Parametric Amplifiers Wabs

—Parametric amplifiers are probably the most practical way for hams to get a low noise figure at VHF and UHF, This book is the only one available that covers both theory and practice. $2.00


W6VAT.—Every ham needs to have and know how to use test equipment. This book tells you how to make valuable ham test gear easily and cheaply.

It clso covers the use of test equipment. 50c

HAM^RTTY.—This is the most complete book on the subject. Written for the beginning TT'er as well as the expert. Pictures and descriptions of □II popular machines, where io get them, how much, etc. $2.00

Marty Feeney, Jr+ KIOYB

IQ Test

Are yon kept off the air because of TVI? Is your puny 25 watts drowned in the vast wasteland of 20 meter QRM? Does your rig suffer from tired blood? In short, are you

somewhat dissatisfied with amateur radio? Relax, then. This article wont solve all (or any) of your problems, but it will give you an enjoyable few hours.

There are five ham shacks, each of a different color. Each of the operators runs a different rig, each belongs to a different organization, and each reads a different magazine. Below are certain facts about each of these hams. By proceeding in a logical manner, you should be able to determine the answers to these questions:

1) Who reads "Playboy"?

2) Who belongs to the "Munjoy Hill Society for the Preservation of Spark on the Eleven Meter Band"?

Note, The relationships described need not necessarily be reasonable. For example, the DXer may or may not belong to dxec, the lid doesn't necessarily read CQ, etc. The houses are in a straight line, numbered from left to right.

1) There are five ham shacks,

2) The lid operates from the red ham shack.

3) The DXer reads CQ.

4) The operator in the green ham shack belongs to DXCC,

5) The VHFer belongs to ARRL.

6) The green ham shack is immediately to the right of the ivory one.

7) The owner of the DX-20 reads QST.

8) The ham in the yellow shack operates a Communicator II.

9) The operator in the middle house belongs to the IoAR.

10) The ham in the first house is a RTTTer,

11) The ham who operates the home-brew rig has liis ham shack next to the ham who reads 73,

12) The "Proceedings of the IRE° are read in the ham shack next to the ham shack where the Communicator II is operated.

13) The operator of the KWM 2 belongs to the Certificate Haters Club.

14) The Rag-Chewer runs an ARCS.

15) The RTTY'er built his ham shack next to the blue one. . . . KIOYB

Answers on p. 78

73 Technical Feature

Understanding the Schmitt Trigger Circuit-

Jim Kyle K5JKX 1236 N.E. 44th St. Oklohoma City, Okla

Recent publication of RTTY converters built around "Schmitt Trigger" circuitry has revealed, through reader response, that many of us hams don't know very much about this exceptionally useful circuit, and consequently can't follow our usual practice of cut-and-try modification.

Which is distinctly not a good state ol affairs, because the Schmitt Trigger is one of the most versatile and useful circuits ever devised outside the basic amplifier stage! It can act as a peak clipper to provide virtually infinite clipping; it can convert sine waves into square waves should you need them; and it is especially useful for RTTY converters because it combines several functions into a single stage.

Essentially, the Schmitt is a regenerative switch which flips on or off in a matter of microseconds (or even fractions of a microsecond when so designed) yet will remain on so long as the input signal tells it to, it's a variant of the general family of multivibrators; the characteristic which makes it unique in the family is that it doesn't 'vibrate" like all the rest. The routine multivibrator of the free-running variety oscillates continually, the 4 mono-stable" or "one-shot" delivers a single timed output pulse for each input signal applied, regardless of length of the input signal, and the "bistable" or flip-flop alternately turns on or off. Only the Schmitt exhibits the relay type of operation by remaining on so long as input is present.

Unlike the mechanical relay, though9 'he Schmitt can operate nicely up in the megacycle range. It has negligible time delay between input signal and output action, and similarly small delay between removal of input and "drop-out." To boot, it's a high-impedance device which can be connected almost anywhere without loading down the driving source, and it automatically provides an "inverted" or "rev erse" output as well as its normal output.

The output is a voltage at one of two levels; the upper level is equal to the supply voltage (when the Schmitt is not loaded down by the following stage) while the lower is determined by the trigger design but is usually much much lower. A low-voltage level of about 50 volts is typical for vacuum-tube Schinitts; something well below 1 volt is typical for the transistor version of the circuit.

These output levels hold true regardless of the input level. If input signal is large enough to trigger the circuit, output rises to the upper level and stays there. If the input signal is not large enough to trigger, the output remains low.

At the "inverted" output terminals, conditions are reversed. Voltage is at the lower level when the circuit is triggered, and goes liigh when the input signal is absent.

These charaeteristices make the Schmitt a perfect clipper, since any signal above the trigger point will produce an output of known level. Signals below the trigger point don't get through. It also is a wave-squarer, since the trigger point can be set at virtually zero volts. For half of a sine-wave input, the trigger is on, and for the other half it s off. he output, consequendy, is a square wave.

And since a 2-volt change in input level can cause a 150-volt change in output level (in the vacuum-tube version) the Schmitt turns out to be a pretty good amplifier as well Thus in RTTY it can combine the functions of limit-* ing? inverting for "bilateral" copy, and amplifying the teletype signal all in one stage. The transistor version is even more sensitive, producing a change in output of some 10 volts with an input variation of only 0.01 volt. This equals a gain of 1,000 times through the stage, with limiting action thrown in.

To see how it works, take a look at Fig. 1. This shows a typical vacuum-tube Schmitt Trigger circuit, with all circuit components identified. A bit farther on we'll find out how to determine proper values for all these things.

Vacuum Tube Schmitt Trigger

First let's assume that one of the two triode sections is at zero Mas, and conducting as much current as the tube and its plate resistor will allow. Don't worry about the effect of R6, the cathode resistor, because it really doesn't have much to do with the amount of current *vhich flows.

However, the voltage across R6, or Ek, will je equal to the current times the value of R6 n ohms. Let's make sure this amounts to enough volts to completely cut off either triode ¡ection; how much this will have to be can be letermined from a glance at the tube handbooks.

Now let's adjust the relative value of R1 md R2 to fix the voltage at Vla's grid, Eel, ight at cutoff. Remember that cutoff voltage s measured between grid and cathode, and he cathode is positive to ground by Ek volts, rherefore, Eel should l>e equal to Ek minus he cutoff voltage (all voltages expressed as Positive numbers).

This ensures that Via is cut of!:, and no cur* -ent passes tlirougl the tube. With no current lowing, the plate voltage Epl should rise to he same level as the supply B -h If it were iot for the shunt path to ground made up of \4 and R5> this would irxlee*1 be so? and if and R5 are both large compared to R3 the lifference between B 4- and Epl wall be small ndeed-

The grid of V l b, though, is connected to the unction of R4 arid R5T arid so is receiving a raction of the plate voltage of Via. The exact ralue of Ec2 under these conditions is de-ermined by the ratio of R5 to the total of R3, and R5, and can be made almost any-hing desired. For most positive and sensitive ction, Ec2 under these conditions should be et about Y> volt positive with regard to Ek. 7hat is, if Ek is 10 volts, Ec2 should be 10^ to 1 volts.

Remembering that the effective bias is measured between grid and cathode, with the ;rid at Ec2 and the cathode at Ek, the effective bias on Vlb will be about )z volt positive, his locks Vlb into conduction, which estab-ishes our original assumption and assures us hat Ek does really exist.

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