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T EVIS i - AmericanRadioHistory.Com

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Decay Constant<br />

(Continued from page 11)<br />

The decaying waveform, which must<br />

be essentially d.c. in character, is impressed<br />

on one of the cathodes of a<br />

6AL5 twin diode through the primary<br />

windings of a pulse transformer. The<br />

anode is maintained at a selected potential<br />

supplied by a battery external to<br />

the circuit. As long as the cathode potential<br />

exceeds that of the anode, no<br />

conduction can take place. When the<br />

cathode and anode potentials become<br />

equal, a small current flows and a voltage<br />

appears across a differentiating<br />

circuit in the grid of a 6BA6 high -gain<br />

pentode amplifier. This voltage, initially<br />

a small pulse, is amplified and its<br />

phase is inverted by the pentode. The<br />

amplified pulse is impressed upon the<br />

cathode of the diode through a capacitor<br />

and the secondary of the pulse<br />

transformer. The combination of tubeand<br />

transformer -phase inversion drives<br />

the cathode voltage lower and increases<br />

the difference in potential between it<br />

and the anode. This action forces heavy<br />

conduction in the diode within a very<br />

short time after the small initial pulse<br />

appears. It is also cumulative and becomes<br />

oscillatory in a time determined<br />

by the reactances in the regenerative<br />

circuit of the 6BA6. Thus, within a very<br />

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short time after the reference voltage<br />

on the anode and the "signal" voltage<br />

on the cathode have become equal, a<br />

sharply rising oscillatory wave is generated.<br />

The impedance level at the anode of<br />

the 6BA6 pentode is high, and any<br />

loading of the circuit would adversely<br />

influence the speed of response of the<br />

complete circuit. Therefore, the circuits<br />

following this type of regenerative voltage<br />

comparator convert the waveform<br />

into a single pulse at an impedance<br />

level suitable for triggering a precision<br />

counter chronoscope. A diode rectifier<br />

-one-half of another 6AL5-of short<br />

time constant is connected to the anode<br />

of the pentode amplifier to convert the<br />

input oscillations into a step function<br />

containing only a slight ripple. This<br />

impulse waveform is amplified in a two -<br />

section rise -rate triode amplifier (a<br />

12ÁT7) that further increases its speed<br />

of rise. Another differentiating circuit<br />

and an overshoot diode clipper complete<br />

the output section.<br />

At the instant the decaying waveform<br />

is applied to the circuit described<br />

above, it is also impressed on a similar<br />

circuit. The second circuit is identical<br />

to the first except that the reference<br />

voltage on the anode of the comparison<br />

diode prevents the circuit from functioning<br />

until the input voltage has decayed<br />

to a lower<br />

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first circuit, usually<br />

22% volts.<br />

The output pulse<br />

of the second circuit<br />

is then used<br />

to stop the chronoscope.<br />

The time<br />

difference between<br />

the 45 volt -<br />

triggering and<br />

the 22% volt -<br />

stopping pulses is<br />

a measure of the<br />

damping coefficient<br />

of the vibrating<br />

system.<br />

The accuracy<br />

of the NBS decay -<br />

constant measurement<br />

method was<br />

determined by using<br />

precision RC<br />

networks, the<br />

time constants of<br />

which are simply<br />

calculated. The<br />

error in measuring<br />

the time increment<br />

was found<br />

to be proportional<br />

to the length of<br />

the increment.<br />

Thus, for a 3 -second<br />

increment,<br />

the error is 3%;<br />

and a 0.2 second increment has an error<br />

of less than ', %.<br />

Stability over long periods of time is<br />

insured by compensating the comparator<br />

diodes for thermal drift due to<br />

heater aging and temperature variations.<br />

The drift voltages are cancelled<br />

by using two diodes in the same envelope.<br />

In measurements of decay time in<br />

the region of 0.15 second, the compensation<br />

maintains the drift to 1 part in<br />

1500 for periods of several hours. * ..<br />

Coaxial Cavity Element<br />

i,C'ontinued from page 16)<br />

is attached to a small driving motor<br />

which frequency- modulates the output.<br />

A 60 -cycle sweep source, whose phase is<br />

adjustable for proper phasing of sweep<br />

and swept signal on the viewing scope,<br />

is also provided. Two versions of the<br />

unit have been designed: one provides<br />

an output of approximately 2 volts into<br />

72 ohms unbalanced, which is capable of<br />

being attenuated approximately 30 db;<br />

and a second provides an output of approximately<br />

1 volt into 72 ohms unbalanced,<br />

with an output capable of being<br />

attenuated over 100 db. Provision for<br />

internal and external modulation is<br />

made on the latter unit, so that it may<br />

be used as a combination sweep and<br />

signal generator.<br />

In order to fill the need for final testing<br />

of converters, it was found necessary<br />

to have a source of u.h.f. television<br />

signals covering the entire u.h.f. band,<br />

since -unlike v.h.f. -no area has more<br />

than one or two stations. To cover this<br />

deficiency, a relatively low -cost composite<br />

picture, sound, and sync signal generator<br />

using the video and sound signals<br />

of existing v.h.f. stations was designed.<br />

A block diagram of the picture signal<br />

generator is shown in Fig. 6. The unit<br />

is provided with a piston attenuator<br />

capable of a maximum output of approximately<br />

1 volt into 75 ohms and<br />

over 100 db attenuation. It has level -set<br />

meters and controls for both input and<br />

output signals, and is capable of external<br />

modulation by means of a monoscope<br />

and synchronizing generator rack.<br />

The generator makes it possible to provide<br />

a composite picture at any u.h.f.<br />

channel. In effect, it is a miniature u.h.f.<br />

station wired directly to the receiver<br />

being tested. It may be used for u.h.f.<br />

demonstrations, interference studies,<br />

hum modulation and microphonic investigations,<br />

and other applications.<br />

PAGE<br />

PHOTO CREDITS<br />

CREDIT<br />

3, 4....Industrial Nucleonics Corp.<br />

11, 12, 13, 14, 32..National Bureau<br />

of Standards<br />

15. 16 Granco Products Inc.<br />

30<br />

RADIO -ELECTRONIC ENGINEERING APRIL, 1953

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