Heathkit IG-18 User Manual

FROM
SQUARE WAVE
GENERATOR
A DIODE
CAN ALSO BE
© ADDED
~
. =-~~~, .--H)\- --1"-r -----L~t~-~.---~----
-9::J- f®~ k ______ A_.
'---- -V-------
OUTPUT PULSES
Figure 11
PULSE MEASUREMENTS
Fast rise time trigger pulses can easily be
formed at the square wave output of the Gen-
erator by the addition of a singh~ resistor and
capacitor. See Figure 11. The addition of a diode
will eliminate the positive or negative pulses
that are not wanted.
The size of the resistor and capacitor depends
on
the desired frequency and how wide a pulse
you wish to have. A rule of thumb for pulse
width is that it should be about 1/10 as wide
as the positive one-half of the square wave, at
the frequency desired. The sizes of the resistor
and capacitor are generally found experi-
menting, but a close approximation can be found
in the following manner:
1
-------- x 1/2 = time of 1/2 cycle
freq. of square wave of square wave
The above x 1/10 width of average pulse
quantity in fractions of a second
Collecting the above together we get:
1
freq. of square x 1/20 = time of pulse width
for average pulse.
wave
Page 55
To find the size of the pulse that a resistor and
capacitor will give, multiply the resistance in
ohms by the capacitance in "farads" (1 iJ.F =
.000001 farads). The answer will be the time
constant of the two parts and a portion of time
usually expressed in "microseconds" (one-
millionths of a second). If the "time constant" of
the reSistor and capaCitor you choose does not
produce the pulse width you want, change the
value of either or both parts.
For example, if you want positive pulses at a
frequency of 10 kHz:
1 x
1/20 = 5 microseconds
10,000
The following calculations show that the time
constant of a 10 kn resistor and .0005 iJ.F
capacitor will give the desired 5 microsecond
pulse.
-12
500 x 10 (capacitance, .0005 iJ.F) x
1 x
10
4
(reSistance, 10 kn) =
500 x 10-
8
= 5 X 10-
6
= .000005
= 5 microseconds
Another way this time constant can be calculated
is as follows:
.0000000005 (capacitance, .0005 iJ.F)
x 10,000 (reSistance, 10 kn)
.000005 = 5 microseconds

Heathkit IG-18 Specifications

General

Brand	Heathkit
Model	IG-18
Category	Test Equipment
Language	English