fig. 59 is inexpensive, stable with temperature and supply voltage variations yet extends the range of any VOM so it can be used effectively in semiconductor circuit measurements*

In this circuit transistors Q1 and (¿4 constitute an emitter coupled amplifier; Q5 is an emitter follower connected so the circuit's entire output voltage is fed back to Q4. Transistors Q2 and Q3 are constant current sources in the negative and positive lines respectively. These constant current sources reduce the sensitivity of the amplifier to voltage supply variations and result in substantially lower drift. To control the gain of the amplifier for different voltage ranges, a portion of the output voltage is fed back to the base of transistor Q4 through the voltage divider selected by the


The simple VHF monitor/detector illustrated in Fig, 60 may be used for measuring field strength, monitoring modulation or even in hidden transmitter hunts. The frequency of use may be simply changed by changing the length of the dipole antenna. In some cases where the rf field is strong enough, a simple vertical pickup antenna will be sufficient for signal monitoring purposes. Furthermore, the use of this monitor/detector is not limited to the VHF bands; the addition of an appropriate antenna will permit its use at any frequency up to about 500 MHz. For lower frequencies where the size of the dipole antenna would be impractical, a simple vertical pickup antenna and tuned resonant circuit may be substituted at the input.

1 kHz oscillator

The simple 1 kHz oscillator shown in Fig. 61 is very useful for many testing devices around the shop. This circuit is simply a Colpitis oscillator, but the circuit values and feedback have been chosen for maxi-

2n2925 Circuits

Fig, 6L Simple 1000 Hi oscillator is very useful for testing and measurement around the shack. The Colpitts circuit is used with component values chosen for maximum stability and good waveform.

Fig. 62. The 100 fcHi calibrator shown here is ¡ust about the simplest circuit that will provide usable results. For zeroing In with WWV a small padder capacitor may be added in series with the 100 kHz crystal.

01 -2N70B, 2N£7llh 2*2923, 2N369I

Fig. 62. The 100 fcHi calibrator shown here is ¡ust about the simplest circuit that will provide usable results. For zeroing In with WWV a small padder capacitor may be added in series with the 100 kHz crystal.

mum stability along with good waveform. It may be used for testing speech and modulation equipment, or even as a driver for a code practice oscillator.

100 kHz crystal calibrators

Thr 100 kHz crystal oscillator illustrated

■r in Fig. 62 is just about as simple as you can build and still get a usable output, This circuit will provide usable harmonics up to about 30 MHz but it has no pro-\ision for zeroing in with WWW This feature may be added by simply placing a small variable padder capacitor in series with the 100 kHz crystal.

The calibrator circuit shown in Fig, 63 is only sightly more complicated than its counterpart in Fig. 62, but provides usable harmonics up to 150 MHz and has a built in voltage regulator. In this case the base b+

100 Khz Crystal Calibrator

Q\ ANY NPN PLANAR SJLlCOlM TRANSISTOR 02. 2N2925, 2N3392, 2N3565, SE4002

Fig, 63, This 100 kHz crystal calibrator is only slightly more complicated than the one shown in Fig, 62, but has a built in voltage regulator (Ql) and provides usable harmonics up to 150


Fig, 64. This 100 ItHi crystal calibrator uses a crystal in the parallel mode and provides either a sinusoidal or square wave output. The calibrator may be zeroed to WWV with the 4-30 pF trimmer, to emitter junction of an NPN silicon planar transistor is connected as a zener diode. When connected in this manner, these transistors provide a regulated voltage of approximately 11 volts. The value of the series dropping resistor may be determined by using the formula shown in the schematic. The 5—80 pF capacitor is for zeroing in with WWV.

The 100 kHz crystal controlled oscillator

shown in Fig. 64 uses a low cost silicon transistor» and provides both a square wave and sine wave output with excellent frequency stability. The oscillator circuit is basically the Hartley type with positive feedback from the collector to base through a phase reversal in the tapped tank circuit, consisting of two 240 mH chokes and a 1700 pi-1 silver mica capacitor, The oscillator frequency is determined by the resonant frequency of the very high Q series LC network in the feedback loop. This network is made np of the 4—30 pF variable capacitor and quartz crystal which operates in the parallel mode. The variable capacitor provides a fine frequency adjustment control. Feedback is sufficiently large to assure normal circuit operation almost completely independent of transistor gain. The large amount ot feedback drives Hie collector from cutoff to saturation* making a square

Was this article helpful?

0 0
Wood Working for Amateur Craftsman

Wood Working for Amateur Craftsman

THIS book is one of the series of Handbooks on industrial subjects being published by the Popular Mechanics Company. Like Popular Mechanics Magazine, and like the other books in this series, it is written so you can understand it. The purpose of Popular Mechanics Handbooks is to supply a growing demand for high-class, up-to-date and accurate text-books, suitable for home study as well as for class use, on all mechanical subjects. The textand illustrations, in each instance, have been prepared expressly for this series by well known experts, and revised by the editor of Popular Mechanics.

Get My Free Ebook

Post a comment