T EVIS i - AmericanRadioHistory.Com
T EVIS i - AmericanRadioHistory.Com
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 />
value than in the<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